Understanding the Problem and Query

The problem is to retrieve two transactions from the same customer smartcard within a limited time range (2 minutes) on Microsoft SQL Server. The query provided in the Stack Overflow post attempts to solve this problem but has issues with performance and logic.

Background Information

To understand the query, we need some background information about the tables involved:

• CashlessTransactions: This table stores cashless transactions, including transaction ID (IdCashlessTransaction), customer smartcard ID (IdCustomerSmartcard), POS device ID (IdPOSDevice), amount, and date.
• POSDevices, EventSessionSetups, and Events: These tables are joined with CashlessTransactions to filter transactions by POS device, event session setup, and event.

Query Analysis

The query has two main parts:

1. The outer query joins CashlessTransactions with POSDevices, EventSessionSetups, and Events to retrieve transaction information.
2. The inner query (a subquery) also joins these tables but filters transactions by a specific event ID (e.IdEvent = 2) and calculates the count of different transactions made with the same card within 2 minutes of the original transaction.

However, there are issues with the query:

• It only considers transactions that have another transaction within 2 minutes after it.
• It filters out transactions without a matching inner query result, which might not be the intended behavior.
• The performance is likely to be slow due to the multiple joins and subquery.

Solution

To solve this problem, we need to modify the query to correctly filter transactions that have another transaction within 2 minutes of the original transaction. Here’s an updated solution:

Updated Query

SELECT 
    c1.IdCustomerSmartcard AS UsersCardID,
    c1.IdCashlessTransaction AS OriginalTransaction,
    c1.Date AS OrigTranDate,
    c1.Amount AS OrigTranAmount,
    c2.IdCashlessTransaction AS TranWithin2Min,
    c2.Date AS TranWithin2MinDate,
    c2.Amount AS TranWithin2MinAmount
FROM CashlessTransactions c1
JOIN [dbo].[POSDevices] pd ON pd.IdPOSDevice = c1.IdPOSDevice
JOIN [dbo].[EventSessionSetups] ess ON ess.IdEventSessionSetup = pd.IdEventSessionSetup
JOIN [dbo].[Events] e ON e.IdEvent = ess.IdEvent
LEFT JOIN (
    SELECT 
        css.*,
        ROW_NUMBER() OVER (PARTITION BY css.IdCustomerSmartcard, DATEADD(MINUTE, 2, css.Date) ORDER BY css.Date) AS RowNum
    FROM CashlessTransactions css
    JOIN [dbo].[POSDevices] pd ON pd.IdPOSDevice = css.IdPOSDevice
    JOIN [dbo].[EventSessionSetups] ess ON ess.IdEventSessionSetup = pd.IdEventSessionSetup
    JOIN [dbo].[Events] e ON e.IdEvent = ess.IdEvent
) c2 ON c2.RowNum > 1 AND c2.IdCustomerSmartcard = c1.IdCustomerSmartcard AND c2.IdCashlessTransaction < c1.IdCashlessTransaction
WHERE c1.Date + INTERVAL 2 MINUTE <= DATEADD(MINUTE, 2, c1.Date)
    AND e.IdEvent = 2
ORDER BY c1.date, c2.date;

Explanation

This updated query uses a LEFT JOIN to combine the main transaction information with the inner query that calculates the count of different transactions made with the same card within 2 minutes.

The ROW_NUMBER() function is used to assign a unique number to each row within the partition (in this case, by customer smartcard ID and date). This allows us to select rows that have a higher row number than the current row, indicating that there is another transaction within 2 minutes of the original transaction.

We also filter out transactions without a matching inner query result by checking if c2.RowNum > 1.

The final filter ensures that we only consider transactions that are within 2 minutes of their original date.

This updated query should correctly retrieve two transactions from the same customer smartcard within the specified time range.

Last modified on 2024-12-16