After learning how to insert and edit data in a database, thesecond part of our SQL tutorial takes an in-depth look at the SELECTquery,and explains how to use joins, sub-queries and built-in functions tofocus in on the data you need.
SQL also allows you to nest one query within another, such that the result of the inner query provides data for the outer query. Such a query is referred to as a sub-query, and it allows a great deal of flexibility when formulating long and complex queries.
Let's suppose you want to find out who's rented "The Phantom Menace" this weekend. You could of course perform a join, as described on the previous page, and get the answer to your question. Or you could formulate a subquery, which would look like this:
mysql> select fname, lname from members where member_id=(select member_id
from status where video_id=1);
In this case, SQL will first execute the inner query
SELECT member_id FROM status WHERE video_id=1;
+-----------+
| member_id |
+-----------+
| 1 |
+-----------+
and then assign the return value to the outer query, which
will display the result.
SELECT fname, lname FROM members WHERE member_id=1;
+-------+-------+
| fname | lname |
+-------+-------+
| John | Doe |
+-------+-------+
1 row in set (0.00 sec)
There is a limit on the number of subqueries you can use in a
single SQL statement, but it's usually quite a comfortable number. Note, however, that mySQL does not currently support SQL subqueries.
And that's about it. I hope this introduction to SQL helped you get some idea of how to go about creating and using a database, and that you now have a better understanding of the language. Till next time - stay healthy!
This article copyright Melonfire 2001. All rights reserved.
