mybatis 动态sql

if标签

<if test="name !=null and name!='1' ">

</if>

<if test="name !=null or name!='1' ">

</if>

test中写判断内容

choose、when、otherwise标签

<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>

where标签

如果有以下语句

<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>

</select>

如果state等于null，SQL语句就会变成"SELECT * FROM BLOG WHERE",这明显是错误的，如果state等于null，titile不等于null则语句变成"SELECT * FROM BLOG AND TITLE LIKE "，也是不对的，针对这种情况可以加where标签

<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>

where 元素只会在子元素返回内容的情况下才插入 “WHERE” 子句。而且，若子句的开头为 “AND” 或 “OR”，where 元素也会将它们去除。

set标签

set 标签可以用于动态包含需要更新的列，忽略其它不更新的列

<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>

foreach标签

SELECT * FROM POST P

WHERE ID in

#{item}

</foreach>

</select>

bind标签

元素允许你在 OGNL 表达式以外创建一个变量，并将其绑定到当前的上下文。

SELECT * FROM BLOG

WHERE title LIKE #{pattern}

</select>

_parameter:

_parameter:代表整个参数

单个参数：_parameter就是这个参数

多个参数：参数会被封装为一个map:_parameter就是代表这个map