SELECT Employees.*
FROM Employees LEFT JOIN Assets ON Employess.EmployeeID =
Assets.EmployeeID
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:bobsharp@stripped]
Sent: Wednesday, 18 March 2009 3:13 AM
To: mysql@stripped
Cc: win32@stripped; mysql-help@stripped
Subject: Negated SELECT query
3 tables are related by one-many links.
Employees ---<- Assets ---<- Maintenance
Employees can be assigned => 0 Assets
Assets can have => 0 occurances of Maintenance.
Assets table contains EmployeeIDs and MaintenanceIDs, but no Foreign
Key contraints.
Queries ...
1) which Employees do not have any Assets ?
2) which Assets have not had any Maintenance ?
These have been written successfully with Sub-Queries,
I would like to know how they can be done with only JOINs ?
(that also means without the EXCEPT statement)
Is that too much of a challenge ? (MySQL 5.0.67)
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