mybatis复杂查询

Mybatis支持各种复杂查询，以下是一些示例：

1. 多条件查询

<select id="selectUsersByNameAndAge" parameterType="map" resultType="User">
  select * from users where name = #{name} and age = #{age}
</select>

2. 嵌套查询

<select id="selectUsersWithOrders" resultMap="userWithOrders" >
  select * from users
  <where>
    id in (
      select user_id from orders
    )
  </where>
</select>

<resultMap id="userWithOrders" type="User">
  <result property="id" column="id"/>
  <result property="name" column="name"/>
  <collection property="orders" ofType="Order">
    <id property="id" column="id"/>
    <result property="name" column="name"/>
  </collection>
</resultMap>

3. 左连接查询

<select id="selectUsersWithOrders" resultMap="userWithOrders" >
  select * from users left join orders on users.id = orders.user_id
</select>

<resultMap id="userWithOrders" type="User">
  <result property="id" column="id"/>
  <result property="name" column="name"/>
  <collection property="orders" ofType="Order">
    <id property="id" column="id"/>
    <result property="name" column="name"/>
  </collection>
</resultMap>

4. 动态SQL查询

<select id="selectUsers" parameterType="User" resultMap="user">
  select * from users
  <where>
    <if test="id != null">
      id = #{id}
    </if>
    <if test="name != null">
      and name = #{name}
    </if>
  </where>
</select>

<resultMap id="user" type="User">
  <result property="id" column="id"/>
  <result property="name" column="name"/>
</resultMap>

这些只是一些示例，Mybatis还支持更多复杂的查询语句，具体使用方法可以查看官方文档。