Insert SQL statements into Table A with four fields: A, B, C, and D. Table B has only three fields: A, B, and C.

If there are two tables (a, B), table A has four fields: A, B, C, and D. Table B only has three fields: A, B, and C, if I want to merge the contents of Table B into Table A, but there is. A = B. in case of a, a is the primary key and repeated values are not allowed. how to do this.
Insert into a select *, null from B where not exists (select 1 from a where a. A = B.),Only 1 is returned, and * can be used, but not exists (select 1 from a where. A = B. a) than exists (select * from a where. A = B. a) higher efficiency.
When the exists keyword introduces a subquery, it is equivalent to a existence test. Simply put, it is equivalent to the WHERE clause of an external query to test whether the row returned by the subquery exists.
Insert into establishes an internal Association.