How to Handle Depleting Values in SQL Rows Using Recursive CTEs?

SQL: Handling decrementing values ​​in rows

In SQL, when working with multiple rows of data, incrementally subtracting values ​​from a set of rows can be challenging, especially if the subtracted values ​​exhaust the source value. Here's how to fix this type of problem:

Suppose you have two tables:

• Pooled_Lots: Contains inventory information (Id, Pool, Lot, Quantity)
• Pool_Consumption: Storage consumption (Id, PoolId, QuantityConsumed)

You need a result set, subtracting the QuantityConsumed values ​​from the Pooled_Lots table, taking into account the following rules:

• For non-last rows, if QuantityConsumed is less than or equal to Quantity, subtract QuantityConsumed from Quantity.
• For multiple rows, subtract QuantityConsumed from Quantity.
• Loop until the last row.
• For the last row, subtract the remaining QuantityConsumed from the Quantity.

To achieve this, a recursive common table expression (CTE) can be used:

<code class="language-sql">WITH Amos AS (
    -- 从每个Pool的Lot 1开始。
    SELECT
        PL.Pool,
        PL.Lot,
        PL.Quantity,
        PC.QuantityConsumed,
        CASE
            WHEN PC.QuantityConsumed IS NULL THEN PL.Quantity
            WHEN PL.Quantity >= PC.QuantityConsumed THEN PL.Quantity - PC.QuantityConsumed
            WHEN PL.Quantity = PC.QuantityConsumed THEN 0
            WHEN PL.Quantity < PC.QuantityConsumed THEN 0  --处理消耗量大于库存量的情况
            ELSE PL.Quantity
        END AS RunningQuantity,
        CASE
            WHEN PC.QuantityConsumed IS NULL THEN PL.Quantity
            ELSE PC.QuantityConsumed
        END AS RemainingDemand,
        CASE
            WHEN PL.Quantity >= PC.QuantityConsumed THEN 0
            ELSE PC.QuantityConsumed - PL.Quantity
        END AS SurplusOrDeficit
    FROM
        Pooled_Lots PL
    LEFT JOIN
        Pool_Consumption PC ON PL.Pool = PC.PoolId AND PL.Lot = 1  -- 关联到Lot 1
    UNION ALL
    SELECT
        a.Pool,
        a.Lot + 1,
        PL.Quantity,
        PC.QuantityConsumed,
        CASE
            WHEN a.RunningQuantity >= PC.QuantityConsumed THEN a.RunningQuantity - PC.QuantityConsumed
            WHEN a.RunningQuantity < PC.QuantityConsumed THEN 0
            ELSE a.RunningQuantity
        END AS RunningQuantity,
        CASE
            WHEN PC.QuantityConsumed IS NULL THEN 0
            ELSE PC.QuantityConsumed
        END AS RemainingDemand,
        CASE
            WHEN a.RunningQuantity >= PC.QuantityConsumed THEN 0
            ELSE PC.QuantityConsumed - a.RunningQuantity
        END AS SurplusOrDeficit
    FROM
        Amos a
    INNER JOIN
        Pooled_Lots PL ON a.Pool = PL.Pool AND a.Lot + 1 = PL.Lot
    LEFT JOIN
        Pool_Consumption PC ON PL.Pool = PC.PoolId AND PL.Lot = a.Lot + 1
)
SELECT * FROM Amos;</code>

This CTE iteratively subtracts the QuantityConsumed value from the Pooled_Lots rows until it reaches the last row. It calculates RunningQuantity, RemainingDemand and SurplusOrDeficit according to the specified rules.

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