How to Calculate Date Differences Between Consecutive Rows for a Specific Account Number in SQL?

Efficiently Calculating Date Differences Between Consecutive Rows in SQL for a Specific Account

This article demonstrates two SQL queries to calculate the date difference between consecutive rows for a given account number. The methods presented offer varying degrees of efficiency.

Method 1: Using a LEFT JOIN and MIN()

This approach employs a LEFT JOIN to compare each row with subsequent rows for the same account. The MIN() function finds the next date.

<code class="language-sql">SELECT  T1.ID, 
        T1.AccountNumber, 
        T1.Date, 
        MIN(T2.Date) AS Date2, 
        DATEDIFF("D", T1.Date, MIN(T2.Date)) AS DaysDiff
FROM    YourTable T1
        LEFT JOIN YourTable T2
            ON T1.AccountNumber = T2.AccountNumber
            AND T2.Date > T1.Date
GROUP BY T1.ID, T1.AccountNumber, T1.Date;</code>

This query joins the table (YourTable) to itself, comparing each row (T1) with all following rows for the same account number (T2). The MIN(T2.Date) finds the earliest subsequent date, and DATEDIFF computes the difference in days. Grouping ensures a result for each original row.

Method 2: Using a Subquery for Improved Efficiency

For larger datasets, a subquery approach often proves more efficient. This method directly incorporates the date calculation within the main query.

<code class="language-sql">SELECT  ID,
        AccountNumber,
        Date,
        NextDate,
        DATEDIFF("D", Date, NextDate) AS DaysDiff
FROM    (   SELECT  ID, 
                    AccountNumber,
                    Date,
                    (   SELECT  MIN(Date) 
                        FROM    YourTable T2
                        WHERE   T2.AccountNumber = T1.AccountNumber
                        AND     T2.Date > T1.Date
                    ) AS NextDate
            FROM    YourTable T1
        ) AS T</code>

Here, a subquery is nested to find the NextDate for each row. This avoids the potentially less efficient JOIN operation. The outer query then calculates the DaysDiff using DATEDIFF.

Both methods achieve the same result: calculating the date difference between consecutive records for each account number. However, for optimal performance with large tables, Method 2 (using the subquery) is generally recommended. Choose the method best suited to your data volume and database system.

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