CASE condition and how to code it? - sql

Question

I am creating a column in a new table( Table B) called Number of different locations. This column is derived from two columns in a Table A - Customer and Location.
Sample Data from Table A .
Customer Location
Mr James Smith Los Angeles
Mr David Jones London
Mr James Smith Paris
So the pseudo code ?
[Number of Different Locations] =
CASE
When Customer has more than one location ( count greater than 1 of for distinct customer)
Then populate those entries as 'Y'
Else 'N'
Now I have tried a few ways to code the 1st condition but it does not work .
CASE
When EXISTS ( select distinct customer, count ( Location ) from Table B
group by customer)
then 'Y'
Else 'N'
What am I doing wrong ? All the values are coming out in resultant table as 'Y'

Answer 1

SELECT Customer, Location, [Number of different locations] =
CASE
When EXISTS ( select distinct customer, count ( Location ) from Table B
group by customer
having count(location)>1)
then 'Y'
Else 'N'
END
FROM [Table]
you didn't specify "Having > 1"

Answer 2

You don't really need a separate subselect for that. This can be handled entirely within a GROUP BY clause and using the count of locations to determine if there are multiple locations.
SELECT Customer
, MultipleLocations =
CASE WHEN COUNT(Location) > 1
THEN 'Y'
ELSE 'N'
END
FROM YourTable
GROUP BY
Customer
Should your table contain multiple records for a customer with the same location, you can add a DISTINCT clause to accomodate for this.
SELECT Customer
, MultipleLocations =
CASE WHEN COUNT(DISTINCT Location) > 1
THEN 'Y'
ELSE 'N'
END
FROM YourTable
GROUP BY
Customer

Answer 3

Maybe this will help did it in another way:
DECLARE #tbl TABLE
(
Customer VARCHAR(100),
Location VARCHAR(100)
)
INSERT INTO #tbl
SELECT 'Mr James Smith','Los Angeles'
UNION ALL
SELECT 'Mr David Jones','London'
UNION ALL
SELECT 'Mr James Smith','Paris'
;WITH CTE AS
(
SELECT
COUNT(*) OVER(PARTITION BY tbl.Customer) AS NbrOf,
tbl.Customer,
tbl.Location
FROM
#tbl AS tbl
)
SELECT
CTE.Customer,
CTE.Location,
(
CASE
WHEN CTE.NbrOf>1
THEN 'Y'
ELSE 'N'
END
) AS newColumn
FROM
CTE

Answer 4

WITH TableA
AS
(
SELECT *
FROM (
VALUES ('Mr James Smith', 'Los Angeles'),
('Mr David Jones', 'London'),
('Mr James Smith', 'Paris')
) AS T (Customer, Location)
),
TableACustomerTallies
AS
(
SELECT Customer, COUNT(DISTINCT Location) AS Tally
FROM TableA
GROUP
BY Customer
)
SELECT Customer,
'Y' AS HasMultipleLocations
FROM TableACustomerTallies
WHERE Tally > 1
UNION
SELECT Customer,
'N' AS HasMultipleLocations
FROM TableACustomerTallies
WHERE Tally <= 1;