Count number of unique values

Question

If I have three columns:

orderNumber, name, email

and I would like to count how many unique emails are in the table how would I go about do

Answer 1

use

SELECT count( DISTINCT(email) ) FROM orders

Distinct provide unique email ids and then simply count them.

Answer 2

For best performance you should use:

SELECT 
sub.email, 
count(1) as 'count_unique' 
FROM 
(SELECT email FROM orders GROUP by email) sub

Answer 3

The accepted soultion doesn't work for me - it returns a "1" for each unique email address in the table.

This is what I had to do to get the info I needed:

select email, count(email) AS total from sysAccessLog group by email order by total desc

Which returns a list of email addresses and the number of occurrences.

Answer 4

SELECT  count(DISTINCT(email)) FROM orders

its different from your posting, since its filters out the duplicates before counting it