学习视频：【编程不良人】Mybatis-Plus整合SpringBoot实战教程,提高的你开发效率,后端人员必备!

查询方法详解

• 普通查询

  // 根据主键id去查询单个结果的。
  @Test
  public void selectById() {
      User user = userMapper.selectById(1739970502337392641L);
      System.out.println(user);
  }
  //根据多个主键id批量查询结果的
  @Test
  public void selectIds() {
      List list = Arrays.asList(1739970502337392641L, 1739983903621038082L, 1739984905459900417L);
      List userList = userMapper.selectBatchIds(list);
      userList.forEach(System.out::println);
  }
  // 根据多个条件查询结果的
  @Test
  public void selectByMap() {
      // map.put("name","小明")
      // map.put("age",30)
      // 相当于 where name ="小明" and age=30
      Map columnMap = new HashMap<>();
      columnMap.put("name", "小刚");
      columnMap.put("age", "18");
      List userList = userMapper.selectByMap(columnMap);
      userList.forEach(System.out::println);
  }
  

• 条件构造器查询 【重要】

  AbstractWrapper 是 MyBatis Plus 中的一个抽象类，用于构建 SQL 查询条件。定义了泛型 T、C 和 Children。其中，T 表示实体类的类型，C 表示查询条件的类型，Children 表示子类的类型，用于支持链式调用。它提供了一系列方法，用于构建 SQL 查询条件，包括设置查询字段、设置查询条件、排序等。

  常用实现类包括 QueryWrapper 和 UpdateWrapper。这两个类都是 MyBatis Plus 提供的具体实现，用于构建查询条件和更新条件。

• 条件构造器查询示例

  @SpringBootTest
  public class SelectTests {
      @Autowired
      private UserMapper userMapper;
      /**
       * 1.名字中包含刘且年龄大于20
       * name like '%刘%' and age>20
       */
      @Test
      public void selectByWrapper() {
          // 创建queryWrapper的两种方式
          QueryWrapper queryWrapper = new QueryWrapper<>();
  //        QueryWrapper queryWrapper1 = Wrappers.query();
          queryWrapper.like("name", "刘").gt("age", 20);
          List list = userMapper.selectList(queryWrapper);
          list.forEach(System.out::println);
      }
      /**
       * 2.名字中包含刘且年龄大于等于20且小于30并且email不为空
       * name like ‘%刘’ and age between 20 and 30 and email is not null
       */
      @Test
      public void selectByWrapper2() {
          QueryWrapper queryWrapper = new QueryWrapper<>();
          queryWrapper.like("name", "刘").between("age", 20, 30).isNull("email");
          List list = userMapper.selectList(queryWrapper);
          list.forEach(System.out::println);
      }
      /**
       * 3.名字中包含刘或年龄大于等于20,按照年龄降序排序，年龄相同按照id升序排列
       * name like ‘%刘’ and age between 20 and 30 and email is not null
       */
      @Test
      public void selectByWrapper3() {
          QueryWrapper queryWrapper = new QueryWrapper<>();
          queryWrapper.like("name", "刘").or().ge("age",20)
                  .orderByDesc("age")
                  .orderByAsc("user_id");
          List list = userMapper.selectList(queryWrapper);
          list.forEach(System.out::println);
      }
      /**
       * 4.创建日期为2023年12月17日并且直属上级为王姓
       * date_format(create_time,'%Y-%m-$d') and manager_id in(select id from user where name like '王%'
       */
      @Test
      public void selectByWrapper4() {
          QueryWrapper queryWrapper = new QueryWrapper<>();
          queryWrapper.apply("date_format(create_time,'%Y-%m-%d')={0}", "2023-12-27") //  使用占位符避免sql注入的风险
                  .inSql("manager_id","select user_id from mp_user where name like '王%'");
          List list = userMapper.selectList(queryWrapper);
          list.forEach(System.out::println);
      }
      /**
       * 5.名字为刘姓且年龄小于30或者邮箱不为空
       * name like '王%' and (age<20 or email is not null)
       */
      @Test
      public void selectByWrapper5() {
          QueryWrapper queryWrapper = new QueryWrapper<>();
          queryWrapper.likeRight("name", "刘")
                  .and(wq -> wq.lt("age", 30).or().isNotNull("email"));
          List list = userMapper.selectList(queryWrapper);
          list.forEach(System.out::println);
      }
      /**
       * 6.名字为刘姓或者（年龄小于30且大于20并且邮箱不为空）
       * name like '刘%' or (age<30 and age>20 and email is not null)
       */
      @Test
      public void selectByWrapper6() {
          QueryWrapper queryWrapper = new QueryWrapper<>();
          queryWrapper.likeRight("name", "刘")
                  .or(wq -> wq.lt("age", 30).gt("age", 20)
                          .isNotNull("email"));
          List list = userMapper.selectList(queryWrapper);
          list.forEach(System.out::println);
      }
      /**
       * 7.（年龄小于30且邮箱不为空）并且名字为刘姓
       *  (age<30 or email is not null) and name like '刘%'
       */
      @Test
      public void selectByWrapper7() {
          QueryWrapper queryWrapper = new QueryWrapper<>();
          queryWrapper.nested(wq -> wq.lt("age", 30)
                          .or().isNotNull("email"))
                  .likeRight("name", "刘");
          List list = userMapper.selectList(queryWrapper);
          list.forEach(System.out::println);
      }
      /**
       * 8. 年龄为 21 25 33
       *  age in(21,25,33)
       */
      @Test
      public void selectByWrapper8() {
          QueryWrapper queryWrapper = new QueryWrapper<>();
          queryWrapper.in("age", 21, 25, 33);
          List list = userMapper.selectList(queryWrapper);
          list.forEach(System.out::println);
      }
      /**
       * 9. 只返回满足条件的其中一条语句即可
       *  limit 1
       */
      @Test
      public void selectByWrapper9() {
          QueryWrapper queryWrapper = new QueryWrapper<>();
          queryWrapper.in("age", 21, 25, 33).last("limit 1"); // last() 有sql注入的风险，谨慎使用
          List list = userMapper.selectList(queryWrapper);
          list.forEach(System.out::println);
      }
  }
  

• select 不列出全部字段

   /**
     * 10.名字中包含刘且年龄小于30（需求1加强版）
     */
    @Test
    public void selectByWrapper() {
        QueryWrapper queryWrapper = new QueryWrapper<>();
  //        queryWrapper.select("user_id","name","age").like("name", "刘").lt("age", 30); // 包含字段
        queryWrapper.like("name", "刘").lt("age", 30)
                .select(User.class, info ->
                        !info.getColumn().equals("create_time") && !info.getColumn().equals("manager_id"));// 排除字段
        List list = userMapper.selectList(queryWrapper);
        list.forEach(System.out::println);
    }
  

• condition作用

  根据判断决定查询条件是否加到sql语句里面

  @Test
      public void testCondition() {
          String name = "东";
          String email = "";
          condition(name, email);
      }
      private void condition(String name, String email) {
          QueryWrapper queryWrapper = new QueryWrapper<>();
  //        if (StringUtils.isNotEmpty(name)) {
  //            queryWrapper.like("name", name);
  //        }
  //        if (StringUtils.isNotEmpty(email)) {
  //            queryWrapper.like("email", email);
  //        }
          queryWrapper.like(StringUtils.isNotEmpty(name), "name", name)
                  .like(StringUtils.isNotEmpty(email), "email", email);
          List list = userMapper.selectList(queryWrapper);
          list.forEach(System.out::println);
      }
  

• 实体作为条件构造器构造方法的参数

  @Test
  public void selectByWrapperEntity() {
      User user = new User();
      user.setRealName("小刚");
      user.setAge(18);
      QueryWrapper queryWrapper = new QueryWrapper<>(user);
      List list = userMapper.selectList(queryWrapper);
      list.forEach(System.out::println);
  }
  运行结果的sql语句：SELECT * FROM mp_user WHERE **name=? AND age=?**
  会把set注入的属性和值映射到where条件中
  

  • 通过SqlCondition 更改查询条件

    

    用法
    @TableField(condition = SqlCondition.LIKE)
    private String name;
    @TableField(condition = "%s<#{%s}") //根据定义的常量，自己也可以参考并更改
    private Integer age;
    运行结果：SELECT * FROM mp_user WHERE **name LIKE CONCAT('%',?,'%')** AND AND age 

  • allEq

    **allEq**是一个查询条件配置方法，用于构建查询条件。它可以根据传入的条件参数，生成一个包含所有等值条件的查询条件对象。

    @Test
    public void selectByWrapperAllEq() {
        QueryWrapper queryWrapper = new QueryWrapper<>();
        Map params = new HashMap<>();
        params.put("name", "小明");
        params.put("age", null);
        **queryWrapper.allEq(params,false)**; // false代表忽略值为null的字段，即不参与where条件
        List list = userMapper.selectList(queryWrapper);
        list.forEach(System.out::println);
    }
    

  • 其他条件构造器

    **selectMaps 返回**

    @Test
        public void selectByWrapperMaps() {
            QueryWrapper queryWrapper = new QueryWrapper<>();
            queryWrapper.select("user_id","name").like("name", "刘").lt("age", 30);
    //        List list = userMapper.selectList(queryWrapper);// 返回值为实体类的话，没设置的值还是会返回，只不过值为null
            List> userList = userMapper.selectMaps(queryWrapper);// 只返回限定的字段
            userList.forEach(System.out::println);
        }
        /** 示例
         * 11.按照直属上级分组，查询每组的平均年龄、最大年龄、最小年龄，并且至取年龄总和为500的组
         * select avg(age)avg_age,min(age) min_age,max(age) max_age from user group by manager_id having sum(age)<500
         */
        @Test
        public void selectByWrapperMaps2() {
            QueryWrapper queryWrapper = new QueryWrapper<>();
            queryWrapper.select("avg(age) avg_age", "min(age) min_age", "max(age) max_age")
                    .groupBy("manager_id").having("sum(age)<{0}", 500);
            List> userList = userMapper.selectMaps(queryWrapper);
            userList.forEach(System.out::println);
        }
    

  • lambda条件构造器

    @Test
    public void selectLambda() {
        //创建lambda条件构造器的三种方式
        LambdaQueryWrapper lambda = new LambdaQueryWrapper<>();
        LambdaQueryWrapper lambda2 = new QueryWrapper().lambda();
        LambdaQueryWrapper lambda3= Wrappers.lambdaQuery();
        lambda3.like(User::getName, "刘")
                .lt(User::getAge, 30); // where name like '%雨%' and age<30
        List userList = userMapper.selectList(lambda3);
        userList.forEach(System.out::println);
    }
    /**
     * 5.名字为刘姓且（年龄小于30或邮箱不为空）
     * name like '刘%' and (age<30 or email is not null)
     */
    @Test
    public void selectLambda2() {
        LambdaQueryWrapper lambda= Wrappers.lambdaQuery();
        lambda.like(User::getName, "刘")
                .and(wq -> wq.lt(User::getAge, 20)
                        .or().isNotNull(User::getEmail));
        List userList = userMapper.selectList(lambda);
        userList.forEach(System.out::println);
    }
    // 链式调用多个条件 LambdaQueryChainWrapper
    @Test
    public void selectLambda3() {
        List userList = new LambdaQueryChainWrapper<>(userMapper)
                .like(User::getName, "刘").ge(User::getAge, 20).list();
        userList.forEach(System.out::println);
    }
    

    下一章：Spring Boot学习随笔- 集成MyBatis-Plus（三）自定义SQL、分页实现（PaginationInterceptor ）、更新和删除详细方法

   
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