SQL Server SELECT first occurrence OR if no occurrence SELECT other criteria - sql

Question

I am having an issue trying to form the proper SQL query for the job here. I have two tables, one is called CUSTOMER and the other is called CUSTOMER_CONTACT. To simplify this, I will only include the relevant column names.
CUSTOMER columns: ID, CUSTOMERNAME
CUSTOMER_CONTACT columns: ID, CUSTOMER_ID, CONTACT_VC, EMAIL
CUSTOMER_ID is the foreign key to link to the CUSTOMER table from CUSTOMER_CONTACT. CONTACT_VC is just the entry number for their contact information. There could be multiple CUSTOMER_CONTACT records for each customer, but they will have a unique CONTACT_VC.
EMAIL can be null/blank on some or all as well.
I need to select the first CUSTOMER_CONTACT entry where EMAIL is NOT NULL/blank but if none of the CUSTOMER_CONTACT entries have an email address, then select CUSTOMER_CONTACT WHERE CONTACT_VC = 1
Any suggestions on how to accomplish this?

Answer 1

The following approach uses ROW_NUMBER to retrieve a number based on your ordering logic within each CUSTOMER_ID group, then filters by the first record retrieved.
You may try the following:
SELECT
*
FROM (
SELECT
*,
ROW_NUMBER() OVER (
PARTITION BY CUSTOMER_ID
ORDER BY (CASE WHEN EMAIL IS NOT NULL THEN 0 ELSE 1 END),CONTACT_VC
) as rn
FROM
CUSTOMER_CONTACT
) t
WHERE rn=1
If you would like to join this to the customer table you may use the above query as a subquery eg
SELECT
c.*,
contact.*
FROM
CUSTOMER c
INNER JOIN (
SELECT
*,
ROW_NUMBER() OVER (
PARTITION BY CUSTOMER_ID
ORDER BY (CASE WHEN EMAIL IS NOT NULL THEN 0 ELSE 1 END),CONTACT_VC
) as rn
FROM
CUSTOMER_CONTACT
) contact ON c.ID = contact.CUSTOMER_ID and contact.rn=1

Answer 2

Here is almost the same answer as ggordon, but I used a common table expression and I think the ordering in the subquery portion should go by CONTACT_VS first then by non-NULL email addresses. I created some very simple test data to run this:
DECLARE #CUSTOMER AS TABLE
(
[ID] INT NOT NULL,
[CUSTOMERNAME] VARCHAR(10) NOT NULL
);
INSERT INTO #CUSTOMER
(
[ID],
[CUSTOMERNAME]
)
VALUES
(1, 'Alice'),
(2, 'Bob'),
(3, 'Cathy');
DECLARE #CUSTOMER_CONTACT AS TABLE
(
[ID] INT NOT NULL,
[CUSTOMER_ID] INT NOT NULL,
[CONTACT_VC] INT NOT NULL,
[EMAIL] VARCHAR(40) NULL
);
INSERT INTO #CUSTOMER_CONTACT
(
[ID],
[CUSTOMER_ID],
[CONTACT_VC],
[EMAIL]
)
VALUES
(1, 1, 1, 'alice#email.com'),
(2, 1, 2, 'alice#gmail.com'),
(3, 2, 1, NULL),
(4, 2, 2, 'bob#work.com'),
(5, 3, 1, NULL),
(6, 3, 2, NULL),
(7, 3, 3, NULL);
;WITH [cc]
AS (SELECT [ID],
[CUSTOMER_ID],
[CONTACT_VC],
[EMAIL],
ROW_NUMBER() OVER (PARTITION BY [CUSTOMER_ID]
ORDER BY [CONTACT_VC],
(CASE WHEN [EMAIL] IS NOT NULL THEN
0
ELSE
1
END
)
) AS [rn]
FROM #CUSTOMER_CONTACT)
SELECT [c].[ID], [c].[CUSTOMERNAME], [cc].[ID], [cc].[CUSTOMER_ID], [cc].[CONTACT_VC], [cc].[EMAIL]
FROM #CUSTOMER AS [c]
INNER JOIN [cc]
ON [c].[ID] = [cc].[CUSTOMER_ID]
AND [cc].[rn] = 1;

Answer 3

select * from CUSTOMER_CONTACT where EMAIL IS NOT NULL
union all
select * from CUSTOMER_CONTACT where
(CONTACT_VC=1 and NOT EXISTS (select 1 FROM CUSTOMER_CONTACT where EMAIL IS NOT NUL)
order by CONTACT_VC asc limit 1