[sql] SQL: How to properly check if a record exists

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

SELECT COUNT(*) :

• Counts the number of rows.
• Often is improperly used to verify the existence of a record.

Is SELECT COUNT(*) really that bad?

What's the proper way to verify the existence of a record?

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

I would prefer not use Count function at all:

IF [NOT] EXISTS ( SELECT 1 FROM MyTable WHERE ... )
     <do smth>

For example if you want to check if user exists before inserting it into the database the query can look like this:

IF NOT EXISTS ( SELECT 1 FROM Users WHERE FirstName = 'John' AND LastName = 'Smith' )
BEGIN
    INSERT INTO Users (FirstName, LastName) VALUES ('John', 'Smith')
END

Other option:

SELECT CASE
    WHEN EXISTS (
        SELECT 1
        FROM [MyTable] AS [MyRecord])
    THEN CAST(1 AS BIT) ELSE CAST(0 AS BIT)
END

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.

The other answers are quite good, but it would also be useful to add LIMIT 1 (or the equivalent, to prevent the checking of unnecessary rows.

I'm using this way:

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

You can use:

SELECT 1 FROM MyTable WHERE <MyCondition>

If there is no record matching the condition, the resulted recordset is empty.

SELECT COUNT(1) FROM MyTable WHERE ...

will loop thru all the records. This is the reason it is bad to use for record existence.

I would use

SELECT TOP 1 * FROM MyTable WHERE ...

After finding 1 record, it will terminate the loop.