Modifying data and using WHERE clause in MySQL

By: Sathya Narayana Emailed: 1698 times Printed: 2196 times    

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You can modify data using the UPDATE...SET command. When using this syntax, you simply specify the table where you want to change column values after the UPDATE statement and specify which column or columns are to be affected after the SET statement.

Hence, you could change the status column for all books to O (for out of stock) using this command:

mysql> UPDATE book SET status = 'O';

You should see a confirmation that all three rows in the table were affected by the command, and you can verify that by executing the SELECT statement you used earlier.

Note 

Likewise, you could use an UPDATE to modify the title or any other fields. Note that it isn't a good idea to modify the id field once you've set it. Changing a primary key field could impact other parts of your system that rely on the id keeping the same value, such as maintaining referential integrity.

Using the WHERE Clause

Although the UPDATE command you've just seen is a quick way to change all values for a given column or set of columns in a table, you generally want to change only certain rows. To specify criteria that rows must match for a command to be applied, you append a WHERE clause.

For instance, the following command would change the price of the book with an ID of 1 to $8.99:

mysql> UPDATE book SET price = 8.99 WHERE id = 1;

Run it now and see. Can you guess what SQL command you should use to retrieve the following information for just that book?

+----+--------------------------------------------+-------+
| id | title                                      | price |
+----+--------------------------------------------+-------+
| 1  | Lord of the Things                         | 8.99  |
+----+--------------------------------------------+-------+

You simply use the same WHERE clause on a SELECT statement:

mysql> SELECT id, title, price FROM book WHERE id = 1;

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