相关和非相关查询区别

相关和非相关查询区别
--求各部门人数
--格式：select d.*,0 人数 from dept d
select d.*,
(
  select count(*) from emp e
  where e.deptno=d.deptno
) 人数
from dept d

select d.* from dept d
部门：
10
select count(*) from emp e   where e.deptno=10
20
select count(*) from emp e   where e.deptno=20
30
select count(*) from emp e   where e.deptno=30
40
select count(*) from emp e   where e.deptno=40
50
select count(*) from emp e   where e.deptno=50
60
select count(*) from emp e   where e.deptno=60
对于部门表的每一条记录，都要单独执行一次子查询，所以称为相关子查询。

列出薪金比“SMITH”多的所有员工

select * from emp e
where e.sal>
(
  select e.sal from emp e  where e.ename='SMITH'
)
smith的值只求一次，这个值对于emp中所有记录都是一样的，即不与各记录相关，所以称为非相关子查询。

分析：效率高的是：非相关。
原因：相关子查询执行统计的次数太多。

评论

1 楼 java1573 2007-10-12  

--测试数据(Oracle的scott模型)
prompt PL/SQL Developer import file
prompt Created on 2007-10-12 by Administrator
set feedback off
set define off
prompt Dropping DEPT...
drop table DEPT cascade constraints;
prompt Dropping EMP...
drop table EMP cascade constraints;
prompt Creating DEPT...
create table DEPT
(
  DEPTNO NUMBER(2) not null,
  DNAME  VARCHAR2(14),
  LOC    VARCHAR2(13)
)
;
alter table DEPT
  add constraint PK_DEPT primary key (DEPTNO);

prompt Creating EMP...
create table EMP
(
  EMPNO    NUMBER(4) not null,
  ENAME    VARCHAR2(10),
  JOB      VARCHAR2(9),
  MGR      NUMBER(4),
  HIREDATE DATE,
  SAL      NUMBER(7,2),
  COMM     NUMBER(7,2),
  DEPTNO   NUMBER(2)
)
;
alter table EMP
  add constraint PK_EMP primary key (EMPNO);
alter table EMP
  add constraint FK_DEPTNO foreign key (DEPTNO)
  references DEPT (DEPTNO);

prompt Disabling triggers for DEPT...
alter table DEPT disable all triggers;
prompt Disabling triggers for EMP...
alter table EMP disable all triggers;
prompt Disabling foreign key constraints for EMP...
alter table EMP disable constraint FK_DEPTNO;
prompt Loading DEPT...
insert into DEPT (DEPTNO, DNAME, LOC)
values (10, 'ACCOUNTING', 'NEW YORK');
insert into DEPT (DEPTNO, DNAME, LOC)
values (20, 'RESEARCH', 'DALLAS');
insert into DEPT (DEPTNO, DNAME, LOC)
values (30, 'SALES', 'CHICAGO');
insert into DEPT (DEPTNO, DNAME, LOC)
values (40, 'OPERATIONS', 'BOSTON');
insert into DEPT (DEPTNO, DNAME, LOC)
values (50, 'zhangyi', 'china');
insert into DEPT (DEPTNO, DNAME, LOC)
values (60, '陈伟林', null);
commit;
prompt 6 records loaded
prompt Loading EMP...
insert into EMP (EMPNO, ENAME, JOB, MGR, HIREDATE, SAL, COMM, DEPTNO)
values (7369, 'SMITH', 'CLERK', 7902, to_date('17-12-1980', 'dd-mm-yyyy'), 800, null, 20);
insert into EMP (EMPNO, ENAME, JOB, MGR, HIREDATE, SAL, COMM, DEPTNO)
values (7499, 'ALLEN', 'SALESMAN', 7698, to_date('20-02-1981', 'dd-mm-yyyy'), 1600, 300, 30);
insert into EMP (EMPNO, ENAME, JOB, MGR, HIREDATE, SAL, COMM, DEPTNO)
values (7521, 'WARD', 'SALESMAN', 7698, to_date('22-02-1981', 'dd-mm-yyyy'), 1250, 500, 30);
insert into EMP (EMPNO, ENAME, JOB, MGR, HIREDATE, SAL, COMM, DEPTNO)
values (7566, 'JONES', 'MANAGER', 7839, to_date('02-04-1981', 'dd-mm-yyyy'), 2975, null, 20);
insert into EMP (EMPNO, ENAME, JOB, MGR, HIREDATE, SAL, COMM, DEPTNO)
values (7654, 'MARTIN', 'SALESMAN', 7698, to_date('28-09-1981', 'dd-mm-yyyy'), 1250, 1400, 30);
insert into EMP (EMPNO, ENAME, JOB, MGR, HIREDATE, SAL, COMM, DEPTNO)
values (7698, 'BLAKE', 'MANAGER', 7839, to_date('01-05-1981', 'dd-mm-yyyy'), 2850, null, 30);
insert into EMP (EMPNO, ENAME, JOB, MGR, HIREDATE, SAL, COMM, DEPTNO)
values (7782, 'CLARK', 'MANAGER', 7839, to_date('09-06-1981', 'dd-mm-yyyy'), 2450, null, 10);
insert into EMP (EMPNO, ENAME, JOB, MGR, HIREDATE, SAL, COMM, DEPTNO)
values (7788, 'SCOTT', 'ANALYST', 7566, to_date('19-04-1987', 'dd-mm-yyyy'), 3000, null, 20);
insert into EMP (EMPNO, ENAME, JOB, MGR, HIREDATE, SAL, COMM, DEPTNO)
values (7839, 'KING', 'PRESIDENT', null, to_date('17-11-1981', 'dd-mm-yyyy'), 5000, null, 10);
insert into EMP (EMPNO, ENAME, JOB, MGR, HIREDATE, SAL, COMM, DEPTNO)
values (7844, 'TURNER', 'SALESMAN', 7698, to_date('08-09-1981', 'dd-mm-yyyy'), 1500, 0, 30);
insert into EMP (EMPNO, ENAME, JOB, MGR, HIREDATE, SAL, COMM, DEPTNO)
values (7876, 'ADAMS', 'CLERK', 7788, to_date('23-05-1987', 'dd-mm-yyyy'), 1100, null, 20);
insert into EMP (EMPNO, ENAME, JOB, MGR, HIREDATE, SAL, COMM, DEPTNO)
values (7900, 'JAMES', 'CLERK', 7698, to_date('03-12-1981', 'dd-mm-yyyy'), 950, null, 30);
insert into EMP (EMPNO, ENAME, JOB, MGR, HIREDATE, SAL, COMM, DEPTNO)
values (7902, 'FORD', 'ANALYST', 7566, to_date('03-12-1981', 'dd-mm-yyyy'), 3000, null, 20);
insert into EMP (EMPNO, ENAME, JOB, MGR, HIREDATE, SAL, COMM, DEPTNO)
values (7934, 'MILLER', 'CLERK', 7782, to_date('23-01-1982', 'dd-mm-yyyy'), 1300, null, 10);
commit;
prompt 14 records loaded
prompt Enabling foreign key constraints for EMP...
alter table EMP enable constraint FK_DEPTNO;
prompt Enabling triggers for DEPT...
alter table DEPT enable all triggers;
prompt Enabling triggers for EMP...
alter table EMP enable all triggers;
set feedback on
set define on
prompt Done.