SQL Server: Categorize data within table - sql

Question

I am trying to compare a table to itself in SQL Server 2012. The table consists of among other columns Client Name, Client Number and Fiscal Year. Within the table there is data for FY15 and FY16 and clients can repeat multiple times.
I am trying to add a column to the table that will stipulate whether the client in each row is either "New" (exists in FY16 but not FY15), "Existing" (exists in FY16 and FY15), or "Lost" (exists in FY15 but not FY16).
I've tried joining the table to itself and using case statements but no luck.

Answer 1

You can do it using a LEFT JOIN:
SELECT c1.Name, c1.Number, c1.FiscalYear,
CASE
WHEN c1.FiscalYear = 2015 AND c2.FiscalYear = 2016 THEN 'Existing'
WHEN c1.FiscalYear = 2015 AND c2.FiscalYear IS NULL THEN 'Lost'
WHEN c1.FiscalYear = 2016 AND c2.FiscalYear = 2015 THEN 'Existing'
WHEN c1.FiscalYear = 2016 AND c2.FiscalYear IS NULL THEN 'New'
END AS category
FROM clients AS c1
LEFT JOIN clients AS c2
ON c1.Name = c2.Name AND
((c1.FiscalYear = 2015 AND c2.FiscalYear = 2016) OR
(c1.FiscalYear = 2016 AND c2.FiscalYear = 2015))
Demo here
If you just want to get category per client name, then you can use a GROUP BY:
SELECT Name,
CASE
WHEN COUNT(DISTINCT FiscalYear) = 2 THEN 'Existing'
WHEN SUM(FiscalYear = 2015) > 0 AND SUM(FiscalYear = 2016) = 0 THEN 'Lost'
WHEN SUM(FiscalYear = 2016) > 0 AND SUM(FiscalYear = 2015) = 0 THEN 'New'
END AS category
FROM clients
WHERE FiscalYear IN (2015, 2016)
GROUP BY Name
Demo here
You also can use the above query to perform an UPDATE:
UPDATE clients
INNER JOIN (
SELECT Name,
CASE
WHEN COUNT(DISTINCT FiscalYear) = 2 THEN 'Existing'
WHEN SUM(FiscalYear = 2015) > 0 AND SUM(FiscalYear = 2016) = 0 THEN 'Lost'
WHEN SUM(FiscalYear = 2016) > 0 AND SUM(FiscalYear = 2015) = 0 THEN 'New'
END AS category
FROM clients
WHERE FiscalYear IN (2015, 2016)
GROUP BY Name
) AS t ON clients.name = t.name
SET clients.category = t.category
Demo here

Answer 2

Version with update:
update customer
set status = case when
(select count(*) from customer C2 where C2. ClientName = customer.ClientName and C2.Year in (2015,2016) ) = 2 then 'Existing'
when
(select count(*) from customer C2 where C2.clientName = customer.ClientName and C2.Year = 2015 ) = 1 then 'Lost'
when
(select count(*) from customer C2 where C2.clientName = customer.ClientName and C2.Year in (2016) ) = 1 then 'New'
end;
Sample in SQL Fiddle here