SQL, conditional column values? - sql

Question

One of the columns in my table is "DataSource". Datasource can have two values, lets say "A" or "B". I would like to always take the row when Datasource = "A", however, if there isn't an entry for A I would like to take "B".
How does one do this in SQL Server?
EDIT:
So for a partucular product (Product ID) one Product ID may have two rows, each containing a different DataSource, wheres another ProductID may only have one DataSource:
{ProductID DataSource}
{1 A},
{1 B},
{2 B}
Here I would wish to select the top and bottom row

Answer 1

A few options following your edit...
SELECT
*
FROM
table
WHERE
DataSource = 'A'
OR DataSource = 'B' AND NOT EXISTS (SELECT * FROM table AS lookup WHERE ProductID = table.ProductID AND DataSource = 'A')
SELECT
*
FROM
table
INNER JOIN
(SELECT ProductID, MAX(DataSource) AS DataSource FROM table) AS lookup
ON lookup.ProductID = table.ProductID
AND lookup.DataSource = table.DataSource
WITH
sequenced AS
(
SELECT
*,
ROW_NUMBER() OVER (PARTITION BY ProductID ORDER BY DataSource) AS sequence_id
FROM
table
)
SELECT
*
FROM
sequenced
WHERE
sequence_id = 1

Answer 2

select ProductID from product where DataSource= 'A'
UNION
select id from DataSource where product not in (select ProductID from product where DataSource = 'A')

Answer 3

SELECT top 1 *
from YourTable
ORDER BY DataSource ASC
Else
if the real values are NOT 'A' or 'B', which are just placeholders of #A and #B variables
SELECT top 1 *
from YourTable
ORDER BY CASE DataSource WHEN #A THEN 0 ELSE 1 END ASC
or
if there more variances than AB
SELECT top 1 *
from YourTable
ORDER BY CASE DataSource WHEN #A THEN 0 WHEN #B THEN 1 ELSE 2 END ASC
OR
If you need to group by ProductId
select * from (
SELECT *,
ROW_NUMBER OVER(PARTITION BY ProductId ORDER BY CASE DataSource WHEN #A THEN 0 WHEN #B THEN 1 ELSE 2 END ASC) ordinal
from YourTable
) t
WHERE t.Ordinal = 1

Answer 4

Based on your sample data, this works:
SELECT ProductID, MIN(DataSource)
FROM #tab
GROUP BY ProductID;
And here is my test data:
declare #tab table (ProductID int, DataSource char(1))
insert into #tab values (1, 'A');
insert into #tab values (1, 'B');
insert into #tab values (2, 'B');
If there are more columns in the table than the two you show then:
SELECT T1.*
FROM #tab T1
JOIN
(
SELECT ProductID, MIN(DataSource) AS DataSource
FROM #tab
GROUP BY ProductID
) T2 ON T1.ProductID = T2.ProductID AND T1.DataSource = T2.DataSource

Answer 5

ok, if I understood you requirement correctly then this would work
select TOP 1 * from yourtable where DATASOURCE = ISNULL(SELECT MAX(DATASOURCE) WHERE DATASOURCE="A","B")
The idea here is simple, you want to change DATASOURCE condition based on whether you have any rows of your first preference ("A") are available or not, if yes, then apply "A" condition or apply "B" condition