SQL: How to properly check if a record exists

Question

While reading some SQL Tuning-related documentation, I found this:

SELECT COUNT(*) :

• Counts the number of rows.
• Often is improper

Answer 1

I'm using this way:

IIF(EXISTS (SELECT TOP 1 1 
                FROM Users 
                WHERE FirstName = 'John'), 1, 0) AS DoesJohnExist

Answer 2

You can use:

SELECT COUNT(1) FROM MyTable WHERE ... 

or

WHERE [NOT] EXISTS 
( SELECT 1 FROM MyTable WHERE ... )

This will be more efficient than SELECT * since you're simply selecting the value 1 for each row, rather than all the fields.

There's also a subtle difference between COUNT(*) and COUNT(column name):

• COUNT(*) will count all rows, including nulls
• COUNT(column name) will only count non null occurrences of column name

Answer 3

You can use:

SELECT 1 FROM MyTable WHERE... LIMIT 1

Use select 1 to prevent the checking of unnecessary fields.

Use LIMIT 1 to prevent the checking of unnecessary rows.