RE: Negated SELECT query

SELECT Employees.*
FROM Employees LEFT JOIN Assets ON Employess.EmployeeID =3D
Assets.EmployeeID =20
WHERE Assets.EmployeeID IS NULL

The one for assets with no maintenance is similar. The point is the left
join above produces in its output all rows from the Employees table
regardless of whether anything matches in the assets table. By selecting
only rows where the foreign key field in the assets table is null gives
you the employees having no assets.

John Bonnett

-----Original Message-----
From: BobSharp [mailto:[email protected]]=20
Sent: Wednesday, 18 March 2009 3:13 AM
To: [email protected]
Cc: [email protected]; [email protected]
Subject: Negated SELECT query

3 tables are related by one-many links.=20
Employees  ---<-  Assets  ---<-  Maintenance=20

Employees can be assigned  =3D> 0   Assets=20
Assets can have  =3D> 0  occurances of  Maintenance. =20

Assets table contains  EmployeeIDs  and  MaintenanceIDs, but no Foreign
Key contraints. =20


Queries ... =20
1)  which Employees do not have any  Assets ?=20
2)  which  Assets  have not had any  Maintenance ?=20
These have been written successfully with Sub-Queries, =20

I would like to know how they can be done with only JOINs  ?=20
(that also means  without the EXCEPT statement)=20
Is that too much of a challenge ?      (MySQL  5.0.67) =20







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