-Left? Join? Mode (best efficiency in case of big data)

Choice? Answer. *? From where? Answer? Left? Join? b？ Open? B.levelcode=A.levelcode？

Where is it? B.levelcode？ Is it? empty

-Isn't it? Are you online? Method (actually not recommended because A has as many rows as possible)

Select *? From where? Answer? Where is it? levelcode？ Isn't it? Are you online? (choose? levelcode？ From where? b)

-Right? Join? Road (than left? Join is inefficient, and it is most suitable for giving large tables after from.

Choice? Answer. *? From where? b？ Right? Join? Answer? Open? B.levelcode=A.levelcode？

Where is it? B.levelcode？ Is it? empty

-Isn't it? Existence (efficiency is greater than no? In, check as many rows as possible)

Select *? From where? Answer? Where is it? Isn't it? Exist (select? levelcode？ From where? b？ Where is it?

B. Level code = A. Level code)