问题:
A . select sal,decode(sal>3000,'A',sal>2000,'B','C') grade from emp;
B . select sal,decode(sal,3000,'A',2000,'B','C') grade from emp;
C . select sal,(case when sal>3000 then 'A' when sal>2000 then 'B' else 'C' end) grade from emp;
D . select sal,(if sal>3000 then 'A' elsif sal>2000 then 'B' else 'C' end if) grade from em
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