How to use exists for a complete list of a product? - sql

Question

This is the query to find out if a particular car has W1, W2, WA, WH conditions. How can I modify my query so that I can get a list of all the cars as yes or no for these conditions?
NOTE: Here, I have put v.[carnumber] = 't8302' but I need a complete list.
SELECT
CASE
WHEN EXISTS (
SELECT co.[alias]
FROM [MTI_TAXI].[vehicle] v
LEFT JOIN [MTI_SYSTEM].[Conditions] co with (nolock) on v.DispatchSystemID = co.DispatchSystemID and (v.Conditions & co.conditionvalue > 0)
WHERE co.[alias] in ('W1', 'W2', 'WA', 'WH') and v.[DispatchSystemID] = 6 and v.[CarNumber] = 't8302')
THEN cast ('Yes' as varchar)
ELSE cast ('No' as varchar)
END AS [WATS]
OUTPUT - ( WATS - No )
But, here are all the cars but I am getting yes to WATS condition which is incorrect
enter image description here

Answer 1

Simply utilizing your provided filters and moving the EXISTS to be used in an OUTER APPLY statement:
SELECT
CASE
WHEN [find_wats].[Found] = 1
THEN 'Yes'
ELSE 'No'
END AS [WATS]
FROM
[MTI_TAXI].[vehicle] AS v
OUTER APPLY (SELECT TOP (1)
1 AS [Found]
FROM
[MTI_SYSTEM].[Conditions] AS co
WHERE
v.DispatchSystemID = co.DispatchSystemID
AND
(v.Conditions & co.conditionvalue > 0)
AND
co.[alias] IN ('W1', 'W2', 'WA', 'WH')
AND
v.[DispatchSystemID] = 6) AS [find_wats];
Using this set up, you can then use [find_wats].[Found] = 1 to determine that your record within the table [MTI_TAXI].[vehicle] found a match in [MTI_TAXI].[Conditions] (using your provided criteria) while still maintaining a single record in your final result set for each record originally in the table [MTI_TAXI].[vehicle].

Answer 2

Use count(*) to assert that there was exactly 1 row found by adding:
group by co.[alias]
having count(*) = 1
So the whole query becomes:
SELECT
CASE
WHEN EXISTS (
SELECT co.[alias]
FROM [MTI_TAXI].[vehicle] v
LEFT JOIN [MTI_SYSTEM].[Conditions] co with (nolock)
on v.DispatchSystemID = co.DispatchSystemID
and (v.Conditions & co.conditionvalue > 0)
WHERE co.[alias] in ('W1', 'W2', 'WA', 'WH')
and v.[DispatchSystemID] = 6
and v.[CarNumber] = 't8302'
group by co.[alias]
having count(*) = 1
) THEN cast ('Yes' as varchar)
ELSE cast ('No' as varchar)
end AS [WATS]