如何判断哪个查询是使用UNION在MySQL语句中获取的数据

Question

桌子

id    string
------------
1     aaa
2     bbb
3     ccc
4     ddd

查询

(SELECT string FROM table WHERE id > 1 ORDER BY id ASC LIMIT 1)    /* num_row = 1 */
UNION
(SELECT string FROM table WHERE id < 1 ORDER BY id DESC LIMIT 1)   /* null */
UNION
(SELECT string FROM table WHERE id > 4 ORDER BY id ASC LIMIT 1)    /* null */
UNION
(SELECT string FROM table WHERE id < 4 ORDER BY id DESC LIMIT 1)   /* num_row = 2 */

上面的查询将返回2 rows因为没有id = 5和id = 0。

如何判断从这两行中取出哪些查询？

即， num_row = 1从1st SELECT ，和num_row = 2从4th SELECT

Answer 1

你可以试试

(SELECT 1, string FROM table WHERE id > 1 ORDER BY id ASC LIMIT 1)   
UNION
(SELECT 2, string FROM table WHERE id < 1 ORDER BY id DESC LIMIT 1)  
UNION
(SELECT 3, string FROM table WHERE id > 4 ORDER BY id ASC LIMIT 1)   
UNION
(SELECT 4, string FROM table WHERE id < 4 ORDER BY id DESC LIMIT 1) 

Answer 2

您可以使用常用别名添加额外的常量列：

(SELECT string, 'query_1' as query_num FROM table WHERE id > 1 ORDER BY id ASC LIMIT 1)    
UNION
(SELECT string, 'query_2' as query_num FROM table WHERE id < 1 ORDER BY id DESC LIMIT 1)
UNION
(SELECT string, 'query_3' as query_num FROM table WHERE id > 4 ORDER BY id ASC LIMIT 1)    
UNION
(SELECT string, 'query_4' as query_num FROM table WHERE id < 4 ORDER BY id DESC LIMIT 1)   

Answer 3

使用第二列来指示数据来自何处

(SELECT string, '1st query' as from_where FROM table WHERE ...)
UNION
(SELECT string, '2nd query' as from_whereFROM table WHERE ...)

Answer 4

你可以尝试（虽然不优雅）：

（SELECT'1'，string FROM table WHERE id> 1 ORDER BY id ASC LIMIT 1）/ * num_row = 1 / UNION（ SELECT'2 '，string FROM table WHERE id <1 ORDER BY id DESC LIMIT 1）/ null / UNION（SELECT'3'，字符串FROM表WHERE id> 4 ORDER BY id ASC LIMIT 1）/ null / UNION（ SELECT'4 '，字符串FROM表WHERE id <4 ORDER BY id DESC LIMIT 1）/ num_row = 2