In SQL, what's the difference between count(column) and count(*)? -

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i have following query:

select column_name, count(column_name) table group column_name having count(column_name) > 1;

what difference if replaced calls count(column_name) count(*)?

this question inspired how find duplicate values in table in oracle?.

to clarify accepted answer (and maybe question), replacing count(column_name) count(*) return row in result contains null , count of null values in column.

count(*) counts nulls , count(column) not

[edit] added code people can run it

create table #bla(id int,id2 int) insert #bla values(null,null) insert #bla values(1,null) insert #bla values(null,1) insert #bla values(1,null) insert #bla values(null,1) insert #bla values(1,null) insert #bla values(null,null)  select count(*),count(id),count(id2) #bla

results 7 3 2


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