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MySQL Tutorial - Use Existing Column Values in the SET Clause

By: FYIcenter.com

(Continued from previous topic...)

How To Use Existing Column Values in the SET Clause?

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.

(Continued on next topic...)

  1. What Are DML Statements?
  2. How To Create a Testing Table?
  3. How To Insert a New Row into a Table?
  4. How To Specify Default Values in INSERT Statement?
  5. How To Omit Columns with Default Values in INSERT Statement?
  6. What Happens If Unique Value Constraints Are Violated?
  7. How To Insert Multiple Rows with One INSERT Statement?
  8. How To Update Values in a Table?
  9. How To Update Column Values on Multiple Rows?
  10. How To Use Existing Column Values in the SET Clause?
  11. Is the Order of Columns in the SET Clause Important?
  12. How To Use Values from Other Tables in UPDATE Statements?
  13. What Happens If the UPDATE Subquery Returns No Rows?
  14. What Happens If the UPDATE Subquery Returns Multiple Rows?
  15. How To Delete an Existing Row from a Table?
  16. How To Delete Multiple Rows from a Table?
  17. How To Delete All Rows in a Table?

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