ClickHouse/docs/en/sql-reference/aggregate-functions/reference/maxmap.md

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maxMap

Syntax: maxMap(key, value) or maxMap(Tuple(key, value))

Calculates the maximum from value array according to the keys specified in the key array.

Passing a tuple of keys and value arrays is identical to passing two arrays of keys and values.

The number of elements in key and value must be the same for each row that is totaled.

Returns a tuple of two arrays: keys and values calculated for the corresponding keys.

Example:

SELECT maxMap(a, b)
FROM values('a Array(Char), b Array(Int64)', (['x', 'y'], [2, 2]), (['y', 'z'], [3, 1]))

┌─maxMap(a, b)───────────┐
│ [['x','y','z'],[2,3,1]]│
└────────────────────────┘