SQL count of distinct union - mysql

Question

I have a query that captures customer ids from three tables (each table is a different contact method).
I want to get the count of distinct customer ids after the unions.
The SQL statement below is working and returns a list of unique customer ids (no dups):
SELECT DISTINCT customer_id
FROM email_contact
WHERE info_id = 1
AND status = 'SENT'
UNION
SELECT DISTINCT customer_id
FROM call_contact
WHERE info_id = 1
AND status = 'CALLED'
UNION
SELECT DISTINCT customer_id
FROM mail_contact
WHERE info_id = 1
AND status = 'MAILED'
From that query I want a count of customers, but my attempts to wrap the query in a select count keep producing syntax errors. How can wrap the unions to provide me with a count of the clients?

Answer 1

I would recommend:
SELECT COUNT(DISTINCT customer_id)
FROM (
SELECT customer_id FROM email_contact WHERE info_id = 1 AND status = 'SENT'
UNION ALL SELECT customer_id FROM call_contact WHERE info_id = 1 AND status = 'CALLED'
UNION ALL SELECT customer_id FROM mail_contact WHERE info_id = 1 AND status = 'MAILED'
) t
I removed the DISTINCT and I changed the UNIONs to UNION ALL, so the database just gathers all the rows from the 3 union members without attempting to manage duplicates (this is fast). Then, you can use COUNT(DISTINCT ...) in the outer query.

Answer 2

You can wrap it like this
SELECT COUNT(*)
FROM
( SELECT DISTINCT customer_id
FROM email_contact
WHERE info_id = 1
AND status = 'SENT'
UNION
SELECT DISTINCT customer_id
FROM call_contact
WHERE info_id = 1
AND status = 'CALLED'
UNION
SELECT DISTINCT customer_id
FROM mail_contact
WHERE info_id = 1
AND status = 'MAILED') t1

Answer 3

Is this what you ant?
SELECT COUNT(*)
FROM ((SELECT customer_id
FROM email_contact
WHERE info_id = 1 AND status = 'SENT'
) UNION -- on purpose to remove duplicates
(SELECT customer_id
FROM call_contact
WHERE info_id = 1 AND status = 'CALLED'
) UNION
(SELECT customer_id
FROM mail_contact
WHERE info_id = 1 AND status = 'MAILED'
)
) c;
Note that all your DISTINCTs are unnecessary because UNION removes duplicates.