SQL Server 2022

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CREATE TABLE some_table (
    "product"       VARCHAR(50) NOT NULL,
    "some_date"     DATE        NOT NULL,
    "some_value"    FLOAT       NOT NULL
);
INSERT INTO some_table VALUES
('Cheese', '2023-01-01', 1.23),
('Cheese', '2023-02-01', 1.19),
('Cheese', '2024-01-01', 1.20),
('Cheese', '2024-08-01', 1.28),
('Cheese', '2025-01-01', 1.33)
;
5 rows affected

WITH base_input AS (
    SELECT
        product         AS "itm",
        YEAR(some_date) AS "yr",
        AVG(some_value) AS "val"
    FROM some_table
    GROUP BY product, YEAR(some_date)
)
SELECT
    itm,
    yr,
    val,
    val / NULLIF(
        LAG(val) OVER (
            PARTITION BY itm
            ORDER BY yr ASC),
        0
    ) AS "infl"
FROM base_input


itm
yr
val
infl


Cheese
2023
1.21
null

Cheese
2024
1.24
1.02479338842975

Cheese
2025
1.33
1.07258064516129

itm	yr	val	infl
Cheese	2023	1.21	null
Cheese	2024	1.24	1.02479338842975
Cheese	2025	1.33	1.07258064516129