How to use case after where clause in sql server - sql

Question

category Item Price
A Pen NULL
B Pen 10
A Pencil 10
B Pencil 8
C Pencil 7
A Note Book 40
B Note Book 30
C Note Book 20
A Bottle NULL
B Bottle 80
A Ball 50
B Ball 40
A Bag 1000
B Bag 800
This is My data i want to Show only category A data if A price is null
then show category B price. I was tried but don't know how i show the
Data
select * from tbl1
where category = case when price is null then 'B' else 'A' end
When Run this query it's show only Category A data
category Item Price
A Pencil 10
A Note Book 40
A Ball 50
A Bag 1000

Answer 1

Something like this
SELECT price
FROM table
WHERE
category = CASE
WHEN price is not null THEN 'A'
ELSE 'B'
END
`

Answer 2

Using a left join and coalesce() (you could also use isnull()):
select a.Category, a.Item, coalesce(a.Price,b.Price) as Price
from yourtable a
left join yourtable b
on a.Item = b.Item
and b.category = 'B'
where a.category = 'A'
coalesce() will return the first non null value from the parameters in order from left to right.
rextester demo: http://rextester.com/FZO89906
returns:
+----------+-----------+-------+
| Category | Item | Price |
+----------+-----------+-------+
| A | Pen | 10 |
| A | Pencil | 10 |
| A | Note Book | 40 |
| A | Bottle | 80 |
| A | Ball | 50 |
| A | Bag | 1000 |
+----------+-----------+-------+
Reference:
coalesce()
isnull()

Answer 3

;WITH cte(category,Item,Price)
AS
(
SELECT 'A','Pen' ,NULL UNION ALL
SELECT 'B','Pen' ,10 UNION ALL
SELECT 'A','Pencil' ,10 UNION ALL
SELECT 'B','Pencil' ,8 UNION ALL
SELECT 'C','Pencil' ,7 UNION ALL
SELECT 'A','Note Book' ,40 UNION ALL
SELECT 'B','Note Book' ,30 UNION ALL
SELECT 'C','Note Book' ,20 UNION ALL
SELECT 'A','Bottle' ,NULL UNION ALL
SELECT 'B','Bottle' ,80 UNION ALL
SELECT 'A','Ball' ,50 UNION ALL
SELECT 'B','Ball' ,40 UNION ALL
SELECT 'A','Bag' ,1000 UNION ALL
SELECT 'B','Bag' ,800
)
SELECT category,Item,Price From
(
SELECT *,ROW_NUMBER()Over(Partition by Price order by Price)seq From
(
SELECT o.* FROM cte i
INNER JOIN cte o
ON o.category=i.category
WHERE o.category='A' AND o.Price!=i.Price
)dt
) Final
where Final.seq=1
OutPut
category Item Price
A Pencil 10
A Note Book 40
A Ball 50
A Bag 1000
Output For value'B'
category Item Price
--------------------------------
B Pencil 8
B Pen 10
B Note Book 30
B Ball 40
B Bottle 80
B Bag 800

Answer 4

I understand it as If an item in a category has a null price it should pick up that specific item from another category having a price
--Filter records having prices
with CTETable as
(
Select distinct category, item, price
from tbl1
where price is not null
)
--distinct items
select * from
(select category, item, price, ROW_NUMBER() over (Partition by Item order by
Category) as RowNo from CTETable)
as c
where c.RowNo=1