How To Use UNION to Merge Outputs from Two Queries Together in MySQL?

If you have two queries that returns the same row fields, you can merge their outputs together with the UNION operator. The following tutorial exercise shows you how to return all links that were created since year 2006 plus the one more link immediately before that:

mysql> SELECT id, url, created FROM fyi_links 
   ORDER BY created DESC;
+-----+-------------------+---------------------+
| id  | url               | created             |
+-----+-------------------+---------------------+
| 102 | dba.fyicenter.com | 2006-07-01 12:00:00 |
| 103 | sqa.fyicenter.com | 2006-07-01 12:00:00 |
| 101 | dev.fyicenter.com | 2006-04-30 00:00:00 |
| 104 | www.mysql.com     | 2006-01-01 00:00:00 |
| 105 | www.oracle.com    | 2005-01-01 00:00:00 |
| 106 | www.php.net       | 2004-01-01 00:00:00 |
| 107 | www.winrunner.com | 2003-01-01 00:00:00 |
+-----+-------------------+---------------------+
7 rows in set (0.00 sec)

mysql> SELECT id, url, created FROM fyi_links 
   WHERE created > '2005-12-31' ORDER BY created DESC;
+-----+-------------------+---------------------+
| id  | url               | created             |
+-----+-------------------+---------------------+
| 102 | dba.fyicenter.com | 2006-07-01 12:00:00 |
| 103 | sqa.fyicenter.com | 2006-07-01 12:00:00 |
| 101 | dev.fyicenter.com | 2006-04-30 00:00:00 |
| 104 | www.mysql.com     | 2006-01-01 00:00:00 |
+-----+-------------------+---------------------+
4 rows in set (0.00 sec)

mysql> SELECT id, url, created FROM fyi_links 
   WHERE created <= '2005-12-31' ORDER BY created DESC
   LIMIT 1;
+-----+----------------+---------------------+
| id  | url            | created             |
+-----+----------------+---------------------+
| 105 | www.oracle.com | 2005-01-01 00:00:00 |
+-----+----------------+---------------------+
1 row in set (0.01 sec)

mysql> (SELECT id, url, created FROM fyi_links 
   WHERE created > '2005-12-31' ORDER BY created DESC)
   UNION 
   (SELECT id, url, created FROM fyi_links 
   WHERE created <= '2005-12-31' ORDER BY created DESC
   LIMIT 1);
+-----+-------------------+---------------------+
| id  | url               | created             |
+-----+-------------------+---------------------+
| 101 | dev.fyicenter.com | 2006-04-30 00:00:00 |
| 102 | dba.fyicenter.com | 2006-07-01 12:00:00 |
| 103 | sqa.fyicenter.com | 2006-07-01 12:00:00 |
| 104 | www.mysql.com     | 2006-01-01 00:00:00 |
| 105 | www.oracle.com    | 2005-01-01 00:00:00 |
+-----+-------------------+---------------------+
5 rows in set (0.00 sec)

mysql> (SELECT id, url, created FROM fyi_links 
   WHERE created > '2005-12-31' ORDER BY created DESC)
   UNION 
   (SELECT id, url, created FROM fyi_links 
   WHERE created <= '2005-12-31' ORDER BY created DESC
   LIMIT 1) ORDER BY created DESC;
+-----+-------------------+---------------------+
| id  | url               | created             |
+-----+-------------------+---------------------+
| 102 | dba.fyicenter.com | 2006-07-01 12:00:00 |
| 103 | sqa.fyicenter.com | 2006-07-01 12:00:00 |
| 101 | dev.fyicenter.com | 2006-04-30 00:00:00 |
| 104 | www.mysql.com     | 2006-01-01 00:00:00 |
| 105 | www.oracle.com    | 2005-01-01 00:00:00 |
+-----+-------------------+---------------------+
5 rows in set (0.00 sec)

Note that the UNION operator damaged the sorting order of each subquery. You need a final "ORDER BY" clause at the UNION level to sort the final output.

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⇐ Returning the Second 5 Rows in MySQL

⇑ SELECT Statements with JOIN and Subqueries in MySQL

⇑⇑ MySQL Database Tutorials