INSERT INTO a.grades (rollno, grade)
    SELECT rollno, 'A' FROM b.students WHERE ssn = 12345;

一些 DBMS 会接受以下内容，并在 SELECT 语句周围加上一组额外的括号:

Some DBMS would accept the following, with an extra set of parenthesis around the SELECT statement:

INSERT INTO a.grades (rollno, grade)
   VALUES((SELECT rollno FROM b.students WHERE ssn = 12345), 'A');