You have the two tables Table1 and Table2, both with a Column. To get the records from the Table1 that are not present in the Table2 (difference between T1-T2) use the following:
SELECT T1.Column
FROM Table1 AS T1
LEFT JOIN
Table2 AS T2
ON
T1.Key = T2.Key
WHERE T2.Column IS NULL
To get all the differences with a single query, use the following:
SELECT
T1.Column
,T2.Column
FROM
Table1 AS T1
FULL OUTER JOIN
Table2 AS T2
ON
T1.Key = T2.Key
WHERE
T1.Column IS NULL
OR
T2.Column IS NULL
When a record is present in Table1 and not in Table2, the Column from the Table2 is NULL (and the other way around). It is not possible to test for NULL values with comparison operators, such as =, <, or <>. Use the IS NULL and IS NOT NULL operators instead.
Alternative Method
SELECT T1.Column
FROM Table1 AS T1
WHERE NOT EXISTS
(SELECT T2.Column
FROM Table2 T2
WHERE T2.Key = T1.Key)
I find the alternate method useful for tables bound using SubscriberKey. For example to find subscribers to whom the email has not been sent. T1 would be a list and the T2 would be the _Sent data view.