Spring Boot学习随笔- 集成MyBatis-Plus(二)条件查询QueryWrapper、聚合函数的使用、Lambda条件查询

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<Long> list = Arrays.asList(1739970502337392641L, 1739983903621038082L, 1739984905459900417L);
        List<User> 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<String, Object> columnMap = new HashMap<>();
        columnMap.put("name", "小刚");
        columnMap.put("age", "18");
        List<User> 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 TC 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<User> queryWrapper = new QueryWrapper<>();
    //        QueryWrapper<User> queryWrapper1 = Wrappers.<User>query();
            queryWrapper.like("name", "刘").gt("age", 20);
            List<User> 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<User> queryWrapper = new QueryWrapper<>();
            queryWrapper.like("name", "刘").between("age", 20, 30).isNull("email");
            List<User> 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<User> queryWrapper = new QueryWrapper<>();
            queryWrapper.like("name", "刘").or().ge("age",20)
                    .orderByDesc("age")
                    .orderByAsc("user_id");
    
            List<User> 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<User> 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<User> 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<User> queryWrapper = new QueryWrapper<>();
            queryWrapper.likeRight("name", "刘")
                    .and(wq -> wq.lt("age", 30).or().isNotNull("email"));
            List<User> 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<User> queryWrapper = new QueryWrapper<>();
            queryWrapper.likeRight("name", "刘")
                    .or(wq -> wq.lt("age", 30).gt("age", 20)
                            .isNotNull("email"));
    
            List<User> 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<User> queryWrapper = new QueryWrapper<>();
            queryWrapper.nested(wq -> wq.lt("age", 30)
                            .or().isNotNull("email"))
                    .likeRight("name", "刘");
            List<User> 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<User> queryWrapper = new QueryWrapper<>();
            queryWrapper.in("age", 21, 25, 33);
            List<User> 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<User> 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<User> 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<User> 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<User> 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<User> 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<User> 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<User> queryWrapper = new QueryWrapper<>(user);
        List<User> 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&lt;#{%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<User> queryWrapper = new QueryWrapper<>();
        Map<String, Object> 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<User> list = userMapper.selectList(queryWrapper);
        list.forEach(System.out::println);
    }
    
  • 其他条件构造器

    **selectMaps 返回**

    @Test
        public void selectByWrapperMaps() {
            QueryWrapper<User> queryWrapper = new QueryWrapper<>();
            queryWrapper.select("user_id","name").like("name", "刘").lt("age", 30);
    //        List<User> 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<Map<String, Object>> 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<User> 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<Map<String, Object>> 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<User> lambda = new LambdaQueryWrapper<>();
        LambdaQueryWrapper<User> lambda2 = new QueryWrapper<User>().lambda();
        LambdaQueryWrapper<User> lambda3= Wrappers.<User>lambdaQuery();
    
        lambda3.like(User::getName, "刘")
                .lt(User::getAge, 30); // where name like '%雨%' and age<30
        List<User> 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<User> lambda= Wrappers.<User>lambdaQuery();
        lambda.like(User::getName, "刘")
                .and(wq -> wq.lt(User::getAge, 20)
                        .or().isNotNull(User::getEmail));
        List<User> userList = userMapper.selectList(lambda);
        userList.forEach(System.out::println);
    }
    // 链式调用多个条件 LambdaQueryChainWrapper
    @Test
    public void selectLambda3() {
        List<User> userList = new LambdaQueryChainWrapper<>(userMapper)
                .like(User::getName, "刘").ge(User::getAge, 20).list();
        userList.forEach(System.out::println);
    }