一文掌握MyBatis Plus的条件构造器方法

1.组装查询条件

组装查询其实很简单，可以支持条件的链式编程：

查询用户名包含a，年龄在 10 - 20 之间并且邮箱不为空的用户：

@Test
void contextLoads() {
   // 查询用户名包含a，年龄在 10 - 20 之间并且邮箱不为空的用户
   QueryWrapper<User> wrapper = new QueryWrapper<>();
   wrapper.like("name","a")
           .between("age",10,20)
           .isNotNull("email");
   List<User> users = userMapper.selectList(wrapper);
   users.forEach(System.out::println);
}

==>  Preparing: SELECT id,name,age,email,is_delete FROM user WHERE is_delete=0 AND (name LIKE ? AND age BETWEEN ? AND ? AND email IS NOT NULL)
==> Parameters: ％a％(String), 10(Integer), 20(Integer)
<==    Columns: id, name, age, email, is_delete
<==        Row: 2, dada, 11, 111@qq.com, 0
<==        Row: 4, dahe, 12, 34567@qq.com, 0
<==      Total: 2

2.组装排序条件

查询用户信息，按照年龄的降序排序，若年龄相同，则按照id升序排序：

@Test
void contextLoads() {
   // 查询用户信息，按照年龄的降序排序，若年龄相同，则按照id升序排序
   QueryWrapper<User> wrapper = new QueryWrapper<>();
   wrapper.orderByDesc("age").orderByAsc("id");
   List<User> users = userMapper.selectList(wrapper);
   users.forEach(System.out::println);
}

==>  Preparing: SELECT id,name,age,email,is_delete FROM user WHERE is_delete=0 ORDER BY age DESC,id ASC
==> Parameters:
<==    Columns: id, name, age, email, is_delete
<==        Row: 3, dahe, 33, 34567@qq.com, 0
<==        Row: 4, dahe, 12, 34567@qq.com, 0
<==        Row: 2, dada, 11, 111@qq.com, 0
<==      Total: 3

3.使用Lambda表达式解决条件优先级

将用户名中包含有a并且（年龄大于20或邮箱为null）的用户信息查询出来：

Lambda中的条件优先执行！

@Test
void contextLoads() {
   // 将用户名中包含有a并且（年龄大于20或邮箱为null）的用户信息查询出来
   QueryWrapper<User> wrapper = new QueryWrapper<>();
   wrapper.like("name", "a")
           .and(i -> i.gt("age", 20).or().isNull("email"));
   List<User> users = userMapper.selectList(wrapper);
   users.forEach(System.out::println);
}

==>  Preparing: SELECT id,name,age,email,is_delete FROM user WHERE is_delete=0 AND (name LIKE ? AND (age > ? OR email IS NULL))
==> Parameters: ％a％(String), 20(Integer)
<==    Columns: id, name, age, email, is_delete
<==        Row: 3, dahe, 33, 34567@qq.com, 0
<==      Total: 1

4.组装select语句

有时我们不想查询数据库某张表的所有字段，可以使用maps集合选择只查询某些字段的值：

@Test
void contextLoads() {
   QueryWrapper<User> wrapper = new QueryWrapper<>();
   wrapper.select("name","email");
   List<Map<String, Object>> maps = userMapper.selectMaps(wrapper);
   maps.forEach(System.out::println);
}

==>  Preparing: SELECT name,email FROM user WHERE is_delete=0
==> Parameters:
<==    Columns: name, email
<==        Row: dada, 111@qq.com
<==        Row: dahe, 34567@qq.com
<==        Row: dahe, 34567@qq.com
<==      Total: 3
Closing non transactional SqlSession [org.apache.ibatis.session.defaults.DefaultSqlSession@237add]
{name=dada, email=111@qq.com}
{name=dahe, email=34567@qq.com}
{name=dahe, email=34567@qq.com}

5.组装子查询

@Test
void contextLoads() {
   QueryWrapper<User> wrapper = new QueryWrapper<>();
   wrapper.inSql("id","select id from user where id > 1");
   List<User> users = userMapper.selectList(wrapper);
   users.forEach(System.out::println);
}

==>  Preparing: SELECT id,name,age,email,is_delete FROM user WHERE is_delete=0 AND (id IN (select id from user where id > 1))
==> Parameters:
<==    Columns: id, name, age, email, is_delete
<==        Row: 2, dada, 11, 111@qq.com, 0
<==        Row: 3, dahe, 33, 34567@qq.com, 0
<==        Row: 4, dahe, 12, 34567@qq.com, 0
<==      Total: 3

6.使用UpdateWrapper实现修改功能

将名字中包含a的记录修改：

@Test
void contextLoads() {
   // 将名字中包含a的记录修改
   UpdateWrapper<User> wrapper = new UpdateWrapper<>();
   wrapper.like("name","a");
   wrapper.set("name","hello").set("age",18);
   userMapper.update(null, wrapper);
}

JDBC Connection [com.mysql.cj.jdbc.ConnectionImpl@36c07c75] will not be managed by Spring
==>  Preparing: UPDATE user SET name=?,age=? WHERE is_delete=0 AND (name LIKE ?)
==> Parameters: hello(String), 18(Integer), ％a％(String)
<==    Updates: 3

7.模拟开发中组装条件的情况

真实的开发中往往有这样的情况：我们不知道前端传来的条件是什么，此时无法正常的拼接sql，那么就需要在后端进行相应的判断：

@Test
void contextLoads() {
    String name = null;
    Integer ageBegin = 18;
    Integer ageEnd = 28;
    QueryWrapper<User> wrapper = new QueryWrapper<>();
    if (StringUtils.isNotBlank(name)) {
        wrapper.like("name", name);
    }
    if (ageBegin != null) {
        wrapper.ge("age", ageBegin);
    }
    if (ageEnd != null) {
        wrapper.le("age", ageEnd);
    }
    List<User> users = userMapper.selectList(wrapper);
    users.forEach(System.out::println);
}

==>  Preparing: SELECT id,name,age,email,is_delete FROM user WHERE is_delete=0 AND (age >= ? AND age <= ?)
==> Parameters: 18(Integer), 28(Integer)
<==    Columns: id, name, age, email, is_delete
<==        Row: 3, hello, 18, 34567@qq.com, 0
<==        Row: 4, hello, 18, 34567@qq.com, 0
<==      Total: 2

但是，这样的判断我们认为过于的简陋和复杂，事实上，MyBatis Plus的条件构造器中为我们封装了解决此类问题的一些重载方法：

@Test
void contextLoads() {
   String name = null;
   Integer ageBegin = 18;
   Integer ageEnd = 28;
   QueryWrapper<User> wrapper = new QueryWrapper<>();
   wrapper.like(StringUtils.isNotBlank(name), "name", name);
   wrapper.ge(ageBegin != null, "age", ageBegin);
   wrapper.le(ageEnd != null, "age", ageEnd);
   List<User> users = userMapper.selectList(wrapper);
   users.forEach(System.out::println);
}

==>  Preparing: SELECT id,name,age,email,is_delete FROM user WHERE is_delete=0 AND (age >= ? AND age <= ?)
==> Parameters: 18(Integer), 28(Integer)
<==    Columns: id, name, age, email, is_delete
<==        Row: 3, hello, 18, 34567@qq.com, 0
<==        Row: 4, hello, 18, 34567@qq.com, 0
<==      Total: 2

8.LambdaQueryWrapper和LambdaUpdateWrapper

使用LambdaQueryWrapper和LambdaUpdateWrapper可以防止我们的字段名修改导致的程序错误或字段名写错

例如：

@Test
void contextLoads() {
    String name = null;
    Integer ageBegin = 18;
    Integer ageEnd = 28;
    LambdaQueryWrapper<User> wrapper = new LambdaQueryWrapper<>();
    wrapper.like(StringUtils.isNotBlank(name), User::getName, name);
    wrapper.ge(ageBegin != null, User::getAge, ageBegin);
    wrapper.le(ageEnd != null, User::getAge, ageEnd);
    List<User> users = userMapper.selectList(wrapper);
    users.forEach(System.out::println);
}

==>  Preparing: SELECT id,name,age,email,is_delete FROM user WHERE is_delete=0 AND (age >= ? AND age <= ?)
==> Parameters: 18(Integer), 28(Integer)
<==    Columns: id, name, age, email, is_delete
<==        Row: 3, hello, 18, 34567@qq.com, 0
<==        Row: 4, hello, 18, 34567@qq.com, 0
<==      Total: 2

LambdaUpdateWrapper的使用方法类似

来源：https://blog.csdn.net/Gherbirthday0916/article/details/128902090