MyBatis---动态 SQL

1.if

2.choose, when, otherwise

3. where

<select id="findActiveBlogLike"
     resultType="Blog">
  SELECT * FROM BLOG 
  <where> 
    <if test="state != null">
         state = #{state}
    </if> 
    <if test="title != null">
        AND title like #{title}
    </if>
   </where>
</select>

4.set

<update id="updateAuthor">
  update Author
    <set>
      <if test="username != null">username=#{username},</if>
      <if test="password != null">password=#{password},</if>
    </set>
  where id=#{id}
</update>

5.trim

select * from user 
　　<trim prefix="WHERE" prefixoverride="AND |OR"><!--prefixoverride：去掉第一个and或者是or-->
　　　　<if test="name != null and name.length()>0"> AND name=#{name}</if>
　　　　<if test="gender != null and gender.length()>0"> AND gender=#{gender}</if>
　　</trim>

假如说name和gender的值都不为null的话打印的SQL为：select * from user where  name = 'xx' and gender = 'xx'

update user
　　<trim prefix="set" suffixoverride="," suffix=" where id = #{id} "><!--suffixoverride：去掉最后一个逗号;suffix：后缀-->
　　　　<if test="name != null and name.length()>0"> name=#{name} , </if>
　　　　<if test="gender != null and gender.length()>0"> gender=#{gender} ,  </if>
　　</trim>

假如说name和gender的值都不为null的话打印的SQL为：update user set name='xx' , gender='xx'   where id='x'

6.foreach

<select id="selectPostIn" resultType="domain.blog.Post">
  SELECT *
  FROM POST P
  WHERE ID in
  <foreach item="item" index="index" collection="list" open="(" separator="," close=")">
        #{item}
  </foreach>
</select>