The Coding Error
The SELECT statement errs by placing:
b.data_date = 199707
in the WHERE clause rather than the ON clause, resulting in the query being processed as a simple inner join rather than the left outer join as intended.
The SELECT statement errs by placing:
b.data_date = 199707
in the WHERE clause rather than the ON clause, resulting in the query being processed as a simple inner join rather than the left outer join as intended.