Calculate rolling adjusted date based on date from parent row - SQL Server lag / recursion

Calculate Rolling Adjusted Date Based on Date from Parent Row in SQL Server: LAG & Recursive CTE Guide

By CodeToCareer | Last updated: October 28, 2024

Introduction

In complex data management, calculating a rolling adjusted date based on a date from a parent row is a common requirement. SQL Server provides powerful tools for handling such tasks, including the LAG function and recursive Common Table Expressions (CTEs). This guide walks through methods for calculating rolling adjusted dates, providing a hands-on approach to handle date dependencies effectively in SQL Server.

Understanding the Challenge

In SQL Server, calculating rolling dates involves adjusting a date based on a previous row’s value. This is useful in scenarios where dates depend on events or processes that occur in sequence. A typical case might include handling overlapping schedules, calculating sequential event dates, or rolling adjustments based on a prior date’s output.

The two primary SQL Server techniques to achieve this include:

• LAG Function – Ideal for referencing prior rows’ values without recursion.
• Recursive CTEs – Useful when building sequential, dependent data based on parent rows.

Method 1: Using SQL Server LAG Function

The LAG function is a window function in SQL Server that allows you to access data from a previous row in the same result set. This is particularly helpful in cases where you want to calculate dates based on the previous row’s date without creating complex recursive queries.

Basic LAG Function Syntax

LAG(column_name, offset, default_value) OVER (PARTITION BY partition_column ORDER BY order_column)
        

Here’s a breakdown of the parameters:

• column_name: The column from which to fetch previous values.
• offset: The number of rows back you want to look.
• default_value: Optional. The value returned if no previous row exists.

Example Scenario: Calculating Rolling Dates with LAG

In this example, we have a table called EventSchedule with columns EventID, EventDate, and AdjustedDate. We want to adjust each EventDate based on the previous row’s AdjustedDate.

CREATE TABLE EventSchedule (
    EventID INT,
    EventDate DATE,
    AdjustedDate DATE
);

INSERT INTO EventSchedule (EventID, EventDate)
VALUES
(1, '2024-10-01'),
(2, '2024-10-05'),
(3, '2024-10-10'),
(4, '2024-10-15');
        

Here’s how to use the LAG function to calculate AdjustedDate:

WITH AdjustedEvents AS (
    SELECT
        EventID,
        EventDate,
        COALESCE(
            DATEADD(DAY, 2, LAG(EventDate) OVER (ORDER BY EventID)),
            EventDate
        ) AS AdjustedDate
    FROM EventSchedule
)
SELECT * FROM AdjustedEvents;
        

This query adjusts each EventDate by adding 2 days to the previous row’s date. The COALESCE function ensures that the first row uses EventDate as-is, avoiding NULL values.

Method 2: Using Recursive CTEs for Rolling Adjusted Dates

While the LAG function works for many use cases, recursive Common Table Expressions (CTEs) are a powerful alternative when calculations depend on a continuous chain of adjustments. Recursive CTEs allow us to build results step-by-step, referencing the output from prior rows.

Example Scenario: Rolling Adjusted Date Calculation with Recursive CTE

Suppose we have the same EventSchedule table and want to calculate AdjustedDate by adding a specific interval to the last calculated AdjustedDate.

WITH RecursiveAdjustedDates AS (
    SELECT
        EventID,
        EventDate,
        EventDate AS AdjustedDate
    FROM EventSchedule
    WHERE EventID = 1

    UNION ALL

    SELECT
        es.EventID,
        es.EventDate,
        DATEADD(DAY, 2, rad.AdjustedDate) AS AdjustedDate
    FROM EventSchedule es
    INNER JOIN RecursiveAdjustedDates rad ON es.EventID = rad.EventID + 1
)
SELECT * FROM RecursiveAdjustedDates
OPTION (MAXRECURSION 0);
        

In this query:

• The RecursiveAdjustedDates CTE starts by selecting the first row with EventID = 1 as the base case.
• The UNION ALL operator connects this base case to a recursive select statement, where each successive row calculates AdjustedDate by adding 2 days to the previous row’s AdjustedDate.
• MAXRECURSION is set to 0 to allow unlimited recursion.

Pros and Cons of LAG vs Recursive CTEs

Each approach has its strengths and trade-offs. Here’s a quick comparison:

LAG Function

• Pros: Simple syntax, efficient, and great for non-recursive calculations.
• Cons: Limited to referencing a fixed number of previous rows, less flexible in complex dependencies.

Recursive CTE

• Pros: Allows for flexible, chain-based calculations and dependencies.
• Cons: May be slower for large datasets, risk of performance issues if recursion depth is high.

When to Use Each Method

Determining which method to use depends on the structure of your data and the complexity of the rolling date calculations:

• Use LAG when date adjustments depend only on a fixed offset of previous rows.
• Opt for a recursive CTE when each row’s date depends on an iterative calculation based on the cumulative results of all previous rows.

Advanced Example: Combining LAG and Recursive CTE

In certain situations, you may benefit from combining both methods. For example, using LAG to initialize a set of dates and then applying recursive logic for dependent calculations.

WITH InitialLag AS (
    SELECT
        EventID,
        EventDate,
        COALESCE(LAG(EventDate) OVER (ORDER BY EventID), EventDate) AS AdjustedDate
    FROM EventSchedule
),
RecursiveAdjusted AS (
    SELECT
        EventID,
        EventDate,
        AdjustedDate
    FROM InitialLag
    WHERE EventID = 1

    UNION ALL

    SELECT
        il.EventID,
        il.EventDate,
        DATEADD(DAY, 2, ra.AdjustedDate) AS AdjustedDate
    FROM InitialLag il
    INNER JOIN RecursiveAdjusted ra ON il.EventID = ra.EventID + 1
)
SELECT * FROM RecursiveAdjusted;
        

Conclusion

Calculating a rolling adjusted date based on a parent row’s date in SQL Server can be achieved effectively using either the LAG function or recursive CTEs. The LAG function offers a simpler approach, while recursive CTEs provide the flexibility needed for more complex, dependent date calculations. Selecting the right method will depend on your specific requirements and data structure. Both techniques are essential for mastering sequential data processing in SQL Server, enabling efficient date adjustments and dependencies in your data workflows.

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