动态 SQL

• 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>