How To Use Existing Values in UPDATE Statements

Q

How To Use Existing Values in UPDATE Statements? - MySQL FAQs - Understanding SQL INSERT, UPDATE and DELETE Statements

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A

If a row matches the WHERE clause in a UPDATE statement, existing values in this row can be used in expressions to provide new values in the SET clause. Existing values are represented by column names in the expressions. The tutorial exercise below shows a good example:

mysql> UPDATE fyi_links SET id = id+200, counts = id*2 
   WHERE id >= 500;
Query OK, 5 rows affected (0.01 sec)
Rows matched: 5  Changed: 5  Warnings: 0

mysql> SELECT id, url, notes, counts, DATE(created) 
   FROM fyi_links WHERE id >= 500;
+-----+-------------------+-------+--------+---------------+
| id  | url               | notes | counts | DATE(created) |
+-----+-------------------+-------+--------+---------------+
| 801 | moc.retneciyf.ved | Wrong |   1602 | 2006-04-30    |
| 802 | moc.retneciyf.abd | Wrong |   1604 | 2006-08-31    |
| 803 | moc.retneciyf.aqs | Wrong |   1606 | 2006-08-31    |
| 810 |                   | Wrong |   1620 | 2006-08-31    |
| 700 | moc.retneciyf.www | Wrong |   1400 | 2006-08-31    |
+-----+-------------------+-------+--------+---------------+
5 rows in set (0.00 sec)

This statement increased values in the id column by 200. It also updated the counts column with the newly increased id value.

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