How to check if a value exists in the database

Question

I have the following code which I am trying to use to check if a value is already stored inside the db.

IF EXISTS
(
    SELECT * FROM [Clock] WHERE [clockIn] ='12/03/2016'
             AND UserName = 'ROSE'
)
BEGIN 
    RAISERROR('CLOCK IS TRUE NOTHING MORE TO DO',16 ,1)
    ROLLBACK TRAN
END
ELSE
IF NOT EXISTS
(
    SELECT * FROM [Clock] WHERE [clockIn] ='12/03/2016'
    AND UserName = 'ROSE'
)
    BEGIN
        INSERT INTO [Clock] ([clockIn], [UserName])
        VALUES(GetDate(), 'Rose')
    END

I'm not entirely sure why this is working as it always seems to insert a new row into the db and I am puzzled. I have used the following as an example SQL Server IF NOT EXISTS Usage? and also SQL Server 2008 - IF NOT EXISTS INSERT ELSE UPDATE as reference.

Answer 1

Code seems ok to me - but the IF NOT EXISTS in the ELSE case is superfluous - you already checked and got back the fact that that value does not exist - so there's really no need to check again .

However: you're checking for existence of a row with a fixed date specified in a string literal - that'll work today , but not anymore tomorrow. I'd recommend to first get the current date into a variable, and then use that variable both for the checking and the possible insert.

-- store today's date in a variable  
DECLARE @Today DATE = SYSDATETIME();

-- check if that row already exists in table
IF EXISTS (SELECT * FROM [Clock] 
           WHERE [clockIn] = @Today AND UserName = 'ROSE')
BEGIN 
    RAISERROR('CLOCK IS TRUE NOTHING MORE TO DO',16 ,1)
    ROLLBACK TRAN
END
ELSE
BEGIN
    INSERT INTO [Clock] ([clockIn], [UserName])
    VALUES(@Today, 'Rose')
END

As a side note: I'd recommend to use the ISO-8601 format when specifying dates as string literals - YYYYMMDD . That format will work with any language and/or regional settings in your (and anybody else's) SQL Server, while any other format can be dependent on the settings and might not work all the time!