CREATE TABLE product_prices
(
    ProductId UInt64,
    TrackedAt Date,
    Price Nullable(Decimal(6, 2))
)
ENGINE MergeTree() ORDER BY (ProductId, TrackedAt)

| ProductId  | Price       | TrackedAt    |
|:-----------|------------:|:------------:|
| 1          |       20    |  2019-01-16  |
| 1          |       19    |  2019-01-17  |
| 2          |        5    |  2019-01-16  |
| 2          |        7    |  2019-01-17  |

| ProductId  |
|:-----------|
| 1          |

select (argMax(Price, TrackedAt) - argMin(Price, TrackedAt)) from (select ProductId, Price, TrackedAt from product_prices where ProductId = 1000 order by TrackedAt DESC limit 2)