高级子查询常见用法及举例

CASE表达式中的标量子查询

查找部门号为1800的部门ID，地点显示为CANADA，其他显示为USA

SELECT EMPLOYEE_ID,LAST_NAME,DEPARTMENT_ID,

    (CASE

        WHEN DEPARTMENT_ID=(SELECT DEPARTMENT_ID FROM DEPARTMENTS WHERE LOCATION_ID=1800)

        THEN 'CANADA'

        ELSE 'USA'

        END )LOCATION

FROM EMPLOYEES;

ORDER BY子句的标量子查询

将EMPLOYEES表按照DEPARTMENTS表中的部门名排序

SELECT EMPLOYEE_ID,LAST_NAME,DEPARTMENT_ID

FROM EMPLOYEES E

ORDER BY (SELECT DEPARTMENT_NAME

FROM DEPARTMENTS D

WHERE D.DEPARTMENT_ID=E.DEPARTMENT_ID);

相关子查询

SELECT COLUMN1,COLUMN2,...

FROM TABLE1 外层表别名

WHERE COLUMN1 OPERATOR

(SELECT COLUMN1,COLUMN2

FROM TABLE2 内层表别名

                             WHERE EXPR1=外层表.EXPR2)

查找薪水比部门平均薪水高的

SELECT LAST_NAME,SALARY,DEPARTMENT_ID

FROM EMPLOYEES OUTER_TABLE

WHERE SALARY>( 

                SELECT AVG(SALARY)

                FROM EMPLOYEES INNER_TABLE

                WHERE INNER_TABLE.DEPARTMENT_ID=OUTER_TABLE.DEPARTMENT_ID);

使用EXISTS操作符查找领导（查出来的这些人都是领导）

SELECT EMPLOYEE_ID,LAST_NAME,JOB_ID,DEPARTMENT_ID

FROM EMPLOYEES OUTER

WHERE EXISTS(SELECT ‘X’

                     FROM EMPLOYEES

     WHERE MANAGER_ID=OUTER.EMPLOYEE_ID);

查找没有任何员工的部门

SELECT DEPARTMENT_ID,DEPARTMENT_NAME

FROM DEPARTMENTS D

WHERE NOT EXISTS (SELECT 'X'

                  FROM EMPLOYEES

                  WHERE DEPARTMENT_ID=D.DEPARTMENT_ID);

相关UPDATE

CREATE TABLE EMP_I AS SELECT * FROM EMP;

ALTER TABLE EMP_I ADD(DNAME VARCHAR2(20));

UPDATE EMP_I

SET DNAME=(SELECT DNAME FROM DEPT WHERE DEPT.DEPTNO=EMP_I.DEPTNO);

SELECT * FROM EMP_I;

相关DELETE

删除两个表相同的内容

CREATE TABLE EMP1 AS SELECT * FROM EMP;

CREATE TABLE EMP2 AS SELECT * FROM EMP;

DELETE EMP2 WHERE EMPNO<7782;

SELECT * FROM EMP1;

SELECT * FROM EMP2;

DELETE EMP1

WHERE EMPNO=(SELECT EMPNO

             FROM EMP2

             WHERE EMP2.EMPNO=EMP1.EMPNO);