一 需求
sql>select sal from emp where empno=7839;
SAL
----------
12101
sql>select sal from emp_back where empno=7839;
SAL
----------
12101
sql> update emp set sal=sal+10where empno=7839;
已更新1行。
sql>select sal from emp where empno=7839;
12111
sql>select sal from emp_back where empno=7839;
SAL
----------
12111