mybatis dynamic sql, if tag, where tag, foreach tag

Dynamic SQL

What is the soul of mybatis? The soul of mybatis is dynamic SQL!

if tag

	在映射配置文件中这样写

<select id="findUserByCondition" resultType="com.yixuexi.entity.User" parameterType="com.yixuexi.entity.User">
	<!-- 使用if标签 where 1=1不能省略 -->
	select * from user where 1 = 1   
	<if test="name != null">
		and name = #{name}
	</if>
</select>

1. The above if means, if the name of the user object is not empty, then put together and name = #{name}
2. The if tag is and or or
3. If there are multiple conditions, multiple if tags can be assembled
4. The name of test is the name attribute of the user object.

Multiple if tags

select * from user where 1 = 1
<if test="name != null">
	and name = #{name}
</if>
<if test="password != null">
	and password = #{password}
</if>

Precautions:

• When using the if tag alone, you must add where 1=1
• Multiple if will be executed

---

where tag

• Use the where tag to omit where1 = 1
• Is to wrap a where tag outside the if tag

<select id="findUserByCondition" resultType="com.yixuexi.entity.User"       parameterType="com.yixuexi.entity.User">
	select * from user
	<where>
		<if test="name != null">
			and name = #{name}
		</if>
		<if test="password != null">
			and password = #{password}
		</if>
	</where>
</select>

It's very simple, just add a layer of where tag

---

foreach tags

<select id="findUserByIds" parameterType="com.yixuexi.entity.QueryVoIds" resultType="com.yixuexi.entity.User">
	select * from user
	<where> <!--添加了where之后 就不用再手写 where 1=1 -->
		<!--如果这个对象里面的list集合不是null，并且 list集合的个数大于0个 则遍历 -->
		<if test="list != null and list.size > 0">
			<foreach collection="list" open="and id in(" close=")" item="id" separator=",">
				#{id}
			</foreach>
		</if>
	</where>
</select>

Detailed label content:

	- collection：遍历哪个集合，集合名称
	- open：开头添加什么  添加   and id in（      
		} open哪里加and是因为  where标签会帮你加 where 1=1 不加and不行             
	- close：在结尾添加什么 添加 ）
	- item：（起个名）把遍历出来的每一项添加到 open和close之间，用，进行分割
	- separator：用什么进行分割
	- #{id}：item起的什么名字   这里就用什么名字
	- 最后拼装出来的sql： select * from user wherer 1 = 1 and id in (1,2,3,4,5)

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