MyBatis-Plus 中 的动态SQL 片段(sqlSegment)详解

// 创建 Wrapper
QueryWrapper<User> wrapper = new QueryWrapper<>();
wrapper.eq("status", 1)
       .like("username", "Tom")
       .orderByDesc("create_time");
// 执行查询（自动拼接条件到 SQL）
List<User> users = userMapper.selectList(wrapper);

SELECT * FROM user 
WHERE status = 1 AND username LIKE '%Tom%' 
ORDER BY create_time DESC

<!-- XML 映射文件 -->
<select id="selectByWrapper" resultType="User">
    SELECT * FROM user
    WHERE age > 18
    ${ew.customSqlSegment}  <!-- 插入 Wrapper 的条件 -->
</select>

QueryWrapper<User> wrapper = new QueryWrapper<>();
wrapper.ne("email", null);  // 添加条件：email IS NOT NULL
List<User> users = userMapper.selectByWrapper(wrapper);

SELECT * FROM user
WHERE age > 18
  AND email IS NOT NULL  <!-- 来自 Wrapper -->

LambdaQueryWrapper<User> lambdaWrapper = new LambdaQueryWrapper<>();
lambdaWrapper.eq(User::getStatus, 1)         // 方法引用代替字符串
             .ge(User::getAge, 18)
             .orderByAsc(User::getCreateTime);

// Mapper 接口  
@Select("SELECT * FROM user ${ew.customSqlSegment}")  
List<User> selectPageCustom(@Param(Constants.WRAPPER) Wrapper<User> wrapper);  
@Select("SELECT * FROM user WHERE ${ew.sqlSegment}")  
List<User> selectPageWhere(@Param(Constants.WRAPPER) Wrapper<User> wrapper);  

public List<User> searchUsers(String name, Integer minAge) {
    LambdaQueryWrapper<User> wrapper = new LambdaQueryWrapper<>();
    wrapper.like(StringUtils.isNotBlank(name), User::getUsername, name)
           .ge(minAge != null, User::getAge, minAge);
    return userMapper.selectList(wrapper);
}

<!-- 定义公共的 WHERE 条件 -->
<sql id="activeUser">
    is_deleted = 0 AND status = 'ACTIVE'
</sql>
<!-- 在查询中复用 -->
<select id="selectActiveUsers" resultType="User">
    SELECT * FROM user
    WHERE <include refid="activeUser"/>
    ${ew.customSqlSegment}  <!-- 追加其他动态条件 -->
</select>

QueryWrapper<User> wrapper = new QueryWrapper<>();
wrapper.like("username", "Admin");
List<User> users = userMapper.selectActiveUsers(wrapper);

SELECT * FROM user
WHERE is_deleted = 0 AND status = 'ACTIVE'  <!-- 公共片段 -->
  AND username LIKE '%Admin%'              <!-- 动态条件 -->

# application.yml
mybatis-plus:
  global-config:
    db-config:
      logic-delete-field: isDeleted   # 实体类字段名
      logic-delete-value: 1           # 删除后的值
      logic-not-delete-value: 0       # 未删除的值

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