How do I use a CASE statement in a JOIN in SQL Server? - sql

Question

I have a stored procedure that I want to select a column if the applicationTypeId is in a list of IDs. I am trying to use the CASE statement but I don't know what I am doing wrong. I created a table and added the ApplicationTypeIds to it. The table name is #RelationshipsTypeApplicationId
In other words, what I am trying to do is the following:
If the applicationTypeId is in the #RelationshipsTypeApplicationId table
Join and select the Name column from the IGCRelationshipTypes table
Here is my original stored procedure
WITH AllCustomers AS
(
SELECT
app.ApplicationId,
cust.Name,
ApplicationTypes.TypeName AS ApplicationType,
ApplicationSources.ApplicationSourceName AS ApplicationSource,
CreatedDate = app.CreatedDate,
LastUpdateDate = app.StatusChangeDate,
app.StatusId,
InProgress = STUFF((SELECT ';'+#aw.ApplicationWorkflowName
FROM #aw
WHERE #aw.ApplicationId = app.ApplicationId
ORDER BY StepNumber
FOR XML PATH('')), 1, 1, ''),
ActionRequired = (SELECT top 1 1 FROM #aw
WHERE #aw.ApplicationId = app.ApplicationId
AND #aw.WorkflowStatusId = 6),
AccountId = STUFF((SELECT ';' + acct.AccountId
FROM #accounts AS acct
WHERE acct.ApplicationId = app.ApplicationId
ORDER BY acct.ParentAccountId
FOR XML PATH('')), 1, 1, '')
FROM
[dbo].[Applications] app
INNER JOIN
[dbo].[Customers] AS cust ON cust.CustomerId = app.CustomerId
INNER JOIN
[dbo].[ApplicationTypes] ON ApplicationTypes.ApplicationTypeId = app.ApplicationTypeId
INNER JOIN
[dbo].[ApplicationSources] ON ApplicationSources.ApplicationSourceId = app.ApplicationSourceId
WHERE
app.StatusId <> '4020-8901-64FFE33F06B1'
AND (#applicationTypeId IS NULL OR app.ApplicationTypeId = #applicationTypeId)
And here is what I updated:
WITH AllCustomers AS
(
SELECT
app.ApplicationId,
cust.Name,
ApplicationTypes.TypeName AS ApplicationType,
ApplicationSources.ApplicationSourceName AS ApplicationSource,
CreatedDate = app.CreatedDate,
LastUpdateDate = app.StatusChangeDate,
app.StatusId,
InProgress = STUFF((SELECT ';' + #aw.ApplicationWorkflowName
FROM #aw
WHERE #aw.ApplicationId = app.ApplicationId
ORDER BY StepNumber
FOR XML PATH('')), 1, 1, ''),
ActionRequired = (SELECT top 1 1 FROM #aw
WHERE #aw.ApplicationId = app.ApplicationId
AND #aw.WorkflowStatusId = 6),
AccountId = STUFF((SELECT ';' + acct.AccountId
FROM #accounts as acct
WHERE acct.ApplicationId = app.ApplicationId
ORDER BY acct.ParentAccountId
FOR XML PATH('')), 1, 1, ''),
CASE
WHEN app.ApplicationTypeId IN (SELECT * FROM #RelationshipsTypeApplicationId)
THEN relationshipType.Name
END
FROM
[dbo].[Applications] app
INNER JOIN
[dbo].[Customers] AS cust ON cust.CustomerId = app.CustomerId
INNER JOIN
[dbo].[ApplicationTypes] ON ApplicationTypes.ApplicationTypeId = app.ApplicationTypeId
INNER JOIN
[dbo].[ApplicationSources] ON ApplicationSources.ApplicationSourceId = app.ApplicationSourceId
INNER JOIN
[dbo].[IGCRelationshipTypes] AS relationshipType
ON CASE
WHEN app.ApplicationTypeId IN (SELECT * FROM #RelationshipsTypeApplicationId)
THEN (relationshipType.Name = #relationshipType)
END
WHERE
app.StatusId <> '4020-8901-64FFE33F06B1'
AND (#applicationTypeId IS NULL OR app.ApplicationTypeId = #applicationTypeId)
Here is a screenshot of the syntax error:
Can someone shed some light on what I am missing here?

Answer 1

CASE in T-SQL is an expression that returns a single scalar value, it is not for control of flow. See Dirty secrets of the CASE expression. You'll need something like this, but I don't have the energy to try to determine why you have a table named #RelationshipsTypeApplicationId with a single column (because you really shouldn't say IN (SELECT *)) or if you want to do something different when the match does not exist.
...
ON relationshipType.Name = CASE
WHEN app.ApplicationTypeId IN
(SELECT * FROM #RelationshipsTypeApplicationId)
THEN #relationshipType END
...