Just expanding on what Phil said, this can be done quite easily with a LEFT JOIN.

DECLARE @months TABLE ([month] VARCHAR(12))

INSERT INTO @months

VALUES ('January'), ('February'), ('March'), ('April'), ('May'), ('June'), ('July'), ('August'), ('September'), ('October'), ('November'), ('December')

DECLARE @payroll TABLE ([month] VARCHAR(12), pay NUMERIC)

INSERT INTO @payroll

VALUES ('January', 1200), ('March', 1500), ('December', 2000)

SELECT

m.[month],

CASE WHEN p.pay IS NULL THEN 00 ELSE p.pay END AS pay

FROM @months m

LEFT JOIN @payroll p ON m.[month] = p.[month]