问题

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 doing so?

A statement like:

SELECT count(email) FROM orders

gives me the total count.

I tried SELECT DISTINCT count(email) FROM orders

but that does not seem to be giving me the numbers I am expecting.

回答1:

use

SELECT count( DISTINCT(email) ) FROM orders

Distinct provide unique email ids and then simply count them.

回答2:

SELECT count(DISTINCT(email)) FROM orders

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

回答3:

For best performance you should use:

SELECT

sub.email,

count(1) as 'count_unique'

FROM

(SELECT email FROM orders GROUP by email) sub

回答4:

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.

来源：https://stackoverflow.com/questions/16697215/mysql-count-number-of-unique-values