SQL Inner Join Returns WAY More Rows Than Expected

问题

The following query returns >7000 rows when each table only has 340 rows.

SELECT Config.Spec, TempTable.Spec FROM Confg INNER JOIN TempTable on Config.Spec = TempTable.Spec

Why would this happen? If an INNER JOIN only returns a row if there is a match in both tables then why would it return multiple rows for a match.

回答1:

If there is more than one row with the same Spec value in TempTable for the same Spec value in Confg, then you will get duplicate rows, and vice versa.

回答2:

Are the Spec field values non unique? This might explain why the query returns too many results; with duplicates you get get an effective cross product for those.

来源：https://stackoverflow.com/questions/4185891/sql-inner-join-returns-way-more-rows-than-expected