mybatisの动态SQL

1. if  根据条件包含 where 子句的一部分

   <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>

2. choose when otherwise 不想应用到所有的条件语句，而只想从中择其一项

   <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. 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>

4. set 元素会动态前置 SET 关键字，同时也会删掉无关的逗号

   <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>

5. 动态 SQL 的另外一个常用的操作需求是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>

   注意：当使用可迭代对象或者数组时，index 是当前迭代的次数，item 的值是本次迭代获取的元素。当使用 Map 对象（或者 Map.Entry 对象的集合）时，index 是键，item 是值。

6. 官网地址 http://www.mybatis.org/mybatis-3/zh/dynamic-sql.html