How do I get all the results not in a set? [closed]

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Im joining two tables and I want to then join a third but I want to get the result of the records that dont join. Dont really know what to use to do this. Can I search for nulls that appear outside the join or something?

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The best-performing way to do this in general is to use NOT IN or NOT EXISTS - they are identical behind the scenes in SQL Server 2005+.

They are preferred over LEFT JOIN...IS NULL because they short circuit - as soon as the matching condition is found, that record is skipped. LEFT JOIN loads the whole data set and relation, then eliminates records afterwards.

SELECT a.*
FROM TableA a
<other joins>
WHERE a.ComparisonField NOT IN (SELECT RelationField FROM OtherTable)

or

SELECT a.*
FROM TableA a
<other joins>
WHERE NOT EXISTS(SELECT 1 
                 FROM OtherTable o
                 WHERE o.Relationfield = a.Comparisonfield)