等值连接 select e.empno, d.deptno from emp e, dept d where e.deptno=d.deptno; 不等值连接 select e.empno, e.ename, e.sal, s.grade from emp e, salgrade s where e.sal between s.losal and s.hisal; 外连接(取交集) select d.deptno, d.dname, count(e.empno) from emp e, dept…