Beginning SQL

Chapter 9
284
The query would work fine; however, the number of members born before that date is very
small, whereas plenty of people live in New State. This means that State = New State will
occur a number of times and the database system will go on to check the second condition,
DateOfBirth < ‘1940-01-01’. If you swap the conditions around, the least likely condition
(DateOfBirth < ‘1940-01-01’) is evaluated first:
SELECT FirstName, LastName
FROM MemberDetails
WHERE DateOfBirth < ‘1940-01-01’ AND State = ‘New State’;
Because the condition is mostly going to be false, the second condition will rarely be executed,
which saves time. It’s not a big deal when there are few records, but it is when there are a lot of
them.
❑ When using OR, put the condition most likely to be true first. Whereas AND needs both sides to
be true for the overall condition to be true, OR needs only one side to be true. If the left-hand
side is true, there’s no need for OR to check the other condition, so you can save time by putting
the most likely condition first. Consider the following statement:
SELECT FirstName, LastName
FROM MemberDetails
WHERE State = ‘New State’ OR DateOfBirth < ‘1940-01-01’;
If New State is true, and it is true more often than DateOfBirth < ‘1940-01-01’ is true, then
there’s no need for the database system to evaluate the other condition, thus saving time.
❑ DISTINCT can be faster than GROUP BY. DISTINCT and GROUP BY often do the same thing:
limit results to unique rows. However, DISTINCT is often faster with some database systems
than GROUP BY. For example, examine the following GROUP BY:
SELECT MemberId
FROM Orders
GROUP BY MemberId;
The GROUP BY could be rewritten using the DISTINCT keyword:
SELECT DISTINCT MemberId
FROM Orders;
❑ Restrict join results. The less information the database has to pull out, the better. This is particularly
true with joins, which are very expensive time-wise. Put the restrictive condition on the
outer table of the join. The outer table is the table on the right of the INNER JOIN keyword. For
example, in the following code, the table with the restriction — a condition in the WHERE clause
restricting results — is the MemberDetails table, which is the outer table in the join:
SELECT MemberDetails.MemberId, FirstName, LastName
FROM Orders INNER JOIN MemberDetails
ON Orders.MemberId = MemberDetails.MemberId
WHERE MemberDetails.DateOfBirth BETWEEN ‘1900-01-01’ AND ‘1970-12-31’;