AVG() function

Calculate the average commission without using AVG() 

In general,we use 

SELECT AVG(COMM) FROM EMP;

But here,there are NULL values under the comm.

SELECT SUM(COMM)/COUNT(COMM) FROM EMP;

There is a chance to commit mistake here.Some people write

SELECT SUM(COMM)/COUNT(*) FROM EMP; 

Here COUNT(*) gives 14 and the average value changes,which is incorrect.