mybatis动态sql

Posted 2020-08-21 swaggyC

　　动态sql是mybatis的特色功能，所谓动态sql,就是支持sql语句的动态拼接，从而使sql语句的使用更加灵活，使代码简洁可复用。

　　以下就常用的动态sql用法做一些总结。

1. 　　if ：通过对参数的判断，动态地插入查询条件

   <select id="findActiveBlogWithTitleLike"
        resultType="Blog">
     SELECT * FROM BLOG 
     WHERE state = ‘ACTIVE’ 
     <if test="title != null">
       AND title like #{title}
     </if>
   </select>

    

2. choose when otherwise 多选一，相当于if else 

<select id="findActiveBlogLike"
     resultType="Blog">
  SELECT * FROM BLOG WHERE state = ‘ACTIVE’
  <choose>
    <when test="title != null">
      AND title like #{title}
    </when>
    <when test="author != null and author.name != null">
      AND author_name like #{author.name}
    </when>
    <otherwise>
      AND featured = 1
    </otherwise>
  </choose>
</select>

　　3.trim where set 智能得插入 where and set ,等关键字，注意where与and配合使用，而set与，配合使用

<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>
    <if test="author != null and author.name != null">
        AND author_name like #{author.name}
    </if>
  </where>
</select>

<trim prefix="WHERE" prefixOverrides="AND |OR ">
  ... 
</trim>

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

<trim prefix="SET" suffixOverrides=",">
  ...
</trim>

　　4.foreach 遍历集合参数

<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>