Get the rows with first phone number - sql

Question

I need to display 100 members per page. Because of multiple phone numbers of a member, I have to pick the first phone number for each member.
Here is the query which one gets every phone numbers of a member:
SELECT * FROM
(
SELECT
row_number() over(order by(1)) rn,
NAME, PHONE
FROM MEMBERS t0
LEFT OUTER JOIN MEMBER_IDENTITY ON MEMBER_IDENTITY.ID=t0.ID
LEFT JOIN MEMBER_PHONE ON MEMBER_PHONE.MEMBER_ID=t0.ID
WHERE
NAME LIKE 'U%'
ORDER BY NAME ASC
)
WHERE rn >= 0
AND rn <= 100
How can I pick first -or MAX, etc- phone number?

Answer 1

You could make a sub query for retrieving the phone numbers together with their row number per member, and then filter out the first of them:
SELECT *
FROM (
SELECT row_number() OVER (ORDER BY name ASC) rn,
name,
phone
FROM members t0
LEFT JOIN member_identity
ON member_identity.id = t0.id
LEFT JOIN (
SELECT member_id,
phone
row_number() OVER (PARTITION BY member_id ORDER BY (1)) ph_rn
FROM member_phone
) member_phone
ON member_phone.member_id = t0.id
AND ph_rn = 1
WHERE name LIKE 'U%'
ORDER BY name ASC
)
WHERE rn BETWEEN 0 AND 100
I would:
use the same order for determining the rn value as the result set (ORDER BY name ASC), otherwise the order will not be consistent across pages;
use BETWEEN in the outer WHERE condition, although the lower bound condition (0) is not necessary for the first page.

Answer 2

When you switch to MAX you can apply the ROW_NUMBER directly on the name (Windowed Aggregate Functions are calulated after GROUP BY/HAVING):
SELECT * FROM
(
SELECT
row_number() over (ORDER BY NAME ASC) rn,
NAME, MAX(PHONE)
FROM MEMBERS t0
LEFT OUTER JOIN MEMBER_IDENTITY ON MEMBER_IDENTITY.ID=t0.ID
LEFT JOIN MEMBER_PHONE ON MEMBER_PHONE.MEMBER_ID=t0.ID
WHERE NAME LIKE 'U%'
GROUP BY NAME
)
WHERE rn >= 0
AND rn <= 100
or move the aggregation into a Derived Table:
SELECT * FROM
(
SELECT
row_number() over (ORDER BY NAME ASC) rn,
NAME, PHONE
FROM MEMBERS t0
LEFT OUTER JOIN MEMBER_IDENTITY ON MEMBER_IDENTITY.ID=t0.ID
LEFT JOIN
( SELECT MEMBER_ID, MAX(PHONE) AS PHONE
FROM MEMBER_PHONE
GROUP BY NAME
) MEMBER_PHONE
ON MEMBER_PHONE.MEMBER_ID=t0.ID
WHERE NAME LIKE 'U%'
)
WHERE rn >= 0
AND rn <= 100