Should I COUNT(*) or not?

If the column in question is NOT NULL, both of your queries are equivalent. When group_id contains null values,

select count(*)

will count all rows, whereas

select count(group_id)

will only count the rows where group_id is not null.

Also, some database systems, like MySQL employ an optimization when you ask for count(*) which makes such queries a bit faster than the specific one.

Personally, when just counting, I'm doing count(*) to be on the safe side with the nulls.

If I remember it right, in MYSQL COUNT(*) counts all rows, whereas COUNT(column_name) counts only the rows that have a non-NULL value in the given column.

COUNT(*) count all rows while COUNT(column_name) will count only rows without NULL values in the specified column.

Important to note in MySQL:

COUNT() is very fast on MyISAM tables for * or not-null columns, since the row count is cached. InnoDB has no row count caching, so there is no difference in performance for COUNT(*) or COUNT(column_name), regardless if the column can be null or not. You can read more on the differences on this post at the MySQL performance blog.

if you try SELECT COUNT(1) FROMgroup_relations it will be a bit faster because it will not try to retrieve information from your columns.

Edit: I just did some research and found out that this only happens in some db. In sqlserver it's the same to use 1 or *, but on oracle it's faster to use 1.

http://social.msdn.microsoft.com/forums/en-US/transactsql/thread/9367c580-087a-4fc1-bf88-91a51a4ee018/

Apparently there is no difference between them in mysql, like sqlserver the parser appears to change the query to select(1). Sorry if I mislead you in some way.