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

Other option:

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

Answer 2

It's better to use either of the following:

-- Method 1.
SELECT 1
FROM table_name
WHERE unique_key = value;

-- Method 2.
SELECT COUNT(1)
FROM table_name
WHERE unique_key = value;

The first alternative should give you no result or one result, the second count should be zero or one.

How old is the documentation you're using? Although you've read good advice, most query optimizers in recent RDBMS's optimize SELECT COUNT(*) anyway, so while there is a difference in theory (and older databases), you shouldn't notice any difference in practice.

Answer 3

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.

Answer 4

You can use:

SELECT 1 FROM MyTable WHERE <MyCondition>

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

Answer 5

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

Answer 6

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.