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Updating Data
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If you want to change the values in a row, you use the UPDATE statement. UPDATE is similar to INSERT, except that like SELECT, it can be modified (or constrained) by conditions. If we want to change the author for the Intro to SQL article, we can do so like this:
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UPDATE articles SET author = 'Eugene' WHERE id = 1;
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APPENDIX B DATABASES 101
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Query OK, 1 row affected (0.07 sec) Rows matched: 1 Changed: 1 Warnings: 0
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MySQL tells us that one row was matched by the query and that the row was successfully changed. The fact that we used the primary key to find and update the row is significant. While we could have matched any value in any column, the only surefire way to ensure we re updating the row we want is to use the primary key. We can confirm that the value was updated with another query.
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SELECT author FROM articles WHERE id = 1;
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+--------+ | author | +--------+ | Eugene | +--------+
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Sure enough, the author field has been updated.
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Deleting Data
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Of course, not all information in a database is going to stay there forever. Sometimes you need to delete records, such as when a product goes out of stock or a user cancels her account. That s the purpose of the DELETE statement. It works a lot like the UPDATE statement in that it accepts conditions, and deletes the rows for any records that match the conditions. If we wanted to delete the article with the id of 1, our DELETE statement would be as follows:
DELETE FROM articles WHERE id = 1;
Query OK, 1 row affected (0.08 sec)
MySQL tells us that one row was affected. And, of course, if we subsequently search for the record, we ll find that it no longer exists.
SELECT * FROM articles WHERE id = 1;
APPENDIX B DATABASES 101
Empty set (0.01 sec)
Understanding Relationships
It s good practice to avoid duplication in your database by creating distinct tables to store certain kinds of information. You relate two tables to one another using an association. This will make more sense when you see it in action, so let s take a look at the articles table again. This time, the table has more data in it.
SELECT * FROM articles;
+----+-----------------------+---------------------+ | id | title | author | +----+-----------------------+---------------------+ | 1 | ActiveRecord Basics | Jeffrey Hardy | | 2 | Advanced ActiveRecord | Cloves Carneiro Jr. | | 3 | Setting up Subversion | Cloves Carneiro Jr. | | 4 | Databases 101 | Jeffrey Hardy | +----+-----------------------+---------------------+
There s quite a bit of duplication in the author field. This can potentially create a few problems. While we could search for all articles by a particular author using a standard SELECT query, what would happen if someone s name were misspelled Any articles by the misspelled author wouldn t show up in our query. And if there were such a typo, we would need to update a lot of records in order to fix it. Moreover, searching on a text field like author is both unreliable and rather slow when compared to searching using an integer type. We could improve this design significantly by putting authors in their own table and referencing each author s unique id (primary key) in the articles table instead of the name. Let s do that now. We ll create a new table called authors and change the author field in the articles table so it can store an integer instead of text. The new authors table looks like this:
SHOW COLUMNS FROM authors;
APPENDIX B DATABASES 101
+------------+--------------+------+-----+---------+----------------+ | Field | Type | Null | Key | Default | Extra | +------------+--------------+------+-----+---------+----------------+ | id | int(11) | NO | PRI | NULL | auto_increment | | name | varchar(255) | YES | | NULL | | +------------+--------------+------+-----+---------+----------------+
The modified articles table looks like this:
SHOW COLUMNS FROM articles;
+------------+--------------+------+-----+---------+----------------+ | Field | Type | Null | Key | Default | Extra | +------------+--------------+------+-----+---------+----------------+ | id | int(11) | NO | PRI | NULL | auto_increment | | author_id | int(11) | YES | | NULL | | | title | varchar(255) | YES | | NULL | | +------------+--------------+------+-----+---------+----------------+
Note how instead of a text field called author, we now have a numeric field that references the author s primary key from the authors table called author_id. This field holds what is called a foreign key, which is a reference to the primary key of the table it relates to; in this case, the author who wrote the article. If we now look at the data from both tables, we ll see that we ve eliminated the duplication.
SELECT * FROM articles;
+----+-----------+-----------------------+ | id | author_id | title | +----+-----------+-----------------------+ | 1 | 1 | ActiveRecord Basics | | 2 | 2 | Advanced ActiveRecord | | 3 | 2 | Setting up Subversion | | 4 | 1 | Databases 101 | +----+-----------+-----------------------+
SELECT * FROM authors;
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