为什么左连接对交叉连接求值?

Lets say I have a table A with columns (Alpha, Beta) which is linked to table B with columns (Beta, Delta, Gamma). I can't explain why the first query is transformed to a cross join. (A.Alpha, A.Beta and B.Delta are unique keys. B.Beta looks up to A.Beta).Lets say I have a table A with columns (Alpha,