Assign session number to a series of transactions - sql

Question

CREATE TABLE [Transaction](
[TransactionID] [bigint] IDENTITY(1,1) NOT NULL,
[LocationID] [int] NOT NULL,
[KioskID] [int] NOT NULL,
[TransactionDate] [datetime] NOT NULL,
[TransactionType] [varchar](7) NOT NULL,
[Credits] [int] NOT NULL,
[StartingBalance] [int] NULL,
[EndingBalance] [int] NULL,
[SessionID] [int] NULL
);
Please refer to this fiddle for the sample data:
Link to SQL Fiddle
I'm trying to figure out if there is a way to assign a session number to a sequence of transactions in a single update.
A "Session" is defined as a number of deposits and purchases ending with a withdrawal. A Session has sequential transactions consisting of:
1 to n deposits (TransactionType = 'D'),
0 to n purchases (TransactionType = 'P') and
0 or 1 withdrawals (TransactionType = 'W')
With the same LocationID and KioskID. A session can end with a 0 balance or a withdrawal. First deposit with no session starts one. Only P transactions have balances. For D and W they are NULL.
LocationID, KioskID, SessionID must be unique.
I'm really hoping that there is a SQL way of doing this. I'd hate to have to loop through hundreds of millions of transactions to set sessions procedurally.

Answer 1

This should do it:
;WITH markSessions as
(
SELECT *,
CASE
WHEN TransactionType='W' THEN 1
WHEN TransactionType='P' And EndingBalance=0 THEN 1
ELSE 0 END As SessionEnd
FROM Transactions
)
SELECT *,
SUM(SessionEnd) OVER(PARTITION BY LocationID, KioskID ORDER BY TransactionID)
+ 1 - SessionEnd As SessionID
FROM markSessions
No triggers, cursors or client code needed.
If you actually want to set the SessionID in the table, then you'd use an UPDATE statement like this:
;WITH markSessions as
(
SELECT *,
CASE
WHEN TransactionType='W' THEN 1
WHEN TransactionType='P' And EndingBalance=0 THEN 1
ELSE 0 END As SessionEnd
FROM Transactions
)
UPDATE markSessions
SET SessionID = SUM(SessionEnd) OVER(PARTITION BY LocationID, KioskID ORDER BY TransactionID)
+ 1 - SessionEnd
I am unable to test it, but the following should take into account pre-existing SessionIDs
;WITH markSessions as
(
SELECT *,
CASE
WHEN TransactionType='W' THEN 1
WHEN TransactionType='P' And EndingBalance=0 THEN 1
ELSE 0 END As SessionEnd
FROM Transactions
)
UPDATE markSessions
SET SessionID = SUM(SessionEnd) OVER(PARTITION BY LocationID, KioskID ORDER BY TransactionID)
+ 1 - SessionEnd
+ COALESCE(MAX(SessionID) OVER (PARTITION BY LocationID, KioskID), 0)
WHERE SessionID Is NULL
Note that this will only work if all new rows (those without SessionIDs) have higher transaction IDs than the Pre-existing rows (those that already have SessionIDs). It definitely NOT work if new rows were added with TransactionIDs, lower than the highest TransactionID already assigned a SessionID.
If you may have that situation, then you likely will have to reassign the old TransactionIDs.