MyBatis常用标签

if 条件判断

<select id="findActiveBlogLike" resultType="Blog">
SELECT * FROM BLOG WHERE state = ‘ACTIVE’ 
<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> 

choose (when, otherwise)条件查询

<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> 

trim (where, set)

• where：元素只会在至少有一个子元素的条件返回 SQL 子句的情况下才去插入“WHERE”子句。而且，若语句的开头为“AND”或“OR”，where 元素也会将它们去除。

  <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:通过自定义 trim 元素来定制 where 元素的功能

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

• set:元素可以用于动态包含需要更新的列，而舍去其它的

  <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：对一个集合进行遍历，通常是在构建 IN 条件语句的时候

<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>

bind：元素可以从 OGNL 表达式中创建一个变量并将其绑定到上下文

<select id="selectBlogsLike" resultType="Blog">
    <bind name="pattern" value="'%' + _parameter.getTitle() + '%'" />
    SELECT * FROM BLOG
    WHERE title LIKE #{pattern}
</select>