Question

I have a table containing a unique ID field. Another field (REF) contains a reference to another dataset’s ID field.
Now I have to select all datasets where REF points to a dataset that doesn’t exist.

SELECT * FROM table WHERE ("no dataset with ID=REF exists")

How can I do this?

Answer 1

3 ways

SELECT * FROM YourTable y WHERE NOT EXISTS 
     (SELECT * FROM OtherTable o WHERE y.Ref = o.Ref)

SELECT * FROM YourTable WHERE Ref NOT IN 
     (SELECT Ref FROM OtherTable WHERE Ref IS NOT NULL)

SELECT y.* FROM YourTable y 
LEFT OUTER JOIN  OtherTable o ON y.Ref = o.Ref
WHERE o.Ref IS NULL

See also Five ways to return all rows from one table which are not in another table