if
<select id="findByCondition" parameterType="user" resultType="user">
select * from user
<where>
<if test="id!=0">
and id=#{id}
</if>
<if test="username!=null">
and username=#{username}
</if>
</where>
</select>
foreach
<select id="findByIds" parameterType="list" resultType="user">
select * from user
<where>
<foreach collection="list" open="id in(" close=")" item="id" separator=",">
#{id}
</foreach>
</where>
</select>
foreach标签的属性含义如下：
collection：代表要遍历的集合元素，注意编写时不要写#{}open：代表语句的开始部分close：代表结束部分item：代表遍历集合的每个元素，生成的变量名sperator：代表分隔符
choose
<select id="queryUserById" parameterType="user" resultType="user">
select * from user
<where>
<choose>
<when test="id==1">
and username="zhangsan"
</when>
<when test="id==2">
and username="lisi"
</when>
<otherwise>
and username="wangwu"
</otherwise>
</choose>
</where>
</select>
choose相当于java中的switch,when相当于其中的case,otherwise相当于其中的的default.
set
注意每一个if标签后的逗号!!!
<update id="updateUser" parameterType="user">
update user
<set>
<if test="username!=null">
username=#{username},
</if>
<if test="password!=null">
password=#{password},
</if>
<if test="birthday!=null">
birthday=#{birthday},
</if>
</set>
<where>
id=#{id}
</where>
</update>
