Posted by Erland Sommarskog on 10/29/07 22:22

(pbassutti@hotmail.com) writes:
> SELECT Employess.EmployeeID
> FROM Employees LEFT OUTER JOIN Timecards on Employees.EmployeeID =
> Timecards.lmpEmployeeID
> WHERE lmpEmployeeID is NULL and lmpTimecardDate = '10/24/2007'
>
> But it doesn't work. However, when I comment the date condition out
> (lmpTimecardDate = '10/24/2007') it works all right but It's not what

In addition to David's post, here is what is happening:

The FROM ... LEFT JOIN operators define a table that includes all rows
in the outer table, Employees in this case. This table includes the columns
from the Timecards table, but for the employees there there is no timecard,
all columns have NULL. Which you apparently have understood, since you
the condition "lmpEmployeeID IS NULL". But then there is a lapse, and you
filter lmpTimecardDate despite it is not likely that there is a row in
Timecards where the date is non-NULL and the employee ID is NULL. (At least
one would hope so!) Moving the date condition to the ON clause addresses
the issue, as it now will be part of the condition that builds the
table that is then filtered by WHERE.

Personally, I would prefer to write this query with NOT EXISTS:

SELECT E.Employee
FROM Employees E
WHERE NOT EXISTS (SELECT *
FROM Timecards T
WHERE E.EmployeeID = T.lmpEmployeeID
AND T.lmpTimecardDate = '20071014')

Simply because this clearly express what this is all about.

And I would also use a date format that is safe from misinterpretations.


--
Erland Sommarskog, SQL Server MVP, esquel@sommarskog.se

Books Online for SQL Server 2005 at
http://www.microsoft.com/technet/prodtechnol/sql/2005/downloads/books.mspx
Books Online for SQL Server 2000 at
http://www.microsoft.com/sql/prodinfo/previousversions/books.mspx

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