UNION操作符用于合并两个或多个SELECT语句的结果集,这里需要注意的是:UNION内部的SELECT语句必须拥有相同数量的
列,列也必须拥有相似的数据类型,同时,每条SELECT语句中列的顺序必须相同。
UNION语法:
- SELECT column_name(s) FROM table_name1
- UNION
- SELECT column_name(s) FROM table_name2
union操作符合并的结果集,不会允许重复值,如果允许有重复值的话,使用UNION ALL.
UNION ALL语法:- SELECT column_name(s) FROM table_name1
- UNION ALL
- SELECT column_name(s) FROM table_name2
这里我就不举例子说明,重点总结下我在项目开发中遇到的问题:1、由于需要合并十个select语句,写法上需要用到sql中<foreach>;.2、在写sql语句是,用到了order by,要用括号区分开,要不会报错。
sql语句写法:
- <select id="getFourteenHotPost" parameterType="map" resultMap="productCommentsInfoAndroid">
- select t.comments_id,t.product_id,t.comment,t.order_path from (
- <foreach collection="tableNames" item="item" separator="UNION ALL">
- (SELECT c.comments_id,c.product_id,c.comment,i.order_path,c.p_index,c.t_index,c.title,c.time
- FROM ${item} as c left join `gshop_comments_img` as i on c.comments_id = i.comments_id
- where c.object_type=2 and c.display=1 and c.is_show=1
- and c.t_index=1
- GROUP BY c.product_id ORDER BY c.p_index asc,c.t_index desc,c.title desc,c.time desc limit 14)
- </foreach>) t
- GROUP BY t.product_id ORDER BY t.p_index asc,t.t_index desc,t.title desc,t.time desc limit 14
- </select>