IF EXISTS block - sql

Question

I have 2 tables UserSession and Sale.
Table User has 4 columns, namely UserID, UserSessionID, SessionOpenDate and SessionCloseDate.
Table Sale has 2 columns, which are price, cost, userID and CompletedDate.
When a user logs in, a new row is created in the User table, where the user's login timestamp will be saved in the SessionOpenDate and a new UserSessionID will be assigned to the session. When the user logs off, the log off timestamp will be be saved in SessionCloseDate.
When the user is still logged in, the user can make some sale and the sale information is saved in the Sale table. The timestamp when the sale is finalized in saved in CompletedDate column.
For some reason, I need to get the all sales done in a certain UserSessionID where the CompletedDate must be within the SessionOpenDate and SessionCloseDate. However, if the user has not logged off yet, which means that the value in SessionCloseDate is null, the CompletedDate should be between SessionOpenDate and now.
Here's my query:
SELECT SUM(s.cost) AS Cost, SUM(s.price) AS Price
FROM Sale AS s
INNER JOIN UserSession AS u
ON s.userID = u.userID
WHERE
(s.CompletedDate >=
( SELECT SessionOpenDate
FROM UserSession
WHERE (UserSessionID = u.UserSessionID)
)
)
AND
(s.CompletedDate <
(
IF EXISTS
(
SELECT SessionCloseDate AS closeTime
FROM UserSession AS UserSessionTemp
WHERE (UserSessionID = u.UserSessionID)
)
BEGIN
SET closeTime = SELECT CURRENT_TIMESTAMP
END
)
)
AND u.UserSessionID IN (1)
However, Sql Server says Incorrect syntax near the keyword 'IF'. and Incorrect syntax near ')'.
Can anyone tell me what went wrong with my IF block?

Answer 1

You can't use an IF block inside a SELECT statement. Also, I don't know what you're really trying to accomplish with SET, since closeTime is not a variable/parameter.
You can use IIF in SQL Server 2012 (syntactical sugar for CASE WHEN <condition> THEN <true_value> ELSE <false_value> END - use this syntax for earlier versions):
IIF(EXISTS
(
SELECT SessionCloseDate AS closeTime
FROM UserSession AS UserSessionTemp
WHERE (UserSessionID = u.UserSessionID)
), (
SELECT SessionCloseDate AS closeTime
FROM UserSession AS UserSessionTemp
WHERE (UserSessionID = u.UserSessionID)
),
CURRENT_TIMESTAMP
)
Honestly, without getting too complicated, here's what I would do instead:
SELECT SUM(s.cost) AS Cost, SUM(s.price) AS Price
FROM userSession AS u
INNER JOIN Sale AS s ON u.userID = s.userID
WHERE u.UserSessionID = #UserSessionId
AND s.CompletedDate >= u.SessionOpenDate
AND (u.SessionCloseDate IS NULL OR s.CompletedDate < u.SessionCloseDate)
Or,
SELECT SUM(s.cost) AS Cost, SUM(s.price) AS Price
FROM userSession AS u
INNER JOIN Sale AS s ON u.userID = s.userID
WHERE u.UserSessionID = #UserSessionId
AND s.CompletedDate BETWEEN u.SessionOpenDate
AND COALESCE(u.SessionCloseDate, '12/31/9999 23:59:59.9999')

Answer 2

I'd go with something like this, depending on what you're looking for. I couldn't tell if you wanted the info for a particular user or session, but that's simple enough to add.
SELECT UserSessionID, SUM(cost) AS TotalCost, SUM(price) AS TotalPrice
FROM UserSession LEFT OUTER JOIN sale
ON UserSession.userid = sale.userid AND
((UserSession.SessionCloseDate IS NULL AND sale.CompletedDate BETWEEN UserSession.SessionOpenDate AND GetDate())
OR (sale.SessionCloseDate IS NOT NULL AND sale.CompletedDate BETWEEN UserSession.SessionOpenDate AND UserSession.SessionCloseDate))
WHERE SUM(cost) > 0
GROUP BY UserSessionID
(you can ADD AND UserSessionID = 'mysessionid' or and UserID = 'myuserid' above the group by if you don't want the full list)

Answer 3

Others have explained the specific problem and "given you a fish". I would like to "teach you how to fish", though.
Please see mixed-up statement types for a full discussion (disclosure: my own blog post). Here are some snippets.
An expression consists of one or more literal values or functions tied together with operators, which when evaluated in the correct order result in a value or collection of values.
Snip...
Procedural statements are called that because there is some procedure that must be followed. It isn't a simple case of order-of-operations resulting in a single value. There is in fact no value expressed at all.
Snip...
Now that you know the three main kinds of statements (and I won't rule out the possibility of there being more or of there being subclassifications of these) the key concept you must know to get along well with SQL Server is that when a certain kind of statement is expected, you can't use a different one in its place.