SQL: How to properly check if a record exists

Question

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.

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

You can use:

SELECT 1 FROM MyTable WHERE <MyCondition>

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

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.

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.