- 动态 SQL
    - if 条件判断

        ```xml
        <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

        ```xml
        <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。

            ```xml
            <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

            ```xml
            <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

            ```xml
            <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

        ```xml
        <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

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