MyBatis动态SQL

<![CDATA[ ]]> 的作用

<![CDATA[  ]]>是xml语法，在<![CDATA[ ... ]]>的所有内容都会被解析器忽略（特殊字符不转译）

<if test="">   <where>  <choose> <trim>  这些标签都不会被解析，所有要尽量缩小<![CDATA[ ]]>的使用范围

if

<select id="dynamicFindBook1" resultMap="BookResult" parameterType="Map">
select * from book where 1=1
<if test="bookName!=null">
<![CDATA[and bname like #{bookName}]]>
</if>
<if test="maxPrice!=null">
<![CDATA[ and price <= #{maxPrice} ]]>
</if>
<if test="studentId!=null">
and student_id =#{studentId}
</if>
</select>

choose/when/otherwise

<select id="dynamicFindBook2" resultMap="BookResult" parameterType="Map">
select * from book
<choose>
<!--  注意要加上 ' ' 单引号-->
<when test="searchBy=='bookName'">
where bname like#{bookName}
</when>
<when test="searchBy=='studentId'">
where student_id=#{studentId}
</when>
<otherwise>
<![CDATA[where price<=#{price} ]]>
</otherwise>
</choose>
</select>

where

如果所有if不成立，则不会加where，如果第一条if不成立，那么后面成立的第一个if会剔除and

<select id="dynamicFindBook3" resultMap="BookResult" parameterType="Map">
select * from book
<where>
<if test="bookName!=null">
bname like #{bookName}
</if>
<if test="maxPrice!=null">
<![CDATA[ and price <= #{maxPrice} ]]>
</if>
<if test="studentId!=null">
and student_id =#{studentId}
</if>
</where>
</select>

trim

trim和where类似，更灵活，当基句中已存在where时，可以将改为 prefix=”and”

<select id="dynamicFindBook4" resultMap="BookResult" parameterType="Map">
select * from book
<trim prefix="where" prefixOverrides="and|or">
<if test="bookName!=null">
bname like #{bookName}
</if>
<if test="maxPrice!=null">
<![CDATA[ and price <= #{maxPrice} ]]>
</if>
<if test="studentId!=null">
and student_id =#{studentId}
</if>
</trim>
</select>

foreach

<select id="dynamicFindBook5" resultMap="BookResult" parameterType="Map">
select * from book
<if test="studentIds!=null">
<where>
<foreach item="studentId" collection="studentIds">
or student_id=#{studentId}
</foreach>
</where>
</if>
</select>

上面的例子也可使用in

但是当集合中没有数据时，将会出错，而用or不会

<select id="dynamicFindBook6" resultMap="BookResult" parameterType="Map">
select * from book
<if test="studentIds!=null">
<where>
student_id in
<foreach item="studentId" collection="studentIds" open="(" separator="," close=")">
#{studentId}
</foreach>
</where>
</if>
</select>

set 相对于update修改的

注意在if的内容中 后面有一个 , 逗号

<update id="dynamicFindBook7" parameterType="com.z.entity.Book">
update book
<set>
<if test="name!=null"> bname=#{name}, </if>
<if test="price!=null"> price=#{price}, </if>
</set>
where bid=#{id}
</update>