前言

博客：http://blog.healerjean.com

一、依赖

1、`mybatis-plus`依赖

<!--mybatis-plus-->
<mybatis-plus-boot-starter.version>3.4.1</mybatis-plus-boot-starter.version>

<!--mybatis-plus-->
<dependency>
    <groupId>com.baomidou</groupId>
    <artifactId>mybatis-plus-boot-starter</artifactId>
    <version>${mybatis-plus-boot-starter.version}</version>
</dependency>

2、我的项目依赖

1）主项目

<?xml version="1.0" encoding="UTF-8"?>
<project xmlns="http://maven.apache.org/POM/4.0.0" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
         xsi:schemaLocation="http://maven.apache.org/POM/4.0.0 https://maven.apache.org/xsd/maven-4.0.0.xsd">
    <modelVersion>4.0.0</modelVersion>
    <parent>
        <groupId>org.springframework.boot</groupId>
        <artifactId>spring-boot-starter-parent</artifactId>
        <version>2.2.5.RELEASE</version>
        <relativePath/> <!-- lookup parent from repository -->
    </parent>
    <groupId>com.healerjean.proj</groupId>
    <artifactId>hlj-parent</artifactId>
    <version>0.0.1-SNAPSHOT</version>
    <name>hlj-parent</name>
    <description>Demo project for Spring Boot</description>
    <packaging>pom</packaging>

    <properties>
        <java.version>1.8</java.version>
        <lombok.version>1.18.4</lombok.version>
        <!--swagger 版本-->
        <swagger.version>2.7.0</swagger.version>
        <!--数据源 注意提高下面的版本，否则不支持LocalDate-->
        <com-alibaba-druid.version>1.1.9</com-alibaba-druid.version>
        <!--mybatis plus-->
        <mybatis-plus-boot-starter.version>3.4.1</mybatis-plus-boot-starter.version>
    </properties>

    <modules>
        <module>hlj-client</module>
    </modules>

    <dependencyManagement>
        <dependencies>

            <!--lombok-->
            <dependency>
                <groupId>org.projectlombok</groupId>
                <artifactId>lombok</artifactId>
                <version>${lombok.version}</version>
            </dependency>

            <!--swagger-->
            <dependency>
                <groupId>io.springfox</groupId>
                <artifactId>springfox-swagger2</artifactId>
                <version>${swagger.version}</version>
            </dependency>
            <dependency>
                <groupId>io.springfox</groupId>
                <artifactId>springfox-swagger-ui</artifactId>
                <version>${swagger.version}</version>
            </dependency>


            <!--数据源-->
            <dependency>
                <groupId>com.alibaba</groupId>
                <artifactId>druid-spring-boot-starter</artifactId>
                <version>${com-alibaba-druid.version}</version>
            </dependency>

            <!--mybatis-plus-->
            <dependency>
                <groupId>com.baomidou</groupId>
                <artifactId>mybatis-plus-boot-starter</artifactId>
                <version>${mybatis-plus-boot-starter.version}</version>
            </dependency>

        </dependencies>
    </dependencyManagement>

    <build>
        <plugins>
            <plugin>
                <groupId>org.springframework.boot</groupId>
                <artifactId>spring-boot-maven-plugin</artifactId>
            </plugin>
        </plugins>
    </build>

</project>

1.1.2.2、子工程

<?xml version="1.0" encoding="UTF-8"?>
<project xmlns="http://maven.apache.org/POM/4.0.0" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
         xsi:schemaLocation="http://maven.apache.org/POM/4.0.0 https://maven.apache.org/xsd/maven-4.0.0.xsd">
    <modelVersion>4.0.0</modelVersion>
    <parent>
        <groupId>com.healerjean.proj</groupId>
        <artifactId>hlj-parent</artifactId>
        <version>0.0.1-SNAPSHOT</version>
    </parent>
    <groupId>com.healerjean.proj</groupId>
    <artifactId>hlj-client</artifactId>
    <version>0.0.1-SNAPSHOT</version>
    <name>hlj-client</name>
    <description>Demo project for Spring Boot</description>

    <properties>
        <java.version>1.8</java.version>
    </properties>

    <dependencies>
        <!--web-->
        <dependency>
            <groupId>org.springframework.boot</groupId>
            <artifactId>spring-boot-starter-web</artifactId>
        </dependency>

        <dependency>
            <groupId>org.springframework.boot</groupId>
            <artifactId>spring-boot-starter-test</artifactId>
            <scope>test</scope>
            <exclusions>
                <exclusion>
                    <groupId>org.junit.vintage</groupId>
                    <artifactId>junit-vintage-engine</artifactId>
                </exclusion>
            </exclusions>
        </dependency>
        <dependency>
            <groupId>junit</groupId>
            <artifactId>junit</artifactId>
        </dependency>
        <dependency>
            <groupId>org.springframework</groupId>
            <artifactId>spring-test</artifactId>
            <version>5.1.9.RELEASE</version>
            <scope>text</scope>
        </dependency>


        <!-- Jackson jsonUtils-->
        <dependency>
            <groupId>com.fasterxml.jackson.core</groupId>
            <artifactId>jackson-databind</artifactId>
            <exclusions>
                <exclusion>
                    <artifactId>jackson-annotations</artifactId>
                    <groupId>com.fasterxml.jackson.core</groupId>
                </exclusion>
            </exclusions>
        </dependency>
        <dependency>
            <groupId>com.fasterxml.jackson.core</groupId>
            <artifactId>jackson-core</artifactId>
        </dependency>
        <dependency>
            <groupId>com.fasterxml.jackson.core</groupId>
            <artifactId>jackson-annotations</artifactId>
        </dependency>
        <dependency>
            <groupId>com.fasterxml.jackson.module</groupId>
            <artifactId>jackson-module-parameter-names</artifactId>
        </dependency>
        <dependency>
            <groupId>com.fasterxml.jackson.datatype</groupId>
            <artifactId>jackson-datatype-jdk8</artifactId>
        </dependency>
        <dependency>
            <groupId>com.fasterxml.jackson.datatype</groupId>
            <artifactId>jackson-datatype-jsr310</artifactId>
        </dependency>


        <!--lombok-->
        <dependency>
            <groupId>org.projectlombok</groupId>
            <artifactId>lombok</artifactId>
        </dependency>

        <!--swagger-->
        <dependency>
            <groupId>io.springfox</groupId>
            <artifactId>springfox-swagger2</artifactId>
        </dependency>
        <dependency>
            <groupId>io.springfox</groupId>
            <artifactId>springfox-swagger-ui</artifactId>
        </dependency>

        <!-- 数据源 -->
        <dependency>
            <groupId>com.alibaba</groupId>
            <artifactId>druid-spring-boot-starter</artifactId>
        </dependency>
        <dependency>
            <groupId>mysql</groupId>
            <artifactId>mysql-connector-java</artifactId>
        </dependency>


        <!--mybatis-plus-->
        <dependency>
            <groupId>com.baomidou</groupId>
            <artifactId>mybatis-plus-boot-starter</artifactId>
        </dependency>

        <!--StringUtils-->
        <dependency>
            <groupId>org.apache.commons</groupId>
            <artifactId>commons-lang3</artifactId>
        </dependency>

    </dependencies>

    <build>
        <plugins>
            <plugin>
                <groupId>org.springframework.boot</groupId>
                <artifactId>spring-boot-maven-plugin</artifactId>
            </plugin>
        </plugins>
    </build>

</project>

3、配置文件

1）、`application.properties`

spring.application.name=hlj-mybatis-plus
spring.profiles.active=local


####################################
### DB
####################################
#durid
spring.datasource.type=com.alibaba.druid.pool.DruidDataSource
spring.datasource.druid.driver-class-name=com.mysql.jdbc.Driver
spring.datasource.druid.initialSize=5
spring.datasource.druid.minIdle=5
spring.datasource.druid.maxActive=20
spring.datasource.druid.maxWait=60000
spring.datasource.druid.timeBetweenEvictionRunsMillis=60000
spring.datasource.druid.minEvictableIdleTimeMillis=300000
spring.datasource.druid.validationQuery=SELECT 1 FROM DUAL
spring.datasource.druid.testWhileIdle=true
spring.datasource.druid.testOnBorrow=false
spring.datasource.druid.testOnReturn=false

# 本大爷啥也没配
# 配置 mybatis的一些配置，也可以在 application.properties 中配置，如果配置了就不需要了mybatis.xml
#mybatis-plus.config-location=classpath:mybatis.xml
#Maven 多模块项目的扫描路径需以 classpath*: 开头 （即加载多个 jar 包下的 XML 文件）
#mybatis-plus.mapper-locations=classpath*:mapper/*.xml
mybatis-plus.type-aliases-package=com.healerjean.proj.data.entity
##主键类型  0:"数据库ID自增，非常大", 1:"用户输入ID（如果用户不输入，则默认是0）",2:"全局唯一ID (数字类型唯一ID)", 3:"全局唯一ID UUID";
mybatis-plus.id-type: 0
/** mybatis-plus如果希望使用数据库自增 */
// @TableId(value = "id", type = IdType.AUTO)
//private Long id;
    
 #字段策略 0:"忽略判断",1:"非 NULL 判断"),2:"非空判断" （默认2）
mybatis-plus.field-strategy: 2
 #数据库大写下划线转换
mybatis-plus.capital-mode: true
mybatis-plus.refresh-mapper: true

2）`application-local.properties`

server.port=8888


####################################
### DB
####################################
spring.datasource.druid.url=jdbc:mysql://localhost:3306/hlj_mybatis_plus?serverTimezone=CTT&useUnicode=true&characterEncoding=utf-8&allowMultiQueries=true
spring.datasource.druid.username=root
spring.datasource.druid.password=123456
####################################

3）`MybatisPlusConfiguration`

/**
 * mybatisPlusInterceptor
 *
 * @author zhangyujin
 * @date 2023/6/15  11:54.
 */
@Slf4j
@MapperScan("com.healerjean.proj.data.mapper")
@Configuration
public class MybatisPlusConfiguration {
    /**
     * MyBatis支持
     *
     * @return MybatisPlusInterceptor
     */
    @Bean
    public MybatisPlusInterceptor mybatisPlusInterceptor() {
        long t1 = System.currentTimeMillis();
        MybatisPlusInterceptor interceptor = new MybatisPlusInterceptor();
        interceptor.addInnerInterceptor(new PaginationInnerInterceptor());
        long t2 = System.currentTimeMillis();
        log.info("MybatisPlusInterceptor injected! times:{}ms", t2 - t1);
        return interceptor;
    }

}

4、数据库语句

CREATE TABLE `user` (
  `id` bigint(20) NOT NULL COMMENT '主键ID',
  `name` varchar(30) DEFAULT NULL COMMENT '姓名',
  `age` int(11) DEFAULT NULL COMMENT '年龄',
  `email` varchar(50) DEFAULT NULL COMMENT '邮箱',
  PRIMARY KEY (`id`)
) ENGINE=InnoDB DEFAULT CHARSET=utf8;

5、`Mapper` 扫描配置

两种方式，一种是每个mapper上使用注解@Mapper

另一种如下，全局配置

@Configuration
@MapperScan("com.healerjean.proj.data.mapper")
public class MybatisPlusConfig {

}

6、`User`实体类

package com.healerjean.proj.data.entity;

import lombok.Data;

@Data
public class User {
    private Long id;
    private String name;
    private Integer age;
    private String email;
}

7、`Mapper`类

package com.healerjean.proj.data.mapper;

import com.baomidou.mybatisplus.core.mapper.BaseMapper;
import com.healerjean.proj.data.entity.User;
public interface UserMapper  extends BaseMapper<User> {


}

8、测试

@RestController
@RequestMapping("hlj/user")
@Api(description = "用户管理")
@Slf4j
public class UserController {

    @Autowired
    private UserMapper userMapper;

    @GetMapping(value = "selectById", produces = MediaType.APPLICATION_JSON_UTF8_VALUE)
    @ResponseBody
    public ResponseBean selectById(UserDTO userDTO) {
        User user = userMapper.selectById(userDTO.getId());
        log.info("用户管理--------selectById：【{}】", JsonUtils.toJsonString(user));
        return ResponseBean.buildSuccess(user);
    }
    
}

测试结果：

http://127.0.0.1:8888/hlj/user/selectById?id=1235553744515612673

{
  "success": true,
  "result": {
    "id": 1235553744515612700,
    "name": "healer",
    "age": 22,
    "email": "22"
  },
  "msg": "",
  "code": 200,
  "date": "1583469689318"
}

二、 `Wrapper` 对象

1、`QueryWrapper`

继承自 ·AbstractWrapper ,自身的内部属性 entity 也用于生成 where 条件及 LambdaQueryWrapper,

区别	`QueryWrapper`	`LambdaQueryWrapper`
`eq(boolean condition, R column, Object val)`	不支持 `condition` 条件判断	支持条件判断
`orderByDesc` 字符串	支持字符串，不支持方法引用符	不支持排序字符串，只支持方法引用符
` select `字符串	支持字符串，不支持方法引用符	不支持排序字符串，只支持方法引用符

1）、获取`QueryWrapper`对象

a、`Wrappers.<User>lambdaQuery()`

LambdaQueryWrapper<User> userWrapper = Wrappers.<User>lambdaQuery()
   .eq(StringUtils.isNotBlank(query.getName()), UserDemo::getName, query.getName())

List<User> users = userMapper.selectList(userWrapper);
System.out.println(JsonUtils.toJsonString(users));

b、`Wrappers.lambdaQuery(User.class)`

LambdaQueryWrapper<User> userWrapper = Wrappers.lambdaQuery(User.class)
   .eq(StringUtils.isNotBlank(query.getName()), UserDemo::getName, query.getName())

List<User> users = userMapper.selectList(userWrapper);
System.out.println(JsonUtils.toJsonString(users));

c、`new QueryWrapper<User>().lambda()`

LambdaQueryWrapper<User> userWrapper = new QueryWrapper<User>().lambda()
   .eq(StringUtils.isNotBlank(query.getName()), UserDemo::getName, query.getName())


List<User> users = userMapper.selectList(userWrapper);
System.out.println(JsonUtils.toJsonString(users));

d、`new QueryWrapper<UserDemo>()`

public Page<UserDemo> queryUserDemoPage(PageQueryBO<UserDemoQueryBO> pageQuery) {
    UserDemoQueryBO query = pageQuery.getData();
    QueryWrapper<UserDemo> queryWrapper = new QueryWrapper<>();
    if (!CollectionUtils.isEmpty(query.getSelectFields())) {
        queryWrapper.select(StringUtils.join(query.getSelectFields(), ","));
    }
    if (!CollectionUtils.isEmpty(query.getOrderByList())) {
        query.getOrderByList().forEach(item -> queryWrapper.orderBy(Boolean.TRUE, 
                                                                    item.getDirection(), 
                                                                    item.getProperty()));
    }
    LambdaQueryWrapper<UserDemo> lambdaQueryWrapper = queryWrapper.lambda()
            .eq(Objects.nonNull(query.getId()), UserDemo::getId, query.getId())
            .eq(StringUtils.isNotBlank(query.getEmail()), UserDemo::getEmail, query.getEmail())

    Page<UserDemo> page = new Page<>(pageQuery.getCurrPage(), pageQuery.getPageSize(), 
                                     pageQuery.getSearchCountFlag());
    return userDemoDao.page(page, lambdaQueryWrapper);
}

2）`select`：设置查询字段

分法为两类.第二类方法为:过滤查询字段(主键除外),入参不包含 class 的调用前需要wrapper内的entity属性有值!

这两类方法重复调用以最后一次为准

select(String... sqlSelect)
select(Predicate<TableFieldInfo> predicate)
select(Class<T> entityClass, Predicate<TableFieldInfo> predicate)
    
    
select("id", "name", "age")
select(i -> i.getProperty().startsWith("test"))

QueryWrapper<SysDistrict> queryWrapper = new QueryWrapper();
queryWrapper.select("distinct province_code, province_name")
    .lambda()
    .eq(SysDistrict::getProvinceCode, provinceCode)
    .eq(SysDistrict::getStatus, StatusEnum.EFFECT.getCode());
SysDistrict sysDistrict = publicSysDistrictDao.selectOne(queryWrapper);

package com.jd.merchant.business.platform.core.dao;

import com.baomidou.mybatisplus.core.conditions.query.LambdaQueryWrapper;
import com.baomidou.mybatisplus.core.metadata.TableFieldInfo;
import com.baomidou.mybatisplus.core.toolkit.CollectionUtils;

import java.util.List;
import java.util.function.Predicate;

/**
* MybatisPlusUtil
* @author zhangyujin
* @date 2023/6/5  15:23.
*/
public class MybatisPlusUtil {


  /**
   * fieldValues
   * @param fieldValues fieldValues
   * @param queryWrapper queryWrapper
   * @param aClass aClass
   * @param <T>  <T>
   */
  public static <T> void fieldValues(List<String> fieldValues, 
                                     LambdaQueryWrapper<T> queryWrapper, 
                                     Class<T> aClass) {
      if (CollectionUtils.isEmpty(fieldValues)) {
          return;
      }
      Predicate<TableFieldInfo> predicate = null;
      for (String field : fieldValues) {
          predicate = predicate == null ? p -> p.getColumn().equals(field) : predicate.or(p -> p.getColumn().equals(field));
      }
      queryWrapper.select(aClass, predicate);
  }

}

2、`UpdateWrapper、LambdaUpdateChainWrapper`

继承自 AbstractWrapper ,自身的内部属性 entity 、也用于生成 where 条件（主要是这个功能）及 LambdaUpdateWrapper, LambdaUpdateChainWrapper可以通过 new UpdateWrapper().lambda() 方法获取!

1）`UpdateWrapper`

Wrapper<UserDemo> updateWrapper = Wrappers.lambdaUpdate(UserDemo.class)
              .eq(Objects.nonNull(userDemo.getId()), UserDemo::getId, userDemo.getId())
              .set(Objects.nonNull(userDemo.getEndTime()), UserDemo::getEndTime, userDemo.getEndTime());
      return userDemoDao.update(updateWrapper);

Wrapper<BPrizeVenderDetail> updateWrapper = new UpdateWrapper<BPrizeVenderDetail>().lambda()
          .set(BPrizeVenderDetail::getStatus, status).
          eq(BPrizeVenderDetail::getBenefitsId, benefitsId);
  return bPrizeVenderDetailDao.update(updateWrapper);

/**
* userMapper.updateById(user); //根据Id更新
* userLambdaUpdateChainWrapper.update(); //条件更新
*/
@Test
public void update(){

    User user = userMapper.selectById(1L);
    Wrapper userWrapper = new UpdateWrapper<User>().lambda()
        .set(User::getEmail, "h@gmail.com")
        .setSql("age = 24").eq(User::getId, user.getId())
        .eq(User::getId, 2L);
    user.setName("jjjjjk"); //会生效
    user.setEmail("66666"); //不会生效 以updateWrapper里面的为主
    //一般不用这二者的结合，毫无意义，写出来的sql也很有可能是错误的
    userMapper.update(user, userWrapper);


    //使用下面这两个
    userMapper.updateById(user);

    LambdaUpdateChainWrapper<User> userLambdaUpdateChainWrapper = new LambdaUpdateChainWrapper<>(userMapper)
        .set(User::getName,"Name")
        .set(User::getEmail, "update@gmail.com")
        .eq(User::getId, 1L);
    //一般我们使用这个非常好
    userLambdaUpdateChainWrapper.update();

    user.setAge(63); //会生效，
    user.setEmail("66666"); //不会生效 以userLambdaUpdateChainWrapper里面的为主
    //使用下面这个其实也毫无意义
    userLambdaUpdateChainWrapper.update(user);
}

2）`set`：SET 字段

set(String column, Object val)
set(boolean condition, String column, Object val)
    


例: set("name", "老李头")
例: set("name", "")--->数据库字段值变为空字符串
例: set("name", null)--->数据库字段值变为null

3）`setSql`：设置 SET 部分 SQL

setSql(String sql)

例: setSql("name = '老李头'")

三、Wrapper方法的的使用

1、`eq`：等于 =

eq(R column, Object val)
eq(boolean condition, R column, Object val)

Wrapper<User> userWrapper = new QueryWrapper<User>().lambda()
    .eq(User::getName, userDTO.getName());
List<User>  users = userMapper.selectList(userWrapper);




//只有一个条件，如果并且不成立则查询书所有的数据
userWrapper = new QueryWrapper<User>().lambda().
    eq(StringUtils.isNotBlank(userDTO.getName()), User::getName, userDTO.getName());
List<User>  users = userMapper.selectList(userWrapper);

2、`ne`：不等于 <>

ne(R column, Object val)
ne(boolean condition, R column, Object val)
    
例: ne("name", "老王")--->name <> '老王'

3、`gt`：大于 >

gt(R column, Object val)
gt(boolean condition, R column, Object val)
    
例: gt("age", 18)--->age > 18

4、`ge`：大于等于 >=

lt(R column, Object val)
lt(boolean condition, R column, Object val)


ge("age", 18)--->age >= 18

5、`lt`：小于 <

lt(R column, Object val)
lt(boolean condition, R column, Object val)

    
例: lt("age", 18)--->age < 18

6、`le`：小于等于 <=

le(R column, Object val)
le(boolean condition, R column, Object val)
    
例: le("age", 18)--->age <= 18

7、`between`：ETWEEN 值1 AND 值2

between(R column, Object val1, Object val2)
between(boolean condition, R column, Object val1, Object val2)
    
例: between("age", 18, 30)--->age between 18 and 30

8、`notBetween`：NOT BETWEEN 值1 AND 值2

notBetween(R column, Object val1, Object val2)
notBetween(boolean condition, R column, Object val1, Object val2)
    
例: notBetween("age", 18, 30)--->age not between 18 and 30

9、`like`：LIKE ‘%值%’

like(R column, Object val)
like(boolean condition, R column, Object val)
    
例: like("name", "王")--->name like '%王%'

10、`notLike`：NOT LIKE ‘%值%’

notLike(R column, Object val)
notLike(boolean condition, R column, Object val)
    
例: notLike("name", "王")--->name not like '%王%'

11、`likeLeft`：LIKE ‘%值’

likeLeft(R column, Object val)
likeLeft(boolean condition, R column, Object val)
    
例: likeLeft("name", "王")--->name like '%王'

12、`likeRight`：LIKE ‘值%’

likeRight(R column, Object val)
likeRight(boolean condition, R column, Object val)
    
例: likeRight("name", "王")--->name like '王%'

13、`isNull`：字段 IS NULL

isNull(R column)
isNull(boolean condition, R column)
    
例: isNull("name")--->name is null

14、`isNotNull`：字段 IS NOT NULL

isNotNull(R column)
isNotNull(boolean condition, R column)
    
例: isNotNull("name")--->name is not null    

15、`in`：字段 IN

in(R column, Collection<?> value)
in(boolean condition, R column, Collection<?> value)
    
例: in("age",{1,2,3})--->age in (1,2,3)    

in(R column, Object... values)
in(boolean condition, R column, Object... values)

例: in("age", 1, 2, 3)--->age in (1,2,3)

测试

@Test
public void in() {
    Wrapper<User> userWrapper = null;
    List<User> users = null;
    userWrapper = Wrappers.<User>lambdaQuery().in(User::getAge, 20, 18);
    users = userMapper.selectList(userWrapper);
    System.out.println(JsonUtils.toJsonString(users));

    List<Integer> integers = new ArrayList<>();
    integers.add(18);
    integers.add(20);
    userWrapper = Wrappers.<User>lambdaQuery().in(User::getAge, integers);
    users = userMapper.selectList(userWrapper);
    System.out.println(JsonUtils.toJsonString(users));
}

16、`notIn`：字段 IN (value.get(0), value.get(1), …)

notIn(R column, Collection<?> value)
notIn(boolean condition, R column, Collection<?> value)
    
例: notIn("age",{1,2,3})--->age not in (1,2,3)

notIn(R column, Object... values)
notIn(boolean condition, R column, Object... values)

例: notIn("age", 1, 2, 3)--->age not in (1,2,3)

17、`inSql`：字段 IN ( sql语句 )

inSql(R column, String inValue)
inSql(boolean condition, R column, String inValue)
    
例: inSql("id", "select id from table where id < 3")--->id in (select id from table where id < 3)

测试

@Test
public void inSql() {
    Wrapper<User> userWrapper = null;
    List<User> users = null;
    userWrapper = Wrappers.<User>lambdaQuery()
        .inSql(User::getAge, "select age from user where name = 'healer' ");
    users = userMapper.selectList(userWrapper);
    System.out.println(JsonUtils.toJsonString(users));
}

18、`notInSql`：字段 NOT IN ( sql语句 )

notInSql(R column, String inValue)
notInSql(boolean condition, R column, String inValue)
    
例: notInSql("id", "select id from table where id < 3")
    
    --->age not in (select id from table where id < 3)

19、`groupBy`：分组：GROUP BY 字段, .

groupBy(R... columns)
groupBy(boolean condition, R... columns)
    
例: groupBy("id", "name")
    
    --->group by id,name

测试

@Test
public void groupBy() {
    Wrapper<User> userWrapper = null;
    List<User> users = null;
    //这样会报错，肯定的啊，我们是分组
    // userWrapper = Wrappers.<User>lambdaQuery().groupBy(User::getAge);
    userWrapper = Wrappers.<User>lambdaQuery().select(User::getAge).groupBy(User::getAge).having("age = {0}", 18);
    users = userMapper.selectList(userWrapper);
    System.out.println(JsonUtils.toJsonString(users));
}

20、`orderBy/orderByAsc`：ORDER BY 字段, … ASC

orderBy(boolean condition, boolean isAsc, R... columns)

orderByAsc(R... columns)
orderByAsc(boolean condition, R... columns)

例: orderByAsc("id", "name")
    
    --->order by id ASC,name ASC

21、`orderByDesc`：排序：ORDER BY 字段, … DESC

orderByDesc(R... columns)
orderByDesc(boolean condition, R... columns)
   
例: orderByDesc("id", "name")--->order by id DESC,name DESC

22、`having`：HAVING ( sql语句 )

having(String sqlHaving, Object... params)
having(boolean condition, String sqlHaving, Object... params)


例: having("sum(age) > {0}", 11)--->having sum(age) > 1

测试

@Test
public void groupBy() {
    Wrapper<User> userWrapper = null;
    List<User> users = null;
    //这样会报错，肯定的啊，我们是分组
    // userWrapper = Wrappers.<User>lambdaQuery().groupBy(User::getAge);
    userWrapper = Wrappers.<User>lambdaQuery().select(User::getAge).groupBy(User::getAge).having("age = {0}", 18);
    users = userMapper.selectList(userWrapper);
    System.out.println(JsonUtils.toJsonString(users));
}

23、`or`：拼接 OR

注意事项：主动调用or表示紧接着下一个方法不是用and连接!(不调用or则默认为使用and连接)

or()
or(boolean condition)

例: `eq("id",1).or().eq("name","老王")`--->`id = 1 or name = '老王'`

1）`OR` 嵌套

例: or(i -> i.eq("name", "李白")
      .ne("status", "活着"))
    
--->or (name = '李白' and status <> '活着')

@Test
public void or() {
    Wrapper<User> userWrapper = null;
    List<User> users = null;
    userWrapper = Wrappers.<User>lambdaQuery()
        .and(userLambdaQueryWrapper -> userLambdaQueryWrapper
             .eq(User::getAge, 20)
             .eq(User::getAge, 20))
        .or().eq(User::getName, "healer");
    users = userMapper.selectList(userWrapper);
    System.out.println(JsonUtils.toJsonString(users));
}


SELECT id,name,age,email FROM user WHERE ((age = ? AND age = ?) OR name = ?) 

24、`and`：默认为and

and(Consumer<Param> consumer)
and(boolean condition, Consumer<Param> consumer)

1）`and` 嵌套

例:  and(i -> i.eq("name", "李白")
        .ne("status", "活着"))
    
 --->and (name = '李白' and status <> '活着')

25、`nested`：正常嵌套不带 AND 或者 OR

默认都是自动加and

nested(Consumer<Param> consumer)
nested(boolean condition, Consumer<Param> consumer)

例: nested(i -> i.eq("name", "李白").ne("status", "活着"))
    
     --->(name = '李白' and status <> '活着')

@Test
public void or() {
    Wrapper<User> userWrapper = null;
    List<User> users = null;
    userWrapper = Wrappers.<User>lambdaQuery()
        .or().eq(User::getName, "healer")
        .nested(wapper -> wapper.eq(User::getAge, 21 ).eq(User::getAge, 32));
    users = userMapper.selectList(userWrapper);
    System.out.println(JsonUtils.toJsonString(users));
}



SELECT id,name,age,email FROM user WHERE
    ((age = ? AND age = ?) OR name = ? AND (age = ? AND age = ?)) 

26、`apply`：拼接 sql

该方法可用于数据库函数动态入参的params对应前面applySql内部的{index}部分.这样是不会有sql注入风险的,反之会有!

apply(String applySql, Object... params)
apply(boolean condition, String applySql, Object... params)
    
例: apply("id = 1")--->id = 1
例: apply("date_format(dateColumn,'%Y-%m-%d') = '2008-08-08'")
    --->date_format(dateColumn,'%Y-%m-%d') = '2008-08-08'")
    
例: apply("date_format(dateColumn,'%Y-%m-%d') = {0}", "2008-08-08")
    --->date_format(dateColumn,'%Y-%m-%d') = '2008-08-08'")

测试

@Test
public void apply() {
    Wrapper<User> userWrapper = null;
    List<User> users = null;
    userWrapper = Wrappers.<User>lambdaQuery().eq(User::getAge, 22).apply("name =  'healer'");
    users = userMapper.selectList(userWrapper);
    System.out.println(JsonUtils.toJsonString(users));
}



SELECT id,name,age,email FROM user WHERE (age = ? AND name = 'healer')

LambdaQueryWrapper<SystemHelpDoc> queryWrapper = Wrappers.lambdaQuery(SystemHelpDoc.class)
    .like(StringUtils.isNotBlank(queryBo.getTitle()), SystemHelpDoc::getTitle, queryBo.getTitle())
    .in(CollectionUtils.isNotEmpty(queryBo.getIds()), SystemHelpDoc::getId, queryBo.getIds())
    .eq(Objects.nonNull(queryBo.getStatus()), SystemHelpDoc::getStatus, queryBo.getStatus());
if (StringUtils.isNotBlank(queryBo.getUserTypes())) {
		String[] userTypes = queryBo.getUserTypes().split(",");
		for (String userType : userTypes) {
   		 String applySql = "find_in_set(?, user_types)";
   		 queryWrapper.apply(StringUtils.isNotBlank(queryBo.getUserTypes()), applySql, userType);
		}
}
queryWrapper.orderByDesc(SystemHelpDoc::getModifiedTime);

27、`last`：无视优化规则直接拼接到 sql 的最后

只能调用一次,多次调用以最后一次为准有sql注入的风险,请谨慎使用

last(String lastSql)
last(boolean condition, String lastSql)
    
    
例: last("limit 1")  

28、`exists`：拼接 EXISTS ( sql语句 )

exists(String existsSql)
exists(boolean condition, String existsSql)
    
例: exists("select id from table where age = 1")
    
    --->exists (select id from table where age = 1)

29、`notExists`：拼接 NOT EXISTS ( sql语句 )

notExists(String notExistsSql)
notExists(boolean condition, String notExistsSql)
    
    
例: notExists("select id from table where age = 1")
    --->not exists (select id from table where age = 1)

四、复杂`SQL`

1、返回自定义对象

1）Mapper

 public interface UserMapper  extends BaseMapper<User> {


    @Select("select * from user where name = #{name}")
    //写不写下面这行都行
    // @ResultType(UserDTO.class)
    @Results(
            @Result(property = "userId", column = "id")
    )
    List<UserDTO> selectUserDtoList(UserDTO userDTO);
}

2）测试类

@Test
public void userDTO(){
    UserDTO userDTO = new UserDTO();
    userDTO.setName("healer");
    List<UserDTO> userDTOS = userMapper.selectUserDtoList(userDTO);
    System.out.println(JsonUtils.toJsonString(userDTOS));
}


[{"userId":1235553744515612673,"name":"healer","age":22,"email":"22"}]

2、原生·SQL·使用 ·Script· 语句

1）Mapper

@Select({
    "<script>",
    "select * from user where",
    "<if test='ids != null and ids.size > 0'> ",
    " id in",
    "<foreach collection='ids' index='index' item='item' open='(' separator=',' close=')'>",
    " #{item} ",
    "</foreach>",
    "</if>",
    "</script>"
})
List<UserDTO> selectListByScript(UserDTO userDTO);

2）测试类

@Test
public void selectListByScript() {
    UserDTO userDTO = new UserDTO();
    userDTO.setIds(Arrays.asList(1L, 2L));
    List<UserDTO> userDTOS = userMapper.selectListByScript(userDTO);
    System.out.println(JsonUtils.toJsonString(userDTOS));
}


[{"id":1,"name":"healer","age":22,"email":"healerjean@gmial.com"}]

3、配置`mapper.xml`

1）`application.properties`配置

# 配置 mybatis的一些配置，也可以在 application.properties 中配置，如果配置了就不需要了mybatis.xml
#mybatis-plus.config-location=classpath:mybatis.xml
#Maven 多模块项目的扫描路径需以 classpath*: 开头 （即加载多个 jar 包下的 XML 文件）
mybatis-plus.mapper-locations=classpath*:mapper/*.xml
mybatis-plus.type-aliases-package=com.healerjean.proj.data.entity
##主键类型  0:"数据库ID自增，非常大", 1:"用户输入ID（如果用户不输入，则默认是0）",2:"全局唯一ID (数字类型唯一ID)", 3:"全局唯一ID UUID";
mybatis-plus.id-type: 0
/** mybatis-plus如果希望使用数据库自增 */
// @TableId(value = "id", type = IdType.AUTO)
//private Long id;
 #字段策略 0:"忽略判断",1:"非 NULL 判断"),2:"非空判断"
mybatis-plus.field-strategy: 2
 #数据库大写下划线转换
mybatis-plus.capital-mode: true
mybatis-plus.refresh-mapper: true

2）`mybatis.xml`

<?xml version="1.0" encoding="UTF-8"?>
<!DOCTYPE configuration PUBLIC "-//mybatis.org//DTD Config 3.0//EN" "http://mybatis.org/dtd/mybatis-3-config.dtd">
<configuration>
    <settings>
        <setting name="cacheEnabled" value="false"/>
        <setting name="lazyLoadingEnabled" value="false"/>
        <setting name="defaultStatementTimeout" value="25000"/>
    </settings>

</configuration>

3）`mapper.xml`

1584524650597

<?xml version="1.0" encoding="UTF-8"?>
<!DOCTYPE mapper PUBLIC "-//mybatis.org//DTD Mapper 3.0//EN" "http://mybatis.org/dtd/mybatis-3-mapper.dtd" >
<mapper namespace="com.healerjean.proj.data.mapper.UserMapper">


    <select id = "selectByMappeXml" resultType="com.healerjean.proj.dto.UserDTO">
              select * from user where name = #{name}
    </select>

</mapper>

4）`Mapepr.java`

List<UserDTO> selectByMappeXml(UserDTO userDTO);

5）测试

@Test
public void test(){
    UserDTO userDTO = new UserDTO();
    userDTO.setName("healer");
    List<UserDTO> userDTOS = userMapper.selectByMappeXml(userDTO);
    System.out.println(JsonUtils.toJsonString(userDTOS));
}


[{"id":1,"name":"healer","age":22,"email":"healerjean@gmial.com"}]

4、分页查询

1）、准备数据

a、`@Configuration`

@Configuration
@MapperScan("com.healerjean.proj.data.mapper")
public class MybatisPlusConfig {

    @Bean
    public PaginationInterceptor paginationInterceptor() {
        PaginationInterceptor paginationInterceptor = new PaginationInterceptor();
        // 设置请求的页面大于最大页后操作， true调回到首页，false 继续请求  默认false
        // paginationInterceptor.setOverflow(false);
        // 设置最大单页限制数量，默认 500 条，-1 不受限制
        // paginationInterceptor.setLimit(500);
        // 开启 count 的 join 优化,只针对部分 left join
        paginationInterceptor.setCountSqlParser(new JsqlParserCountOptimize(true));
        return paginationInterceptor;
    }

}

b、`BeanUtils`封装

public static <T> PageDTO<T> toPageDTO(IPage iPage, List<T> datas) {
    if (datas == null || datas.isEmpty() || iPage == null) {
        return null;
    } else {
        return new PageDTO(iPage.getCurrent(), iPage.getSize(), iPage.getTotal(), iPage.getPages(), datas);
    }
}

2）简单分页

a、简单分页

/** 传入QueryWrapper分页 */
@Test
public void page1 (){    
    Page<User> page = new Page<>(1, 2);
    Wrapper<User> userWrapper = new QueryWrapper<User>().lambda();
    IPage<User> userIPage = userMapper.selectPage(page, userWrapper);
    System.out.println(userIPage);
}

/** 传入普通参数分页 */
@Test
public void page2(){
    Page<UserDTO> page = new Page<>(1, 2);
    String name = "name";
    IPage<UserDTO> users = userMapper.selectMapperXmlPage(page, name);
    System.out.println(users);
}

b、`Mapper`

public interface UserMapper extends BaseMapper<User> {

    IPage<UserDTO> selectMapperXmlPage(Page<UserDTO> page , String name);

}

<select id="selectMapperXmlPage" resultType="com.healerjean.proj.dto.UserDTO">
    select * from user where name = #{name}
</select>

3、复杂分页

a、`Service`

@Test
public void page3() {
    Page page = new Page<>(1, 2);
    QueryWrapper wrapper = Wrappers.<User>query().eq("name", "name");
    IPage<UserDTO> users = userMapper.selectMapperXmlFZPage(page, wrapper);
    System.out.println(users);
}

b、`Mapper`

public interface UserMapper extends BaseMapper<User> {

    IPage<UserDTO> selectMapperXmlFZPage(Page page,   @Param(Constants.WRAPPER) QueryWrapper<UserDTO> queryWrapper);
}

<select id="selectMapperXmlFZPage" resultType="com.healerjean.proj.dto.UserDTO">
    select * from user  ${ew.customSqlSegment}
</select>

4、传入实体对象分页

a、`Service`

@Test
public void page4() {
    Page page = new Page<>(1, 2);
    UserDTO userDTO = new UserDTO();
    userDTO.setName("name");
    //分页查询
    IPage<UserDTO>  users = userMapper.selectMapperXmlEntity(page, userDTO);
    System.out.println(users);

    //list查询
    List<UserDTO>  user2s = userMapper.selectMapperXmlEntity( userDTO);
    System.out.println(user2s);
}

b、`Mapper`

public interface UserMapper extends BaseMapper<User> {

    IPage<UserDTO> selectMapperXmlEntity(Page page,   @Param("userDTO") UserDTO userDTO);
    List<UserDTO> selectMapperXmlEntity( @Param("userDTO") UserDTO userDTO);
}

<select id="selectMapperXmlEntity" resultType="com.healerjean.proj.dto.UserDTO">
    select * from user where  name = #{userDTO.name}
</select>

3、`Wrapepr`复用

@Test
public void queryManyUse() {
    QueryWrapper<User> queryWrapper = new QueryWrapper();
    for (int i = 0; i < 2; i++) {
        queryWrapper.lambda().clear();
        queryWrapper.lambda().eq(User::getName, "healerjean");
        queryWrapper.lambda().eq(User::getEmail, i==1 ? "healer@gmail.com" : "healerjean@gmail.com");
        List<User> users =  userMapper.selectList(queryWrapper);
        System.out.println(users);
    }
}

4、返回自定义函数和字段

注意大使用双引号，否则出来是大写 MINID、MAXID

QueryWrapper<UserDemo> queryWrapper = new QueryWrapper<>();
queryWrapper.select("min(id) as \"minId\"", "max(id) as \"maxId\"");
LambdaQueryWrapper<UserDemo> lambdaQueryWrapper = queryWrapper.lambda()
        .eq(Objects.nonNull(query.getId()), UserDemo::getId, query.getId())
        .eq(Objects.nonNull(query.getValidFlag()), UserDemo::getValidFlag, query.getValidFlag())
        .like(StringUtils.isNotBlank(query.getName()), UserDemo::getName, query.getName())
        .like(StringUtils.isNotBlank(query.getPhone()), UserDemo::getPhone, query.getPhone())
        .like(StringUtils.isNotBlank(query.getEmail()), UserDemo::getEmail, query.getEmail())
        .like(StringUtils.isNotBlank(query.getLikeName()), UserDemo::getName, query.getLikeName())
        .like(StringUtils.isNotBlank(query.getLikePhone()), UserDemo::getPhone, query.getLikePhone())
        .le(Objects.nonNull(query.getQueryTime()), UserDemo::getStartTime, query.getQueryTime())
        .ge(Objects.nonNull(query.getQueryTime()), UserDemo::getEndTime, query.getQueryTime());
Map<String, Object> map = userDemoDao.getMap(lambdaQueryWrapper);
return ImmutablePair.of(MapUtils.getLong(map, "minId"), MapUtils.getLong(map, "maxId"));

五、问题

1、日期问题

1）日期格式

可能会遇到日期格式的时间段问题，当数据库的时间为 DATE 类型时，MyBatis 的jdbcType应该使用 DATE jdbcType=DATE，而不是使用jdbcType=TIMESTAMP

2）返回 `LocalDate` 和 `LocalDateTime` 报错

提高 druid 版本,我提高到了1.1.21

org.springframework.dao.InvalidDataAccessApiUsageException: Error attempting to get column 'create_date' from result set.  Cause: java.sql.SQLFeatureNotSupportedException
; null; nested exception is java.sql.SQLFeatureNotSupportedException

<!--数据源-->
<com-alibaba-druid.version>1.1.20</com-alibaba-druid.version>


<!--数据源-->
<dependency>
    <groupId>com.alibaba</groupId>
    <artifactId>druid-spring-boot-starter</artifactId>
    <version>${com-alibaba-druid.version}</version>
</dependency>

2、更新 `Null` 字段

updateById() 方法不能更新字段为 null 的原因及解决办法。

Mybatis-plus 的字段策略（FieldStrategy）有三种策略：默认的更新策略是 NOT_NULL，即通过接口更新数据时数据为 NULL值时将不更新进数据库。

| 策略 | 说明 | | —————– | ——————- | | IGNORED： | 0 忽略 | | **NOT_NULL**： | 1 非 NULL，默认策略 | | **NOT_EMPTY`**： | 2 非空 |

1）方案1：在配置文件中修改全局策略

这样做是进行全局配置，在更新时会忽略对所有字段的判断。但是如果一些字段没有传值过来，会被直接更新为null，可能会影响其它业务数据的准确性。不推荐使用此方法。

mybatis-plus.global-config.db-config.field-strategy=ignored

#yml文件格式：
mybatis-plus:
  global-config:
      #字段策略 0:"忽略判断",1:"非 NULL 判断",2:"非空判断"
    field-strategy: 0

2）方案2：对指定的字段单独设置 `field-strategy`

设置好了之后，在更新时就可以直接使用 mybatis-plus 中的 updateById 方法就可以成功将字段更新为 null,但是这样做存在一定的弊端，就是当需要这样处理的字段比较多时，要给对应的字段都要添加上这样的注解。

@TableField(updateStrategy = FieldStrategy.IGNORED)
private String updateBy;

3）方案2：使用 `update`方法结合`UpdateWrapper`方式更新

User user=userService.lambdaQuery().eq(User::getUserId,userId).one();
if(user!=null){
    userService.update(user,new UpdateWrapper<User>().lambda()
               .set(User::getUserName,null)
               .eq(User::getUserId,user.getUserId()));
}

3、返回值问题

1）更新返回 `boolean`

数据情况	说明
数据不存在	用于表示更新操作是否成功。如果数据不存在，方法通常会返回 `false`，因为更新操作没有影响到任何记录。
数据并未修改	这通常意味着没有满足更新条件的记录被更新，因此该方法通常会返回 `false`
返回 `int`	`userDemoDao.getBaseMapper().update(null, updateWrapper)`

4、`SQL` 执行问题

1）多出现 `limit`

原因：在执行 sql 的时候，使用了自定义的分页，但是执行之后自动在后面加上了一个 limit ，导致 sql 执行报错，虽然自己没有使用，但是项目中配置了 PageHelper 分页插件，然后查找资料，才发现只要自定义的 sql 中有 pageSize 和 pageNum参数值，就会在执行之后自动加上 limit

解决：在执行的 sql 中，把 pageSize 和 pageNum 规避掉，可以换成 startIndex 和 maxCount

limit #{pageNum,jdbcType=INTEGER},  #{pageSize,jdbcType=INTEGER}

替换成limit #{maxCount,jdbcType=INTEGER}, #{startIndex,jdbcType=INTEGER}

ContactAuthor

一、依赖

1、mybatis-plus依赖

2、我的项目依赖

1）主项目

1.1.2.2、子工程

3、配置文件

1）、application.properties

2）application-local.properties

3）MybatisPlusConfiguration

4、数据库语句

5、Mapper 扫描配置

6、User实体类

7、Mapper类

8、测试

二、 Wrapper 对象

1、QueryWrapper

1）、获取QueryWrapper对象

a、Wrappers.<User>lambdaQuery()

b、Wrappers.lambdaQuery(User.class)

c、new QueryWrapper<User>().lambda()

d、new QueryWrapper<UserDemo>()

2）select：设置查询字段

2、UpdateWrapper、LambdaUpdateChainWrapper

1）UpdateWrapper

2）set：SET 字段

3）setSql：设置 SET 部分 SQL

三、Wrapper方法的的使用

1、eq：等于 =

2、ne：不等于 <>

3、gt：大于 >

4、ge：大于等于 >=

5、lt：小于 <

6、le：小于等于 <=

7、between：ETWEEN 值1 AND 值2

8、notBetween：NOT BETWEEN 值1 AND 值2

9、like：LIKE ‘%值%’

10、notLike：NOT LIKE ‘%值%’

11、likeLeft：LIKE ‘%值’

12、likeRight：LIKE ‘值%’

13、isNull：字段 IS NULL

14、isNotNull：字段 IS NOT NULL

15、in：字段 IN

16、notIn：字段 IN (value.get(0), value.get(1), …)

17、inSql：字段 IN ( sql语句 )

18、notInSql：字段 NOT IN ( sql语句 )

19、groupBy：分组：GROUP BY 字段, .

20、orderBy/orderByAsc：ORDER BY 字段, … ASC

21、orderByDesc：排序：ORDER BY 字段, … DESC

22、having：HAVING ( sql语句 )

23、or：拼接 OR

1）OR 嵌套

24、and：默认为and

1）and 嵌套

25、nested：正常嵌套 不带 AND 或者 OR

26、apply：拼接 sql

27、last：无视优化规则直接拼接到 sql 的最后

28、exists：拼接 EXISTS ( sql语句 )

29、notExists：拼接 NOT EXISTS ( sql语句 )

四、复杂SQL

1、返回自定义对象

1）Mapper

2）测试类

2、原生·SQL·使用 ·Script· 语句

1）Mapper

2）测试类

3、配置mapper.xml

1）application.properties配置

2）mybatis.xml

3）mapper.xml

4）Mapepr.java

5）测试

4、分页查询

1）、准备数据

a、@Configuration

b、BeanUtils封装

2）简单分页

a、简单分页

b、Mapper

3、复杂分页

a、Service

1、`mybatis-plus`依赖

1）、`application.properties`

2）`application-local.properties`

3）`MybatisPlusConfiguration`

5、`Mapper` 扫描配置

6、`User`实体类

7、`Mapper`类

二、 `Wrapper` 对象

1、`QueryWrapper`

1）、获取`QueryWrapper`对象

a、`Wrappers.<User>lambdaQuery()`

b、`Wrappers.lambdaQuery(User.class)`

c、`new QueryWrapper<User>().lambda()`

d、`new QueryWrapper<UserDemo>()`

2）`select`：设置查询字段

2、`UpdateWrapper、LambdaUpdateChainWrapper`

1）`UpdateWrapper`

2）`set`：SET 字段

3）`setSql`：设置 SET 部分 SQL

1、`eq`：等于 =

2、`ne`：不等于 <>

3、`gt`：大于 >

4、`ge`：大于等于 >=

5、`lt`：小于 <

6、`le`：小于等于 <=

7、`between`：ETWEEN 值1 AND 值2

8、`notBetween`：NOT BETWEEN 值1 AND 值2

9、`like`：LIKE ‘%值%’

10、`notLike`：NOT LIKE ‘%值%’

11、`likeLeft`：LIKE ‘%值’

12、`likeRight`：LIKE ‘值%’

13、`isNull`：字段 IS NULL

14、`isNotNull`：字段 IS NOT NULL

15、`in`：字段 IN

16、`notIn`：字段 IN (value.get(0), value.get(1), …)

17、`inSql`：字段 IN ( sql语句 )

18、`notInSql`：字段 NOT IN ( sql语句 )

19、`groupBy`：分组：GROUP BY 字段, .

20、`orderBy/orderByAsc`：ORDER BY 字段, … ASC

21、`orderByDesc`：排序：ORDER BY 字段, … DESC

22、`having`：HAVING ( sql语句 )

23、`or`：拼接 OR

1）`OR` 嵌套

24、`and`：默认为and

1）`and` 嵌套

25、`nested`：正常嵌套不带 AND 或者 OR

26、`apply`：拼接 sql

27、`last`：无视优化规则直接拼接到 sql 的最后

28、`exists`：拼接 EXISTS ( sql语句 )

29、`notExists`：拼接 NOT EXISTS ( sql语句 )

四、复杂`SQL`

3、配置`mapper.xml`

1）`application.properties`配置

2）`mybatis.xml`

3）`mapper.xml`

4）`Mapepr.java`

a、`@Configuration`

b、`BeanUtils`封装

b、`Mapper`

a、`Service`

b、`Mapper`

a、`Service`

b、`Mapper`

3、`Wrapepr`复用

2）返回 `LocalDate` 和 `LocalDateTime` 报错

2、更新 `Null` 字段

2）方案2：对指定的字段单独设置 `field-strategy`

3）方案2：使用 `update`方法结合`UpdateWrapper`方式更新

1）更新返回 `boolean`

4、`SQL` 执行问题

1）多出现 `limit`