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A.select * from emp where comm=‘‘;
B.select * from emp where comm=null;
C.select * from emp where nvl(comm)=0;
D.select * from emp where comm is null;
E.select * from emp where nvl(comm,0)=0;
A.select * from emp where comm=‘‘;
B.select * from emp where comm=null;
C.select * from emp where nvl(comm)=0;
D.select * from emp where comm is null;
E.select * from emp where nvl(comm,0)=0;
A.select * from emp where hiredate>=‘19820101‘;
B.select * from emp where hiredate>=to_char(‘19820101‘,‘YYYYMMDD‘);
C.select * from emp where to_date(hiredate,‘YYYYMMDD‘)>=‘19820101‘;
D.select * from emp where to_char(hiredate,‘YYYYMMDD‘)>=‘19820101‘;
A.A.select alle_sal from emp
B.B.select distinct e_sal from emp
C.C.select e_sal from emp
D.D.select e_sal where emp
A.student表的一个字段
B.student表的全部字段
C.student表的一条记录
D.student表的全部记录
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