Question

I have a unique column. I also have a known set of elements that are possible values for the column. I need to know which of the possible values are not already in the table, and as such, are suitable for insertion.

Is this possible with SQL or is post processing required?

Currently, I am using the “in” operator to select all rows where the column value equals an element in my set. Then I remove all matched elements from my set via post processing.

Answer 1

Stick the allowed values in a temporary table allowed, then use a subquery using NOT IN:

SELECT *
FROM allowed
WHERE allowed.val NOT IN (
    SELECT maintable.val
)

Some DBs will allow you to build up a table “in-place”, instead of having to create a separate table. E.g. in PostgreSQL (any version):

SELECT *
FROM (
    SELECT 'foo'
    UNION ALL SELECT 'bar'
    UNION ALL SELECT 'baz'    -- etc.
) inplace_allowed
WHERE inplace_allowed.val NOT IN (
    SELECT maintable.val
)

More modern versions of PostgreSQL (and perhaps other DBs) will let you use the slightly nicer VALUES syntax to do the same thing.