Oracle用一个表的数据更新另一个表的列

需求

如下有两张表，部门表dept1和员工表emp
要将员工表中每个部门的人数更新到部门表的person_count中

scott@ORCL>select * from dept1;

    DEPTNO DNAME          LOC           PERSON_COUNT
---------- -------------- ------------- ------------
        10 ACCOUNTING     NEW YORK
        20 RESEARCH       DALLAS
        30 SALES          CHICAGO
        40 OPERATIONS     BOSTON

scott@ORCL>select * from emp;

     EMPNO ENAME      JOB              MGR HIREDATE         SAL       COMM     DEPTNO
---------- ---------- --------- ---------- --------- ---------- ---------- ----------
      7369 SMITH      CLERK           7902 17-DEC-80       1000                    20
      7499 ALLEN      SALESMAN        7698 20-FEB-81       1600        300         30
      7521 WARD       SALESMAN        7698 22-FEB-81       1250        500         30
      7566 JONES      MANAGER         7839 02-APR-81       2975                    20
      7654 MARTIN     SALESMAN        7698 28-SEP-81       1250       1400         30
      7698 BLAKE      MANAGER         7839 01-MAY-81       2850                    30
      7782 CLARK      MANAGER         7839 09-JUN-81       2450                    10
      7788 SCOTT      ANALYST         7566 19-APR-87       3000                    20
      7839 KING       PRESIDENT            17-NOV-81       5000                    10
      7844 TURNER     SALESMAN        7698 08-SEP-81       1500          0         30
      7876 ADAMS      CLERK           7788 23-MAY-87       1100                    20
      7900 JAMES      CLERK           7698 03-DEC-81        950                    30
      7902 FORD       ANALYST         7566 03-DEC-81       3000                    20
      7934 MILLER     CLERK           7782 23-JAN-82       1300                    10

SQL语句

update dept1 d set person_count = (select count(*) from emp e where e.deptno = d.deptno);

scott@ORCL>select * from dept1;

    DEPTNO DNAME          LOC           PERSON_COUNT
---------- -------------- ------------- ------------
        10 ACCOUNTING     NEW YORK                 3
        20 RESEARCH       DALLAS                   5
        30 SALES          CHICAGO                  6
        40 OPERATIONS     BOSTON                   0