MyBatis动态SQL

动态SQL
MyBatis的动态SQL，解决了SQL字符串拼接的痛苦。

<select 
	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。


，when，otherwise

<select 
	parameterType="BLOG" resultType="BLOG">
	SELECT * FROM BLOG
	WHERE
	<choose>
		<when test="title != null">
			AND title like #{title}
		</when>
		<when test="author != null and  != null">
			AND title like #{}
		</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 
	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  != null">
				AND title like #{}
			</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。

<update 
	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 
	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>