MyBatis动态SQL

MyBatis的动态SQL，解决了SQL字符串拼接的痛苦。

1.if

<select id="findActiveBlogWithTitleLike"
    parameterType="Blog" resultType="Blog">
    SELECT * FROM BLOG
    WHERE state = 'ACTIVE'
    <if test="title != null">
        AND title like #{title}
    </if>
</select>

这条一句会提供一个可选的文本查找功能。如果没有传递title，那么所有激活的博客都会被返回。
如果传递了title，那么就会查找相近的title。


2.choose，when，otherwise

<select id="findActiveBlogLike"
    parameterType="BLOG" resultType="BLOG">
    SELECT * FROM BLOG
    WHERE
    <choose>
        <when test="title != null">
            AND title like #{title}
        </when>
        <when test="author != null and author.name != null">
            AND title like #{author.name}
        </when>
        <otherwise>
            AND featured = 1
        </otherwise>
    </choose>
</select>

注：如果上述条件都没有匹配，则会变成SELECT * FROM BLOG WHERE
如果仅有第二个匹配，则会变成SELECT * FROM BLOG WHERE AND title LIKE somelike
显然这样会查询失败。要解决这个问题，mybatis提供了解决方法。

<select id="findActiveBlogLike"
    parameterType="BLOG" resultType="BLOG">
    SELECT * FROM BLOG
    WHERE
    <trim prefix="WHERE" prefixOverrides="AND |OR ">
        <choose>
            <when test="title != null">
                AND title like #{title}
            </when>
            <when test="author != null and author.name != null">
                AND title like #{author.name}
            </when>
            <otherwise>
                AND featured = 1
            </otherwise>
        </choose>
    </trim>
</select>

overrides属性采用管道文本分隔符来覆盖，这里的空白是重要的。它的结果就是移除在InnerText中overrides中指定的内容。


3.set

<update id="updateAuthorIfNecessary"
    parameterType="Author">
    update Author
    <set>
        <if test="username != null">username=#{username},</if>
        <if test="password != null">password=#{password},</if>
        <if test="email != null">email=#{email}</if>
    </set>
    where id=#{id}
</update>

同上的问题，优化后：

<update id="updateAuthorIfNecessary"
    parameterType="Author">
    update Author
    <trim prefix="where" prefixOverrides=",">
    <set>
        <if test="username != null">username=#{username},</if>
        <if test="password != null">password=#{password},</if>
        <if test="email != null">email=#{email}</if>
    </set>
    where id=#{id}
    </trim>
</update>