You can left join to a derived table containing the "numbers" (well, strings with a possible numeric representation actually).
SELECT d.clientnumber
FROM (VALUES ('00602'),
('00897'),
('00940')) AS n
(n)
LEFT JOIN dataentry AS de
ON de.clientnumber = n.n;
That'll give you as many NULL
s as "numbers" in the list that aren't present in the table and any "number" that is in the table the amount of times it is in it.
Or, what may be a bit more of a practical use, use NOT EXISTS
to get the "numbers" not in the table (yet).
SELECT n.n AS clientnumber
FROM (VALUES ('00602'),
('00897'),
('00940')) AS n
(n)
WHERE NOT EXISTS (SELECT *
FROM dataentry AS de
WHERE de.clientnumber = n.n);
Or even an indicator whether they are present or not.
SELECT n.n AS clientnumber,
CASE
WHEN EXISTS (SELECT *
FROM dataentry AS de
WHERE de.clientnumber = n.n) THEN
'true'
ELSE
'false'
END existence
FROM (VALUES ('00602'),
('00897'),
('00940')) AS n
(n);
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