If you want to include NULL values as "not equal" then you need a NULL safe operator. Otherwise, the <> returns NULL , which is treated as " ... ... <看更多>

Not Equal To (Transact SQL) - exclamation ... Tests whether one expression is not equal to another expression (a comparison operator). If either or both operands ... ... <看更多>

I assume you want rows from table 1 that does not exist in table_2 and vice versa: ... This SQL Tutorial explains the EXISTS, NOT EXISTS usage. ... <看更多>