I'm trying to understand how case when exists
expression works, there is two tables one is trOrderHeader
which stores main info about any order. Other is trOrderLine
which stores details about an order and there is a column IsClosed
which indicates Order is somehow closed (canceled or completed).
So my query is below which I am trying to get OrderLineID with IsClosed column (I can do it by joins it is easy but I am trying case when exists
expression) but all IsClosed column returns 1;
SELECT
OrderLineId,
IsClosed =
CASE
WHEN EXISTS (select * from trOrderHeader where IsClosed=1)
THEN 1
WHEN EXISTS (select * from trOrderHeader where IsClosed=0)
THEN 0
END
FROM
trOrderLine
GROUP BY OrderLineId
Isn't this supposed to give me if an Order is Closed write 1, if an Order is not Closed write 0?
Did I misunderstand case when exists
expression?
You would need to correlate the subqueries. Assuming that column OrderId
can be used to relate the tables, then:
SELECT
OrderLineId,
IsClosed =
CASE
WHEN EXISTS (select 1 from trOrderHeader h where h.IsClosed = 1 AND h.OrderId = l.OrderId)
THEN 1
WHEN EXISTS (select 1 from trOrderHeader h where IsClosed = 0 AND h.OrderId = l.OrderId)
THEN 0
END
FROM trOrderLine l
GROUP BY OrderLineId
However, the logic of the query could probably be simplified by using a JOIN
and an aggregate function to compute the status. Something like this could be what you need:
SELECT l.OrderLineId, MAX(h.isClosed) IsClosed
FROM trOrderLine l
INNER JOIN trOrderHeader h ON h.OrderId = l.OrderId
GROUP BY l.OrderLineId
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