MyBatis提供了9种动态SQL标签:trim、where、set、foreach、if、choose、when、otherwise、bind;
1.if 标签
XML
<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
XML
<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
XML
<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
XML
<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>
XML
<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
XML
<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
XML
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 user
WHERE 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
XML
<!-- 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>