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74 lines
2.5 KiB
Markdown
74 lines
2.5 KiB
Markdown
# Higher-order functions
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## `->` operator, lambda(params, expr) function
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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.
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Examples: `x -> 2 * x, str -> str != Referer.`
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Higher-order functions can only accept lambda functions as their functional argument.
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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.
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For all functions other than 'arrayMap' and 'arrayFilter', the first argument (the lambda function) can be omitted. In this case, identical mapping is assumed.
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### arrayMap(func, arr1, ...)
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Returns an array obtained from the original application of the 'func' function to each element in the 'arr' array.
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### arrayFilter(func, arr1, ...)
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Returns an array containing only the elements in 'arr1' for which 'func' returns something other than 0.
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Examples:
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```sql
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SELECT arrayFilter(x -> x LIKE '%World%', ['Hello', 'abc World']) AS res
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```
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```text
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┌─res───────────┐
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│ ['abc World'] │
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└───────────────┘
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```
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```sql
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SELECT
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arrayFilter(
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(i, x) -> x LIKE '%World%',
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arrayEnumerate(arr),
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['Hello', 'abc World'] AS arr)
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AS res
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```
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```text
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┌─res─┐
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│ [2] │
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└─────┘
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```
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### arrayCount(\[func,\] arr1, ...)
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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.
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### arrayExists(\[func,\] arr1, ...)
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Returns 1 if there is at least one element in 'arr' for which 'func' returns something other than 0. Otherwise, it returns 0.
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### arrayAll(\[func,\] arr1, ...)
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Returns 1 if 'func' returns something other than 0 for all the elements in 'arr'. Otherwise, it returns 0.
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### arraySum(\[func,\] arr1, ...)
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Returns the sum of the 'func' values. If the function is omitted, it just returns the sum of the array elements.
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### arrayFirst(func, arr1, ...)
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Returns the first element in the 'arr1' array for which 'func' returns something other than 0.
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### arrayFirstIndex(func, arr1, ...)
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Returns the index of the first element in the 'arr1' array for which 'func' returns something other than 0.
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