Table of Contents for

MySQL in a Nutshell, 2nd Edition

You can change data in a table using a few different methods. The most basic and perhaps the most common method is to use the UPDATE statement. With this statement, you can change data for all rows or for specific records based on a WHERE clause.

Looking back on the results displayed from an earlier query, we can see that Graham Greene’s book Brighton Rock has a copyright year of 1937. That’s not correct; it should be 1938. To change or update that bit of information, we would enter the following SQL statement:

UPDATE books
SET pub_year = '1938'
WHERE book_id = '2';
   
Query OK, 1 row affected (0.00 sec)
Rows matched: 1  Changed: 1  Warnings: 0

First, we state the name of the table that’s being updated. Next, we include the SET keyword with the column to change and its corresponding new value. If we wanted to change the values of more than one column, we would provide a comma-separated list of each column along with the equals sign operator and the new respective values. SET is given only once.

The preceding SQL statement has a WHERE clause limiting the rows that will be updated by specifying a condition the row must meet. In this case, our condition is for a specific value of a unique column, so only one row will be changed. The results of the query show that one row was affected, one row was matched, one row was changed, and there were no problems to generate warnings.

Sometimes inserting data into a table will cause a duplicate row to be created because a row for the data already exists. For instance, suppose that we want to run an SQL statement that inserts data on a few books into the books table and one of the books is already in the table. If we use INSERT, a duplicate row will generally be rejected. To prevent this, we can use the REPLACE statement, which inserts rows that are new and replaces existing rows with new data.

From MySQL’s perspective, duplicates occur only when columns defined as unique contain the same value. Because the book_id column is assigned automatically, it’s unlikely that we would duplicate it, because we wouldn’t tend to assign its value when adding records. What’s unique about each book in the book business is its ISBN number, which is the bar code number on the back of the book. To ensure that we do not have rows with the same ISBN number, we’ll alter our books table again and change the isbn column to a UNIQUE column, a column that requires a unique value. This way we won’t be able to enter data inadvertently on a book more than once:

ALTER TABLE books 
CHANGE COLUMN isbn isbn VARCHAR(20) UNIQUE;

Now we’re ready to insert data for more books without worrying about duplicate rows for books with the same ISBN number. Here is an example in which we attempt to add two more books by Graham Greene, one of which is already in the table:

REPLACE INTO books
(title, author_id, isbn, genre, pub_year)
VALUES('Brighton Rock',1,'0099478471','novel','1938'),
('The Quiet American',1,'0099478393','novel','1955');

The syntax for the REPLACE statement is the same as the INSERT statement. Notice that we’ve added two rows here in one statement. This is the same syntax that you would use if you want to add more than one row using INSERT. Just list each row’s data within parentheses and separate them by commas, as shown. In this example, there is already a row for the book containing the ISBN number 0099478471 (i.e., Brighton Rock), so its data will be replaced and a new row will not be added. There is currently no row for Greene’s book The Quiet American, though, so it will be added.