Mybatis 9种动态 sql 标签使用

MyBatis提供了9种动态SQL标签：trim、where、set、foreach、if、choose、when、otherwise、bind；

1.if 标签

<select id="getUser">select * from User<where><if test=" age != null ">and age > #{age}</if><if test=" name != null ">and name like concat(#{name},'%')</if></where>
</select>

2.choose 标签、when 标签、otherwise 标签

<select id="getUser">select * from User<where><choose><when test=" age != null ">and age > #{age}</when><when test=" name != null ">and name like concat(#{name},'%')</when><otherwise>and sex = '女'</otherwise></choose></where>
</select>

3.foreach 标签

<select id="findAll">select  * from user where ids in<foreach collection="list"item="item" index="index"open="(" separator="," close=")">#{item}</foreach>
</select>最终拼接完整的语句就变成：
select  * from user where ids in (1,2,3,...,100);当然你也可以这样编写：
<select id="findAll">select  * from user where<foreach collection="list"item="item" index="index"open=" " separator=" or " close=" ">id = #{item}</foreach>
</select>最终拼接完整的语句就变成：
select  * from user where id =1 or id =2 or id =3  ... or id = 100;

4.where 标签、set 标签

<select id="getUser">select * from User<where><if test=" age != null ">and age > #{age}</if><if test=" name != null ">and name like concat(#{name},'%')</if></where>
</select>

<update id="updateUser">update user<set><if test="age !=null">age = #{age},</if><if test="username !=null">username = #{username},</if><if test="password !=null">password = #{password},</if></set>where id =#{id}
</update>注意，set 标签之所以能够支持去除前缀逗号或者后缀逗号，
是由于其在构造 trim 标签的时候进行了前缀后缀的去除设置，而 where 标签在构造 trim 标签的时候就仅仅设置了前缀去除。

5.trim 标签

<trim prefix="SET" prefixOverrides="," >...
</trim>或者<trim prefix="SET" suffixesToOverride="," >...
</trim>由于 where 标签和 set 标签这两种 trim 标签变种方案已经足以满足我们实际开发需求，所以直接使用 trim 标签的场景实际上不太很多（其实是我自己使用的不多，基本没用过）。

6.bind 标签

平时你使用 mysql 都是如何拼接模糊查询 like 语句的 ( oracle 不支持)
select * from user where name like concat('%',#{name},'%')<select id="selecUser"><bind name="myName" value="'%' + _parameter.getName() + '%'" />SELECT * FROM userWHERE name LIKE #{myName}
</select>无论使用哪种数据库，这个模糊查询的 Like 语法都是支持的

拓展：sql标签 + include 标签

<!-- 可复用的字段语句块 -->
<sql id="userColumns">id,username,password
</sql>
查询或插入时简单复用：<!-- 查询时简单复用 -->
<select id="selectUsers" resultType="map">select<include refid="userColumns"></include>from user
</select>