if not exists in sql

1At first glance your original attempt seems pretty close. I'm assuming that clockDate is a DateTime fields so try this:
2
3IF (NOT EXISTS(SELECT * FROM Clock WHERE cast(clockDate as date) = '08/10/2012') 
4    AND userName = 'test') 
5BEGIN 
6    INSERT INTO Clock(clockDate, userName, breakOut) 
7    VALUES(GetDate(), 'test', GetDate()) 
8END 
9ELSE 
10BEGIN 
11    UPDATE Clock 
12    SET breakOut = GetDate()
13    WHERE Cast(clockDate AS Date) = '08/10/2012' AND userName = 'test'
14END 
15Note that getdate gives you the current date. If you are trying to compare to a date (without the time) you need to cast or the time element will cause the compare to fail.
16
17If clockDate is NOT datetime field (just date), then the SQL engine will do it for you - no need to cast on a set/insert statement.
18
19IF (NOT EXISTS(SELECT * FROM Clock WHERE clockDate = '08/10/2012') 
20    AND userName = 'test') 
21BEGIN 
22    INSERT INTO Clock(clockDate, userName, breakOut) 
23    VALUES(GetDate(), 'test', GetDate()) 
24END 
25ELSE 
26BEGIN 
27    UPDATE Clock 
28    SET breakOut = GetDate()
29    WHERE clockDate = '08/10/2012' AND userName = 'test'
30END 
31As others have pointed out, the merge statement is another way to tackle this same logic. However, in some cases, especially with large data sets, the merge statement can be prohibitively slow, causing a lot of tran log activity. So knowing how to logic it out as shown above is still a valid technique.