If you have two tables A and B, both with column C, here are the records, which are present in the table A but not in B:
SELECT A.*
FROM A
LEFT JOIN B ON (A.C = B.C)
WHERE B.C IS NULL
To get all the differences with a single query, a full join must be used, like this:
SELECT A.*, B.*
FROM A
FULL JOIN B ON (A.C = B.C)
WHERE A.C IS NULL OR B.C IS NULL
What you need to know in this case is, that when a record can be found in A, but not in B, then the columns which come from B will be NULL, and similarly for those, which are present in B and not in A, the columns from A will be null.
Thanks to Treaschf for the answer.