Mybatis（动态SQL标签、一对一，一对多关联）

动态sql

if 标签

<!-- 演示动态sql-if标签的使用情景 -->
	<select id="getUserByWhere" parameterType="user" resultType="com.itheima.mybatis.pojo.User">
		<!-- SELECT * FROM USER WHERE username LIKE '%${username}%' and id = #{id} -->
		SELECT * FROM USER where 1 = 1
		<!-- if标签的使用 -->
		<if test="id != null">
			and id = #{id}
		</if>
		<if test="username != null and username != ''">
			and username LIKE '%${username}%'
		</if>
	</select>

Where 标签

　<!-- 演示动态sql-where标签的使用情景 -->
	<select id="getUserByWhere2" parameterType="user"
		resultType="com.itheima.mybatis.pojo.User">
		<!-- include:引入sql片段,refid引入片段id -->
		SELECT
		*
		FROM USER
		<!-- where会自动加上where同处理多余的and -->
		<where>
			<!-- if标签的使用 -->
			<if test="id != null">
				and id = #{id}
			</if>
			<if test="username != null and username != ''">
				and username LIKE '%${username}%'
			</if>
		</where>
　　</select>

foreach 标签

<!-- 演示动态sql-foreach标签的使用情景 -->
	<select id="getUserByIds" parameterType="queryvo"
		resultType="com.itheima.mybatis.pojo.User">
		
		SELECT
		*
		FROM USER
		<!-- where会自动加上where同处理多余的and -->
		<where>
			<!-- id IN(1,10,25,30,34) -->
			<!-- foreach循环标签 
				 collection:要遍历的集合，来源入参 
				 open:循环开始前的sql 
				 separator:分隔符 
				 close:循环结束拼接的sql
			-->
			<foreach item="uid" collection="ids" open="id IN(" separator=","
				close=")">
				#{uid}
			</foreach>		</where>
	</select>

Sql 片段

定义一个SQL片段

　　<!-- sql片段 定义，id:片段唯一标识 -->
	<sql id="user_column">
		`id`,
		`username`,
		`birthday`,
		`sex`,
		`address`,
		`uuid2`
	</sql>

使用SQL片段

　　SELECT
		<!-- sql片段的使用：include:引入sql片段,refid引入片段id -->
		<include refid="user_column" />
		FROM USER	

关联查询

一对一关联

• 方法一，使用resultType
• 新建OrderUser的pojo，继承自Order。

public class OrderUser extends Order {
	
	private String username;
	private String address;

	public String getUsername() {
		return username;
	}
	public void setUsername(String username) {
		this.username = username;
	}
	public String getAddress() {
		return address;
	}
	public void setAddress(String address) {
		this.address= address;
	}

}

新增查询方法getOrderUser。

　<!-- 一对一关联查询，使用resultType -->
	<select id="getOrderUser" resultType="orderuser">
		SELECT
		  o.`id`,
		  o.`user_id` userId,
		  o.`number`,
		  o.`createtime`,
		  o.`note`,
		  u.`username`,
		  u.`address`
		FROM `order` o
		LEFT JOIN `user` u
		ON u.id = o.`user_id`
	</select>

• 方法二，使用resultMap
  改造order的pojo
  修改order的映射文件

<!-- 一对一关联查询-resultMap -->
	<resultMap type="order" id="order_user_map">
		<!-- id标签用于绑定主键 -->
		<id property="id" column="id"/>
		<!-- 使用result绑定普通字段 -->
		<result property="userId" column="user_id"/>
		<result property="number" column="number"/>
		<result property="createtime" column="createtime"/>
		<result property="note" column="note"/>
		
		<!-- association:配置一对一关联
			 property:绑定的用户属性
			 javaType:属性数据类型，支持别名
		-->
		<association property="user" javaType="com.itheima.mybatis.pojo.User">
			<id property="id" column="user_id"/>
			
			<result property="username" column="username"/>
			<result property="address" column="address"/>
			<result property="sex" column="sex"/>
		</association>
	</resultMap>
	<!-- 一对一关联查询-使用resultMap -->
	<select id="getOrderUser2" resultMap="order_user_map">
		SELECT
		  o.`id`,
		  o.`user_id`,
		  o.`number`,
		  o.`createtime`,
		  o.`note`,
		  u.`username`,
		  u.`address`,
		  u.`sex`
		FROM `order` o
		LEFT JOIN `user` u
		ON u.id = o.`user_id`
	</select>

一对多关联

• 改造user的pojo
  修改user的映射文件

<!-- 一对多关联查询 -->
	<resultMap type="user" id="user_order_map">
		<id property="id" column="id" />
		<result property="username" column="username" />
		<result property="birthday" column="birthday" />
		<result property="address" column="address" />
		<result property="sex" column="sex" />
		<result property="uuid2" column="uuid2" />
		
		<!-- collection:配置一对多关系
			 property:用户下的order属性
			 ofType:property的数据类型，支持别名
		-->
		<collection property="orders" ofType="order">
			<!-- id标签用于绑定主键 -->
			<id property="id" column="oid"/>
			<!-- 使用result绑定普通字段 -->
			<result property="userId" column="id"/>
			<result property="number" column="number"/>
			<result property="createtime" column="createtime"/>
		</collection>

	</resultMap>
	<!-- 一对多关联查询 -->
	<select id="getUserOrder" resultMap="user_order_map">
		SELECT
		u.`id`,
		u.`username`,
		u.`birthday`,
		u.`sex`,
		u.`address`,
		u.`uuid2`,
		o.`id` oid,
		o.`number`,
		o.`createtime`
		FROM `user` u
		LEFT JOIN `order` o
		ON o.`user_id` = u.`id`
	</select>