1：从update更新语句进行说明：
执行update语句，操作如下：
SQL> conn scott/tiger
Connected.
SQL> select * from emp;
EMPNO ENAME JOB MGR HIREDATE SAL COMM DEPTNO
---------- ---------- --------- - --------- --------- ---------- --------- ----------
7369 SMITH CLERK 7902 17-DEC-80 5000 20
7499 ALLEN SALESMAN 7698 20-FEB-81 5000 300 30
7521 WARD SALESMAN 7698 22-FEB-81 5000 500 30
EMPNO ENAME JOB MGR HIREDATE SAL COMM DEPTNO
---------- ---------- --------- ---------- --------- ---------- ----------
7566 JONES MANAGER 7839 02-APR-81 5000
20
7654 MARTIN SALESMAN 7698 28-SEP-81 5000 1400
30
7698 BLAKE MANAGER 7839 01-MAY-81 5000
30
EMPNO ENAME JOB MGR HIREDATE SAL COMM DEPTNO
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---------- ---------- --------- - --------- --------- ---------- --------- ---------- 7782 CLARK MANAGER 7839 09-JUN-81 5000 10
