ClickHouse/docs/en/sql-reference/functions/higher-order-functions.md

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57	Higher-Order

Higher-order Functions

-> operator, lambda(params, expr) function

Allows describing a lambda function for passing to a higher-order function. The left side of the arrow has a formal parameter, which is any ID, or multiple formal parameters – any IDs in a tuple. The right side of the arrow has an expression that can use these formal parameters, as well as any table columns.

Examples: x -> 2 * x, str -> str != Referer.

Higher-order functions can only accept lambda functions as their functional argument.

A lambda function that accepts multiple arguments can be passed to a higher-order function. In this case, the higher-order function is passed several arrays of identical length that these arguments will correspond to.

For some functions, such as arrayCount or arraySum, the first argument (the lambda function) can be omitted. In this case, identical mapping is assumed.

A lambda function can’t be omitted for the following functions:

• arrayMap
• arrayFilter
• arrayFill
• arrayReverseFill
• arraySplit
• arrayReverseSplit
• arrayFirst
• arrayFirstIndex

arrayMap(func, arr1, …)

Returns an array obtained from the original application of the func function to each element in the arr array.

Examples:

SELECT arrayMap(x -> (x + 2), [1, 2, 3]) as res;

┌─res─────┐
│ [3,4,5] │
└─────────┘

The following example shows how to create a tuple of elements from different arrays:

SELECT arrayMap((x, y) -> (x, y), [1, 2, 3], [4, 5, 6]) AS res

┌─res─────────────────┐
│ [(1,4),(2,5),(3,6)] │
└─────────────────────┘

Note that the first argument (lambda function) can’t be omitted in the arrayMap function.

arrayFilter(func, arr1, …)

Returns an array containing only the elements in arr1 for which func returns something other than 0.

Examples:

SELECT arrayFilter(x -> x LIKE '%World%', ['Hello', 'abc World']) AS res

┌─res───────────┐
│ ['abc World'] │
└───────────────┘

SELECT
    arrayFilter(
        (i, x) -> x LIKE '%World%',
        arrayEnumerate(arr),
        ['Hello', 'abc World'] AS arr)
    AS res

┌─res─┐
│ [2] │
└─────┘

Note that the first argument (lambda function) can’t be omitted in the arrayFilter function.

arrayFill(func, arr1, …)

Scan through arr1 from the first element to the last element and replace arr1[i] by arr1[i - 1] if func returns 0. The first element of arr1 will not be replaced.

Examples:

SELECT arrayFill(x -> not isNull(x), [1, null, 3, 11, 12, null, null, 5, 6, 14, null, null]) AS res

┌─res──────────────────────────────┐
│ [1,1,3,11,12,12,12,5,6,14,14,14] │
└──────────────────────────────────┘

Note that the first argument (lambda function) can’t be omitted in the arrayFill function.

arrayReverseFill(func, arr1, …)

Scan through arr1 from the last element to the first element and replace arr1[i] by arr1[i + 1] if func returns 0. The last element of arr1 will not be replaced.

Examples:

SELECT arrayReverseFill(x -> not isNull(x), [1, null, 3, 11, 12, null, null, 5, 6, 14, null, null]) AS res

┌─res────────────────────────────────┐
│ [1,3,3,11,12,5,5,5,6,14,NULL,NULL] │
└────────────────────────────────────┘

Note that the first argument (lambda function) can’t be omitted in the arrayReverseFill function.

arraySplit(func, arr1, …)

Split arr1 into multiple arrays. When func returns something other than 0, the array will be split on the left hand side of the element. The array will not be split before the first element.

Examples:

SELECT arraySplit((x, y) -> y, [1, 2, 3, 4, 5], [1, 0, 0, 1, 0]) AS res

┌─res─────────────┐
│ [[1,2,3],[4,5]] │
└─────────────────┘

Note that the first argument (lambda function) can’t be omitted in the arraySplit function.

arrayReverseSplit(func, arr1, …)

Split arr1 into multiple arrays. When func returns something other than 0, the array will be split on the right hand side of the element. The array will not be split after the last element.

Examples:

SELECT arrayReverseSplit((x, y) -> y, [1, 2, 3, 4, 5], [1, 0, 0, 1, 0]) AS res

┌─res───────────────┐
│ [[1],[2,3,4],[5]] │
└───────────────────┘

Note that the first argument (lambda function) can’t be omitted in the arraySplit function.

arrayCount([func,] arr1, …)

Returns the number of elements in the arr array for which func returns something other than 0. If ‘func’ is not specified, it returns the number of non-zero elements in the array.

arrayExists([func,] arr1, …)

Returns 1 if there is at least one element in ‘arr’ for which ‘func’ returns something other than 0. Otherwise, it returns 0.

arrayAll([func,] arr1, …)

Returns 1 if ‘func’ returns something other than 0 for all the elements in ‘arr’. Otherwise, it returns 0.

arraySum([func,] arr1, …)

Returns the sum of the ‘func’ values. If the function is omitted, it just returns the sum of the array elements.

arrayFirst(func, arr1, …)

Returns the first element in the ‘arr1’ array for which ‘func’ returns something other than 0.

Note that the first argument (lambda function) can’t be omitted in the arrayFirst function.

arrayFirstIndex(func, arr1, …)

Returns the index of the first element in the ‘arr1’ array for which ‘func’ returns something other than 0.

Note that the first argument (lambda function) can’t be omitted in the arrayFirstIndex function.

arrayCumSum([func,] arr1, …)

Returns an array of partial sums of elements in the source array (a running sum). If the func function is specified, then the values of the array elements are converted by this function before summing.

Example:

SELECT arrayCumSum([1, 1, 1, 1]) AS res

┌─res──────────┐
│ [1, 2, 3, 4] │
└──────────────┘

arrayCumSumNonNegative(arr)

Same as arrayCumSum, returns an array of partial sums of elements in the source array (a running sum). Different arrayCumSum, when then returned value contains a value less than zero, the value is replace with zero and the subsequent calculation is performed with zero parameters. For example:

SELECT arrayCumSumNonNegative([1, 1, -4, 1]) AS res

┌─res───────┐
│ [1,2,0,1] │
└───────────┘

arraySort([func,] arr1, …)

Returns an array as result of sorting the elements of arr1 in ascending order. If the func function is specified, sorting order is determined by the result of the function func applied to the elements of array (arrays)

The Schwartzian transform is used to improve sorting efficiency.

Example:

SELECT arraySort((x, y) -> y, ['hello', 'world'], [2, 1]);

┌─res────────────────┐
│ ['world', 'hello'] │
└────────────────────┘

For more information about the arraySort method, see the Functions for Working With Arrays section.

arrayReverseSort([func,] arr1, …)

Returns an array as result of sorting the elements of arr1 in descending order. If the func function is specified, sorting order is determined by the result of the function func applied to the elements of array (arrays).

Example:

SELECT arrayReverseSort((x, y) -> y, ['hello', 'world'], [2, 1]) as res;

┌─res───────────────┐
│ ['hello','world'] │
└───────────────────┘

For more information about the arrayReverseSort method, see the Functions for Working With Arrays section.

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