使用动态数量的参数创建MySQL存储函数

Question

I am trying to create a MySQL function IS_IN_ENUM('value', 'val1', 'val2', 'val3') which return true if 'value' is in ('val1', 'val2', 'val3'). I know I can do SELECT 'value' IN ('val1', 'val2', 'val3') but that's less intersting because I just want to learn how to create such functions.

I give you an example, consider the following ADD function :

CREATE FUNCTION my_add (
    a DOUBLE,
    b DOUBLE
)
RETURNS DOUBLE
BEGIN

    IF a IS NULL THEN
        SET a = 0;
    END IF;

    IF b IS NULL THEN
        SET b = 0;
    END IF;

    RETURN (a + b);
END;

If I do SELECT my_add(1, 1) , I get 2 (wow!).

How can I improve this function to be able to call :

SELECT my_add(1, 1); -- 2
SELECT my_add(1, 1, 1); -- 3
SELECT my_add(1, 1, 1, 1); -- 4
SELECT my_add(1, 1, 1, 1, 1, 1, .....); -- n

Answer 1

The function example you show is a Stored Function , not a UDF. Stored Functions in MySQL don't support a variable number of arguments, as @Enzino answered.

MySQL UDFs are written in C or C++, compiled into dynamic object files, and then linked with the MySQL server with a different syntax of CREATE FUNCTION .

See http://dev.mysql.com/doc/refman/5.5/en/adding-udf.html for details of writing UDFs. But I don't know if you want to get into writing C/C++ code to do this.

MySQL UDFs do support variable number of arguments. In fact, all UDFs implicitly accept any number of arguments, and it's up to you as the programmer to determine if the number and datatypes of the arguments given are valid for your function.

Processing function arguments in UDFs is documented in http://dev.mysql.com/doc/refman/5.5/en/udf-arguments.html

Answer 2

I am trying to create a MySQL function IS_IN_ENUM('value', 'val1', 'val2', 'val3') which return true if 'value' is in ('val1', 'val2', 'val3').

For this you can use the native function FIELD:

http://dev.mysql.com/doc/refman/5.6/en/string-functions.html#function_field

IS_IN_ENUM means FIELD != 0.

Check also FIND_IN_SET

http://dev.mysql.com/doc/refman/5.6/en/string-functions.html#function_find-in-set

Stored functions do not support variable number of parameters.

Now, if you really want to implement such a native function yourself in the MySQL server code, look for subclasses of Create_native_func in sql/item_create.cc

Answer 3

老问题，但你不需要创建is_in_enum，因为它已经内置。只需从表中选择true，其中值为IN（1,2,3,4）;