Question

the table

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

the query

(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 */

The query above will return 2 rows since there are no id=5 and id=0.

How can I tell which queries are these 2 rows fetched from?

that is, num_row = 1 from 1st SELECT, and num_row = 2 from 4th SELECT

Answer 1

use a second column to indicate from where the data comes from

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