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

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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=30Map<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 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 {@Autowiredprivate UserMapper userMapper;/*** 1.名字中包含刘且年龄大于20* name like '%刘%' and age>20*/@Testpublic 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*/@Testpublic 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*/@Testpublic 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%'*/@Testpublic 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)*/@Testpublic 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)*/@Testpublic 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%'*/@Testpublic 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)*/@Testpublic 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*/@Testpublic 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）*/@Testpublic 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

@Testpublic 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**

@Testpublic 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nullList<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*/@Testpublic 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<30List<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);
}