多表查询求助

[lingyu0303]

我现在有一张主表,几张子表:
主表A,子表B,C,D,E;
A与E之间是一对多关系,A与B之间是一对多关系,B与C之间,C与D之间,也是一对多的关系；要实现通过A的主键查询，SQL要怎么写

[304的的哥]

SQL> select * from emp;
 
EMPNO ENAME      JOB         MGR HIREDATE          SAL      COMM DEPTNO
----- ---------- --------- ----- ----------- --------- --------- ------
 7369 SMITH      CLERK      7902 1980-12-17     800.00               20
 7499 ALLEN      SALESMAN   7698 1981-02-20    1600.00    300.00     30
 7521 WARD       SALESMAN   7698 1981-02-22    1250.00    500.00     30
 7566 JONES      MANAGER    7839 1981-04-02    2975.00               20
 7654 MARTIN     SALESMAN   7698 1981-09-28    1250.00   1400.00     30
 7698 BLAKE      MANAGER    7839 1981-05-01    2850.00               30
 7782 CLARK      MANAGER    7839 1981-06-09    2450.00               10
 7788 SCOTT      ANALYST    7566 1987-04-19    3100.00               20
 7839 KING       PRESIDENT       1981-11-17    5000.00               10
 7844 TURNER     SALESMAN   7698 1981-09-08    1500.00      0.00     30
 7876 ADAMS      CLERK      7788 1987-05-23    1100.00               20
 7900 JAMES      CLERK      7698 1981-12-03     950.00               30
 7902 FORD       ANALYST    7566 1981-12-03    3000.00               20
 7934 MILLER     CLERK      7782 1982-01-23    1300.00               10
 
14 rows selected
 
SQL> select * from dept;
 
DEPTNO DNAME          LOC
------ -------------- -------------
    10 ACCOUNTING     NEW YORK
    20 RESEARCH       DALLAS
    30 SALES          CHICAGO
    40 OPERATIONS     BOSTON
 --dept和emp表存在着1:m的关系，使用等值连接，不就可以查询到deptno对应的员工数目了
SQL> select dept.deptno,count(emp.empno) cnt
  2  from dept,emp
  3  where dept.deptno=emp.deptno
  4  group by dept.deptno
  5  /
 
DEPTNO        CNT
------ ----------
    10          3
    20          5
    30          6