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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。@Testpublic 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@Testpublic 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@Testpublic 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
@SpringBootTestpublic 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 (age20 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
@Testpublic 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。@Testpublic 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<Map> 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<Map> userList = userMapper.selectMaps(queryWrapper); userList.forEach(System.out::println); }lambda条件构造器
@Testpublic 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) */@Testpublic 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@Testpublic void selectLambda3() { List userList = new LambdaQueryChainWrapper(userMapper) .like(User::getName, "刘").ge(User::getAge, 20).list(); userList.forEach(System.out::println);}
作者:扬眉剑出鞘
出处: https://www.cnblogs.com/eyewink/
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