ClickHouse/docs/en/sql-reference/statements/create/function.md

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/en/sql-reference/statements/create/function	38	FUNCTION

CREATE FUNCTION — user defined function (UDF)

Creates a user defined function from a lambda expression. The expression must consist of function parameters, constants, operators, or other function calls.

Syntax

CREATE FUNCTION name [ON CLUSTER cluster] AS (parameter0, ...) -> expression

A function can have an arbitrary number of parameters.

There are a few restrictions:

• The name of a function must be unique among user defined and system functions.
• Recursive functions are not allowed.
• All variables used by a function must be specified in its parameter list.

If any restriction is violated then an exception is raised.

Example

Query:

CREATE FUNCTION linear_equation AS (x, k, b) -> k*x + b;
SELECT number, linear_equation(number, 2, 1) FROM numbers(3);

Result:

┌─number─┬─plus(multiply(2, number), 1)─┐
│      0 │                            1 │
│      1 │                            3 │
│      2 │                            5 │
└────────┴──────────────────────────────┘

A conditional function is called in a user defined function in the following query:

CREATE FUNCTION parity_str AS (n) -> if(n % 2, 'odd', 'even');
SELECT number, parity_str(number) FROM numbers(3);

Result:

┌─number─┬─if(modulo(number, 2), 'odd', 'even')─┐
│      0 │ even                                 │
│      1 │ odd                                  │
│      2 │ even                                 │
└────────┴──────────────────────────────────────┘