有表emp记录员工信息,有字段empid、sal、comm分别记录员工编号、员工基本工资、员工提成。其中comm可能存在null值。现在要计算员工总工资,代码正确的有( )
A. SELECT *, sal+comm as 总工资 from emp;
B. SELECT *, sal + if(comm is null, 0, comm) as 总工资 from emp;
C. SELECT *, ifnull(sal+comm, sal) as 总工资 from emp;
D. SELECT *, sal + ifnull(comm, 0) as总工资 from emp;
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