Question

Consider the following query, where both sets need to be filtered by the same Location. Is there a more efficient/performant approach whereby the Location filter is only applied once across both sets, or are you forced to apply the same filter to each set individually?

SELECT *
FROM   Orders
WHERE  Quantity BETWEEN 1 AND 100
AND Location = 'SE'

EXCEPT

SELECT *
FROM   Orders
WHERE  Quantity BETWEEN 50 AND 75
AND Location = 'SE';

NOTE: Not looking to refactor the set operations per se- it’s just a dummy example. My question is only about how you would handle common WHERE clauses across set operations.

Answer 1

You can do this with a CTE:

with ord (Column1, Column2) as
(
    SELECT Column1, Column2
    FROM Orders 
    WHERE Location = 'SE'
)
SELECT *
FROM ord
WHERE Quantity BETWEEN 1 AND 100

EXCEPT

SELECT *
FROM ord
WHERE Quantity BETWEEN 50 AND 75

If CTEs are not available, you could do the same thing with a view.