SQL Server 2022

By using db<>fiddle, you agree to license everything you submit by Creative Commons CC0.
-- Create the tables
CREATE TABLE Table1 (
    [Month] date,
    Amount decimal(18, 2)
);

CREATE TABLE Table2 (
    [Month] date,
    Amount decimal(18, 2)
);

CREATE TABLE Table3 (
    [Month] date,
    Amount decimal(18, 2)
);
-- Insert data into the tables
INSERT INTO Table1 ([Month], Amount)
VALUES
    ('2023-09-01', 2.00),
    ('2023-10-01', 3.00),
    ('2023-10-01', 1.00);

INSERT INTO Table2 ([Month], Amount)
VALUES
    ('2023-09-01', 1.00),
    ('2023-10-01', 2.00),
    ('2023-10-01', 2.00);

INSERT INTO Table3 ([Month], Amount)
VALUES
    ('2023-09-01', 5.00),
    ('2023-10-01', 2.00),
    ('2023-10-01', 1.00);
9 rows affected

-- Calculate sales growth
WITH CombinedData AS (
    SELECT [Month], SUM(Amount) AS monthly_sales
    FROM (
        SELECT [Month], Amount FROM Table1
        UNION ALL
        SELECT [Month], Amount FROM Table2
        UNION ALL
        SELECT [Month], Amount FROM Table3
    ) AS Combined
    GROUP BY [Month]
),
SalesGrowth AS (
    SELECT
        FORMAT([Month], 'MMM yyyy') [Month],
        monthly_sales,
        LAG(monthly_sales, 1, 0) OVER (ORDER BY  [Month]) AS prev_month_sales,
        CASE
            WHEN LAG(monthly_sales, 1, 0) OVER (ORDER BY [Month]) = 0 THEN 0
            ELSE (monthly_sales - LAG(monthly_sales, 1, 0) OVER (ORDER BY  [Month])) * 100.0 / LAG(monthly_sales, 1, 0) OVER (ORDER BY [Month])
        END AS sales_growth
    FROM CombinedData
)
SELECT [Month], monthly_sales, prev_month_sales, sales_growth
FROM SalesGrowth;


Month
monthly_sales
prev_month_sales
sales_growth


Sep 2023
8.00
0.00
0.000000

Oct 2023
11.00
8.00
37.500000