What is the difference between count (*) and count(attribute_name)?

Question

Is there any difference between COUNT(*) and COUNT(attribute_name)?

I used count(attribute_name) as I thought that it would be specific hence the searching process would be easier. Is that true?

It would be great to see any example with sql code with my issue to help me understand better

Answer

Imagine this table:

select Count(TelephoneNumber) from Calls -- returns 3
select Count(*) from Calls -- returns 4

count(column_name) also counts duplicate values. Consider:

select Count(TelephoneNumber) from Calls -- returns 4

Accepted Answer

Imagine this table:select Count(TelephoneNumber) from Calls -- returns 3select Count(*) from Calls -- returns 4count(column_name) also counts duplicate values. Consider:select Count(TelephoneNumber) from Calls -- returns 4

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Answer