<update id="updateUser" parameterType="com.dy.entity.User">
    update user
        <set>
            <if test="name != null">name = #{name},</if> 
            <if test="password != null">password = #{password},</if> 
            <if test="age != null">age = #{age},</if> 
        </set>
        <where>
            <if test="id != null">
                id = #{id}
            </if>
            and deleteFlag = 0;
        </where>
</update>

2foreach: 你有for, 我有foreach, 不要以為就你才屌！

java中有for, 可通過for迴圈， 同樣， mybatis中有foreach, 可通過它實現迴圈，迴圈的物件當然主要是java容器和陣列。

<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>

3 choose: 我選擇了你，你選擇了我！

Java中有switch, mybatis有choose。

<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>

4 動態SQL解析原理

我們在使用mybatis的時候，會在xml中編寫sql語句。比如這段動態sql程式碼：

<update id="update" parameterType="org.format.dynamicproxy.mybatis.bean.User">
    UPDATE users
    <trim prefix="SET" prefixOverrides=",">
        <if test="name != null and name != ''">
            name = #{name}
        </if>
        <if test="age != null and age != ''">
            , age = #{age}
        </if>
        <if test="birthday != null and birthday != ''">
            , birthday = #{birthday}
        </if>
    </trim>
    where id = ${id}
</update>

6 where, 有了我，SQL語句拼接條件神馬的都是浮雲！

咱們通過where改造一下上面的例子：

<select id="findUserById" resultType="user">
    select * from user 
        <where>
            <if test="id != null">
                id=#{id}
            </if>
            and deleteFlag=0;
        </where>
</select>