MyBatis-Plus中使用Wrapper自定义SQL的详细步骤

public interface UserMapper extends BaseMapper<User> {
    // 使用 @Param("ew") 定义 Wrapper 参数
    List<User> selectUserList(@Param("ew") Wrapper<User> wrapper);
}

<select id="selectUserList" resultType="User">
    SELECT id, name, age, email 
    FROM user 
    ${ew.customSqlSegment} <!-- 自动注入 WHERE 及条件 -->
</select>

QueryWrapper<User> wrapper = new QueryWrapper<>();
wrapper.like("name", "张")       // 模糊查询 name 包含"张"
       .gt("age", 20)            // age > 20
       .orderByDesc("create_time"); // 按创建时间倒序
 
List<User> users = userMapper.selectUserList(wrapper);

SELECT id, name, age, email 
FROM user 
WHERE name LIKE '%张%' AND age > 20 
ORDER BY create_time DESC

@Select("SELECT * FROM user ${ew.customSqlSegment}")
List<User> selectByWrapper(@Param("ew") Wrapper<User> wrapper);

List<UserVO> selectUserWithRole(@Param("ew") Wrapper<User> wrapper);

<select id="selectUserWithRole" resultType="UserVO">
    SELECT u.*, r.role_name 
    FROM user u
    LEFT JOIN role r ON u.role_id = r.id
    ${ew.customSqlSegment} <!-- 注入条件 -->
</select>

LambdaQueryWrapper<User> wrapper = new LambdaQueryWrapper<>();
wrapper.like(User::getName, "王")     // 使用实体类方法引用
       .eq(User::getStatus, 1)        // status = 1
       .nested(i -> i.gt(User::getAge, 18).or().isNotNull(User::getEmail)); // 嵌套条件
 
List<UserVO> list = userMapper.selectUserWithRole(wrapper);

SELECT u.*, r.role_name 
FROM user u 
LEFT JOIN role r ON u.role_id = r.id 
WHERE (
    name LIKE '%王%' 
    AND status = 1 
    AND (age > 18 OR email IS NOT NULL)
)

wrapper.like("u.name", "张"); // XML 中表别名为 u