CREATE FUNCTION (UDF)

Creates a user-defined scalar-valued function (UDF). A UDF is a callable routine that accepts input parameters, executes programmatic logic, and optionally returns a single value.

Syntax

[USING database_name] CREATE [OR REPLACE] FUNCTION function_name ( [parameter_list] )
  [RETURNS { data_type [data_type_modifier] } ] AS
  [DECLARE variable_list] [ ...n ]
  BEGIN
      function_body
  END ;

parameter_list:
  variable_name data_type [data_type_modifier [ ...n ] ]  [, ...]

data_type_modifier:
  DEFAULT default_value | NOT NULL | NULL | COLLATE collation_name

variable_list:
  variable_name data_type [= initial_value] ; [... ;]

Arguments

OR REPLACE

If specified, replaces a UDF if one already exists with the same name.

function_name

The name of the function. Function names must be unique within a database, and cannot be duplicates of names for other UDFs, tables, views, user-defined table-valued functions (TVFs), user-defined aggregate functions (UDAFs), stored procedures (SPs), or built-in functions such as CONCAT(), ABS(), COUNT(), and so on.

You can also specify database_name and function_name together by replacing function_name with database_name.function_name instead of specifying the database in USING database_name. For example, you can write the following:

CREATE FUNCTION db.some_func(a int)
...

Function names are case-insensitive. This is different than the behavior for table names.

parameter_list

Input parameters are optional. Any number of input parameters can be specified, and each must be delimited by a comma (,). Each input parameter must be given a unique identifier name within the scope of the function. A parameter’s name cannot exceed 64 characters in length.

The following example shows how to declare a single input parameter:

CREATE FUNCTION single_param_example(a INT)
...

UDFs allow both scalar data types and non-scalar data types (ARRAY and RECORD) as input parameters. Each valid type is described in the Data Types topic. The following example demonstrates how to declare more than one input parameter, using both scalar and non-scalar data types:

CREATE FUNCTION multi_param_example(a INT, b ARRAY(BIGINT NOT NULL), c DECIMAL(19,4))
...

Default values can be specified for input parameters by using the DEFAULT constraint. Consider the following example:

CREATE FUNCTION default_example(a INT DEFAULT 5)
...

Input parameters can also specify data type modifiers, namely NOT NULL, NULL, and COLLATE. Consider the following example:

CREATE FUNCTION modifier_example(a TEXT NOT NULL COLLATE utf8_bin)
...

data_type

Any scalar-valued or non-scalar valued data type as described in the Data Types topic.

DECLARE variable_list

A list of zero or more variable declaration statements. Variable names must be unique within the scope of the function, and cannot conflict with existing names for other identifiers. Variables may be set with an initial value. The DECLARE clause is only required once before declaring variables, but each variable declaration can be preceded by its own DECLARE statement. So, the following two examples are equivalent:

CREATE FUNCTION single_parameter_example(a INT) AS
  DECLARE
    num INT = 0;
    str VARCHAR(255);
  ...
CREATE FUNCTION multi_declare_example(a INT) AS
  DECLARE num INT = 0;
  DECLARE str VARCHAR(255);
  ...

function_body

The function body of a UDF may contain any of the following statements:

  • Control Flow Statements
  • Variable assignments
  • Calls to other UDFs when assigning them to a variable or in a RETURN statement.

Security and Permissions

The invoker of a UDF must have EXECUTE permissions on the UDF. Also, the UDF’s definer must have EXECUTE permissions on any UDFs used in the function’s body.

Remarks

MemSQL user-defined scalar-valued functions (UDFs) provide a mechanism to create custom programmatic logic that can be called in SQL queries, stored procedures, or in other UDFs.

MySQL Client Delimiters

When creating a UDF using a MySQL-compatible client connected to MemSQL, you must change the client delimiter to ensure that the function definition is correctly passed to the server. By default, all MySQL clients use a semicolon as a statement delimiter. A problem arises when creating UDFs because they use semicolons as statement delimiters within the function body. Therefore, you must change the delimiter setting before creating your function, and then set it back to a semicolon after the alternate delimiter is no longer needed.

To change the MySQL delimiter, execute the following command before creating a new function:

DELIMITER //

When you’ve finished defining UDFs, make sure to change the MySQL delimiter back to a semicolon:

DELIMITER ;

A complete example is shown below, which can be executed directly in a MySQL-compatible client connected to MemSQL:

DELIMITER //
CREATE FUNCTION plus_one(a INT) RETURNS INT AS
  DECLARE
    b INT = a;
  BEGIN
    b += 1;
    RETURN b;
  END //
DELIMITER ;

Each of the examples in this topic and all MemSQL Procedural SQL (MPSQL) topics include this MySQL delimiter consideration.

Limitations

UDFs are limited in the following ways:

Calling Limitations

A UDF cannot call a stored procedure (SP) or a user-defined aggregate function (UDAF).

Function Overloading

A UDF definition cannot be overloaded by changing the function signature, such as adding or removing input parameters or changing the return type.

SQL and Database Object Manipulation

UDFs cannot execute SQL statements, and therefore cannot read or modify any persistent database objects.

Examples

Normalize String

The following example normalizes a string by making all letters lowercase, removing any leading and trailing whitespace characters, and ensuring one whitespace character between words.

DELIMITER //
CREATE FUNCTION normalize_string(input VARCHAR(255)) RETURNS VARCHAR(255) AS
  DECLARE
    result VARCHAR(255) = "";
    i INT;
    previousChar CHAR;
    nextChar CHAR;
    str VARCHAR(255) = input;
  BEGIN
    str = LCASE(TRIM(str));
    IF LENGTH(str) = 0 THEN
      RETURN str;
    END IF;
    previousChar = SUBSTR(str, 1, 1);
    result = CONCAT(result, previousChar);
    i = 2;
    WHILE i <= LENGTH(str) LOOP
      nextChar = SUBSTR(str, i, 1);
      IF NOT(previousChar = ' ' AND nextChar = ' ') THEN
        result = CONCAT(result, SUBSTR(str, i, 1));
      END IF;
      previousChar = nextChar;
      i += 1;
    END LOOP;
    RETURN result;
  END //
DELIMITER ;
memsql> SELECT normalize_string(" THIS is   a test ");
+----------------------------------------+
| normalize_string(" THIS is   a test ") |
+----------------------------------------+
| this is a test                         |
+----------------------------------------+
1 row in set (0.53 sec)

Is Prime Number

The following example function accepts an integer input parameter and returns 1 if it is a prime number or 0 if it is not a prime number.

DELIMITER //
CREATE FUNCTION is_prime(n BIGINT NOT NULL) returns BIGINT AS
  BEGIN
    IF n <= 1 THEN
      RETURN FALSE;
    END IF;
    FOR i IN 2 .. (n-1) LOOP
      EXIT WHEN i * i > n;
      IF n % i != 0 THEN
        CONTINUE;
      END IF;
      RETURN FALSE;
    END LOOP;
    RETURN TRUE;
  END //
DELIMITER ;
memsql> SELECT is_prime(101);
+---------------+
| is_prime(101) |
+---------------+
|             1 |
+---------------+
1 row in set (0.08 sec)

Related Topics

Was this article useful?