1、NVL(comm,0)
NVL函数将NULL 变为指定的内容。
2、DECODE(col/expression,search1,result1,search2,result2,[…])
select decode(1,1,'内容是1',2,'内容是2',3,'内容是3') from dual;
3、 自然连接不显示含有NULL的值
SELECT E.ENAME, E.SAL, D.DNAME, E.DEPTNO
FROM EMP E, DEPT D
WHERE E.DEPTNO = D.DEPTNO
ORDER BY ENAME;
等同于
SELECT * FROM dept NATURAL JOIN emp;
SELECT * FROM emp join dept using(deptno);
4、左连接（+）在右面，以左边的表为准，显示左边含有NULL的数据
SELECT E.ENAME, E.SAL, D.DNAME, E.DEPTNO
FROM EMP E, DEPT D
WHERE E.DEPTNO = D.DEPTNO(+)
ORDER BY ENAME;
等同于
SELECT E.ENAME, E.SAL, D.DNAME, E.DEPTNO
FROM EMP E LEFT OUTER JOIN DEPT D
ON E.DEPTNO = D.DEPTNO ORDER BY ename;
5、右连接（+）在左面，以右边的表为准，显示右边含有NULL的数据
SELECT E.ENAME, E.SAL, D.DNAME, E.DEPTNO
FROM EMP E, DEPT D
WHERE E.DEPTNO(+) = D.DEPTNO
ORDER BY ENAME;
等同于
SELECT E.ENAME, E.SAL, D.DNAME, E.DEPTNO
FROM EMP E RIGHT OUTER JOIN DEPT D
ON E.DEPTNO = D.DEPTNO
ORDER BY ENAME;
6、全连接
SELECT E.ENAME, E.SAL, D.DNAME, E.DEPTNO
FROM EMP E FULL OUTER JOIN DEPT D
ON E.DEPTNO = D.DEPTNO
ORDER BY ENAME;
7、交叉连接
SELECT * FROM t1 CROSS JOIN t2;
8、查询员工的工资等级
SELECT E.ENAME, S.GRADE
FROM EMP E, SALGRADE S
WHERE S.LOSAL <= E.SAL
AND E.SAL <= S.HISAL;
9、exists查询
查询没有员工的部门
SELECT * FROM dept d WHERE NOT EXISTS (SELECT * FROM emp e WHERE e.deptno=d.deptno);
10、自连接查询
查询员工的上级名称
11、嵌套查询
SELECT * FROM EMP WHERE (deptno,job)=(SELECT deptno, job FROM emp WHERE ename='SMITH');
UPDATE emp set (job,sal,comm)=(select job,sal,comm from emp where ename='SMITH') where ename='SCOTT';
来源：https://www.cnblogs.com/nuaa/p/3691566.html