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66 lines
2.1 KiB
Markdown
66 lines
2.1 KiB
Markdown
---
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slug: /en/sql-reference/statements/create/function
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sidebar_position: 38
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sidebar_label: FUNCTION
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title: "CREATE FUNCTION -user defined function (UDF)"
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---
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Creates a user defined function (UDF) from a lambda expression. The expression must consist of function parameters, constants, operators, or other function calls.
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**Syntax**
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```sql
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CREATE FUNCTION name [ON CLUSTER cluster] AS (parameter0, ...) -> expression
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```
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A function can have an arbitrary number of parameters.
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There are a few restrictions:
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- The name of a function must be unique among user defined and system functions.
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- Recursive functions are not allowed.
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- All variables used by a function must be specified in its parameter list.
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If any restriction is violated then an exception is raised.
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**Example**
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Query:
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```sql
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CREATE FUNCTION linear_equation AS (x, k, b) -> k*x + b;
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SELECT number, linear_equation(number, 2, 1) FROM numbers(3);
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```
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Result:
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``` text
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┌─number─┬─plus(multiply(2, number), 1)─┐
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│ 0 │ 1 │
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│ 1 │ 3 │
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│ 2 │ 5 │
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└────────┴──────────────────────────────┘
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```
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A [conditional function](../../../sql-reference/functions/conditional-functions.md) is called in a user defined function in the following query:
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```sql
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CREATE FUNCTION parity_str AS (n) -> if(n % 2, 'odd', 'even');
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SELECT number, parity_str(number) FROM numbers(3);
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```
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Result:
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``` text
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┌─number─┬─if(modulo(number, 2), 'odd', 'even')─┐
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│ 0 │ even │
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│ 1 │ odd │
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│ 2 │ even │
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└────────┴──────────────────────────────────────┘
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```
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## Related Content
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### [Executable UDFs](/docs/en/sql-reference/functions/udf.md).
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### [User-defined functions in ClickHouse Cloud](https://clickhouse.com/blog/user-defined-functions-clickhouse-udfs)
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