you use DELETE to delete a row in one table on which a row in another table is dependent, you’ll have orphaned data. You could execute another DELETE to remove that other row, but it’s usually better to delete rows in both tables in the same DELETE statement,
especially when there may be many rows of data to delete.
The syntax for the DELETE that deletes rows in multiple tables is:
DELETE FROM table[, table]
USING table[, . . . ] [WHERE condition];
In the FROM clause, list the tables in a comma-separated list. The USING clause specifies how the tables are joined together (e.g., based on human_id). The WHERE clause is optional.
Like the UPDATE statement, because this syntax includes multiple tables, the ORDER BY and
LIMIT clauses are not permitted. This syntax can be tricky, but how much so may not be evident from looking at the syntax. Let’s look at an example.
In the example at the end of the previous subsection, we needed to delete rows from two tables that are related. We want to delete the rows for Elena Bokova in which she has a yahoo.com email address in both the humans and the prize_winners tables. To do that efficiently, we’ll enter this from the mysql client:
DELETE FROM humans, prize_winners USING humans JOIN prize_winners WHERE name_first = 'Elena' AND name_last = 'Bokova'
AND email_address LIKE '%yahoo.com'
AND humans.human_id = prize_winners.human_id;
This DELETE statement is similar to other data manipulation statements (e.g., SELECT,
UPDATE). However, there is a difference in the syntax that may be unexpected and
confusing. The FROM clause lists the tables from which data is to be deleted. There is also a
USING clause that lists the tables again and how they are joined. What is significant about this distinction is that we must list the tables in which rows are to be deleted in the FROM clause. If we did not include prize_winners in that list, no rows would be deleted from it
— only rows from humans would be deleted.
There are several contortions and options in the syntax for DELETE. However, at this stage, the methods we reviewed in this chapter will serve well for almost all situations you will encounter as a MySQL and MariaDB developer or administrator.
Summary
The UPDATE and DELETE statements are very useful for changing data in tables; they are essential to managing a MySQL or MariaDB database. They have many possibilities for effecting changes to tables with ease. You can construct very complex SQL statements with them to change precisely the data you want to change or to delete exactly the rows you want to delete. However, it can be confusing and difficult at times. So be careful and learn these SQL statements well.
If you’re nervous at times about using the UPDATE and DELETE statements, it’s because you should be. You can change all of the rows in a table with one UPDATE statement, and you can delete all of the rows in a table with one DELETE statement. On a huge database, that could be thousands of rows of data changed or deleted in seconds. This is why good backups are always necessary. Whenever using these two SQL statements, take your time to be sure you’re right before you execute them. While you’re still learning especially, it can be a good idea to make a duplicate of a table with its data using the CREATE TABLE…
SELECT statement before updating or deleting data. This SQL statement was covered in Essential Changes. This way if you make a major mistake, you can put the data back as it was before you started.
Because of the problems you can cause yourself and others who will use the databases on which you will work, practice using the UPDATE and DELETE statements. More than any other chapter in this book so far, you should make sure to complete the exercises in the next section.
Exercises
Exercises follow for you to practice using the UPDATE and DELETE statements. If you
haven’t already, download the rookery and the birdwatchers databases from the MySQL Resources site). This will give you some good-sized tables on which to practice these SQL statements.
1. Use the CREATE TABLE…SELECT statement (see Essential Changes) to make a copies of the humans and the prize_winners tables. Name the new tables humans_copy and
prize_winners_copy. Once you’ve created the copies, use the SELECT statement to view all of the rows in both of the new tables. You should see the same values as are contained in the original tables.
2. After you’ve done the previous exercise, use the SELECT statement to select all of the members from Austria in the humans table. You’ll need to use a WHERE clause for that SQL statement. The country_id for Austria is au. If you have problems, fix the SQL statement until you get it right.
Next, using the same WHERE clause from the SELECT statement, construct an UPDATE statement to change the value of the membership_type column for Austrian members to premium. In the same UPDATE statement, set the value of the
membership_expiration to one year from the date you execute the SQL statement.
You will need to use the CURDATE() function inside the DATE_ADD() function. The
DATE_ADD() function was shown in an example earlier in this chapter (see Updating Specific Rows). The CURDATE() has no arguments to it, nothing to go inside its
parentheses. Both functions are covered in Chapter 11. If you can’t figure out how to combine these function, you can enter the date manually (e.g., ‘2014-11-03’ for November 3, 2014; include the quote marks). Use the SELECT statement to check the results when you’re done.
3. Using the DELETE statement, delete the rows associated with the member named Barry Pilson from the humans and prize_winners tables. This was explained, along with an example showing how to do it, in Deleting in Multiple Tables. After you do this, use the SELECT statement to view all of the rows in both tables to make sure you deleted both rows.
4. Using the DELETE statement, delete all of the rows in the humans table. Then delete all of the rows of data in the prize_winners tables. Use the SELECT statement to confirm that both tables are empty.
Now copy all of the data from the humans_copy and prize_winners_copy tables to the humans and prize_winners tables. Do this with the INSERT…SELECT statement (covered in Inserting Data from Another Table).
After you’ve restored the data by this method, execute the SELECT statement again to confirm that both tables now have all of the data. If you were successful, use the
DROP TABLE statement to eliminate the humans_copy and prize_winners_copy
tables. This SQL statement was covered in Chapters 4 and 5. If you drop the wrong tables or if you delete data from the wrong tables, you can always download the whole database again from the MySQL Resources site.
Chapter 9. Joining and Subquerying Data
Most of the examples used in this book thus far have intentionally involved one table per SQL statement in order to allow you to focus on the basic syntax of each SQL statement.
When developing a MySQL or MariaDB database, though, you will often query multiple tables. There are a few methods by which you may do that — you’ve seen some simple examples of them in previous chapters. This chapter covers how to merge results from multiple SQL statements, how to join tables, and how to use subqueries to achieve similar results.