Simply SQL query - sql

Question

I have a SQL script and would like to have multiple counts depending on different where clauses, instead of repeating such as this is there a way I can simplify this?
select
UnverifiedEmails =
(
select count(distinct c.ContactRef)
from ContactEmailAddressVerification c
LEFT JOIN EmailAddressVerification e
ON e.EmailAddressVerificationID = c.EmailAddressVerificationID
WHERE DateVerified IS NULL
),
VerifiedEmails =
(
select count(distinct c.ContactRef)
from ContactEmailAddressVerification c
LEFT JOIN EmailAddressVerification e
ON e.EmailAddressVerificationID = c.EmailAddressVerificationID
WHERE DateVerified IS NOT NULL
),
LastMonthVerified =
(
select count(distinct c.ContactRef)
from GDPR_ContactEmailAddressVerification c
LEFT JOIN EmailAddressVerification e
ON e.EmailAddressVerificationID = c.EmailAddressVerificationID
WHERE DateVerified IS NOT NULL
AND DATEPART(m, e.DateAdded) = DATEPART(m, DATEADD(m, -1, getdate()))
),
LastMonthUnverified =
(
select count(distinct c.ContactRef)
from ContactEmailAddressVerification c
LEFT JOIN EmailAddressVerification e
ON e.EmailAddressVerificationID = c.EmailAddressVerificationID
WHERE DateVerified IS NULL
AND DATEPART(m, e.DateAdded) = DATEPART(m, DATEADD(m, -1, getdate()))
)

Answer 1

You can do aggregation with single SELECT statement :
SELECT COUNT(DISTINCT CASE WHEN DateVerified IS NULL THEN c.ContactRef END) UnverifiedEmails,
COUNT(DISTINCT CASE WHEN DateVerified IS NOT NULL THEN c.ContactRef END) VerifiedEmails,
COUNT(DISTINCT CASE WHEN (DateVerified IS NOT NULL AND
DATEPART(m, e.DateAdded) = DATEPART(m, DATEADD(m, -1, getdate()))
)
THEN c.ContactRef
END) LastMonthVerified,
COUNT(DISTINCT CASE WHEN (DateVerified IS NULL AND
DATEPART(m, e.DateAdded) = DATEPART(m, DATEADD(m, -1, getdate()))
)
THEN c.ContactRef
END) LastMonthUnverified
FROM ContactEmailAddressVerification c LEFT JOIN
EmailAddressVerification e
ON e.EmailAddressVerificationID = c.EmailAddressVerificationID;

Answer 2

You may use conditional aggregation:
SELECT
COUNT(DISTINCT CASE WHEN DateVerified IS NULL
THEN c.ContactRef END) AS UnverifiedEmails,
COUNT(DISTINCT CASE WHEN DateVerified IS NOT NULL
THEN c.ContactRef END) AS VerifiedEmails,
COUNT(DISTINCT CASE WHEN DateVerified IS NOT NULL AND
DATEPART(m, e.DateAdded) = DATEPART(m, DATEADD(m, -1, GETDATE()))
THEN c.ContactRef END) AS LastMonthVerified,
COUNT(DISTINCT CASE WHEN DateVerified IS NULL AND
DATEPART(m, e.DateAdded) = DATEPART(m, DATEADD(m, -1, GETDATE()))
THEN c.ContactRef END) AS LastMonthUnverified
FROM ContactEmailAddressVerification c
LEFT JOIN EmailAddressVerification e
ON e.EmailAddressVerificationID = c.EmailAddressVerificationID;
The idea here is to make a single pass over the joined tables, and then take counts/sums conditionally, depending on the logic in each of the WHERE clauses of your original query.

Answer 3

use case when don't need multiple sub-query
select count(distinct case when DateVerified IS NULL then c.ContactRef end) UnverifiedEmails ,
count(distinct case when DateVerified IS not NULL then c.ContactRef end) VerifiedEmails,
count(distinct case when DateVerified IS NOT NULL
AND DATEPART(m, e.DateAdded) = DATEPART(m, DATEADD(m, -1, getdate()))
then c.ContactRef end ) LastMonthVerified,
count(distinct case when DateVerified IS NULL
AND DATEPART(m, e.DateAdded) = DATEPART(m, DATEADD(m, -1, getdate()))
then c.ContactRef end) LastMonthUnverified
from ContactEmailAddressVerification c
LEFT JOIN EmailAddressVerification e
ON e.EmailAddressVerificationID = c.EmailAddressVerificationID