How do I have SQL JOIN done depending on some condition? - sql

Question

I have two tables:
CREATE TABLE Users(
UserId UNIQUEIDENTIFIER NOT NULL PRIMARY KEY,
UserName NVARCHAR(200) NOT NULL,
-- some other columns
CurrentSubscriptionId UNIQUEIDENTIFIER NULL,
)
CREATE TABLE Subscriptions(
SubscriptionId UNIQUEIDENTIFIER NOT NULL PRIMARY KEY,
UserId UNIQUEIDENTIFIER NOT NULL,
-- some other columns
)
and I have a stored procedure that retrieves a list of "users" with their "current subscriptions":
SELECT List-Of-Columns FROM Users LEFT OUTER JOIN Subscriptions ON
Users.CurrentSubscriptionId=Subscriptions.SubscriptionId
WHERE SomeConditionOnUsers
The point is I only need data from "subscriptions" for some calls of the stored procedure, not the others. So I'd add a #includeSubscriptionData bit parameter to the stored procedure (this is pseudocode, I'd have to list each conditionally included column separately):
SELECT List-Of-Columns-FromUsers,
CASE WHEN #includeSubscriptionData THEN Columns-From-Subscriptions ELSE NULL END
FROM Users LEFT OUTER JOIN Subscriptions ON
Users.CurrentSubscriptionId=Subscriptions.SubscriptionId
WHERE SomeConditionOnUsers
to hopefully avoid the JOIN. So I tried this first:
DECLARE #includeSubscriptionData BIT;
SET #includeSubscriptionData=0;
SELECT List-Of-Columns-FromUsers,
CASE WHEN #includeSubscriptionData THEN SubscriptionId ELSE NULL END
FROM Users LEFT OUTER JOIN Subscriptions ON
Users.CurrentSubscriptionId=Subscriptions.SubscriptionId
WHERE SomeConditionOnUsers
and the plan shows that there're still accesses to "subscriptions".
How do I get rid of accesses to the second table when the bit is set to zero?

Answer 1

Have you tried putting it in the join criteria?
DECLARE #includeSubscriptionData BIT;
SET #includeSubscriptionData=0;
SELECT List-Of-Columns-FromUsers,
SubscriptionId
FROM Users LEFT OUTER JOIN Subscriptions ON
Users.CurrentSubscriptionId=Subscriptions.SubscriptionId and #includeSubscriptionData = 1
WHERE SomeConditionOnUsers
OPTION(RECOMPILE)

Answer 2

Method 1 - Control Flow
IF #includeSubscriptionData = 1
BEGIN
SELECT list
, of
, columns
FROM Users
INNER
JOIN Subscriptions
ON Users.CurrentSubscriptionId = Subscriptions.SubscriptionId
WHERE SomeConditionOnUsers
;
END
ELSE
BEGIN
SELECT list
, of
, columns
FROM Users
;
END
;
Method 2 - Modified Join
SELECT list
, of
, columns
FROM Users
LEFT
JOIN Subscriptions
ON Users.CurrentSubscriptionId = Subscriptions.SubscriptionId
AND #includeSubscriptionData = 1
WHERE SomeConditionOnUsers

Answer 3

You can use IF/ELSE for this.
DECLARE #includeSubscriptionData BIT;
SET #includeSubscriptionData=0;
IF #includeSubscriptionData == 0
BEGIN
SELECT List-Of-Columns-FromUsers
FROM Users WHERE SomeConditionOnUsers
END
ELSE
BEGIN
SELECT List-Of-Columns-FromUsers
FROM Users LEFT OUTER JOIN Subscriptions ON Users.CurrentSubscriptionId=Subscriptions.SubscriptionId
WHERE SomeConditionOnUsers
END