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

count(*) counts NULLs and count(column) does not

[edit] added this code so that 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)
from #bla

results 7 3 2

Another minor difference, between using * and a specific column, is that in the column case you can add the keyword DISTINCT, and restrict the count to distinct values:

select column_a, count(distinct column_b)
from table
group by column_a
having count(distinct column_b) > 1;

A further and perhaps subtle difference is that in some database implementations the count(*) is computed by looking at the indexes on the table in question rather than the actual data rows. Since no specific column is specified, there is no need to bother with the actual rows and their values (as there would be if you counted a specific column). Allowing the database to use the index data can be significantly faster than making it count "real" rows.