正如sqlite可以定义自定义函数，它是通过API定义c函数的，不像其他，如这里的mysql。sqlite提供原生接口就可以方便的调用其他语言的方法，同样的mysql也支持调用其它语言的方法。

google "mysql call c function"发现一片文章 MySQL User Defined Functions 

This tutorial explains what an User Defined Function (UDF) is, what it does and why/when they are useful.

1. What is an User Defined Function?

Basically an User Defined Function (UDF) is a piece code that extends the functionality of a MySQL server by adding a new function that behaves just like a native (built-in) MySQL function like abs() or concat(). UDFs are written in C (or C++ if you really need to)! ... well maybe there is a way to write them in BASIC, .NET or whatever but I don't see why anybody would want to do that.

2. Why/When are UDFs useful?

As implied by the name UDFs are useful when you need to extend the functionality of your MySQL server. This little table should make it clear which method is best for a given situation:

Method	Speed	Language	Development
Stored Procedures	slow	SQL	~minutes (for small functions)
UDF	fast	C	~hour
Native Function	fast	C	major pain in the ***

And by "slow" I mean: "slower than the others"! Stored Procedures are still much faster then normal SQL statements!

A little explanation on native functions: The code you have to write here is essentially the same as the one for an UDF. BUT you have to write it in the MySQL source code and recompile the whole thing. This will (believe me) be a lot of work because you have to do it again and again with every new version of MySQL.

3. How to use UDFs?

This part is really easy. When you have your UDF finished you just use it like every other native function. For example : "SELECT MyFunction(data1, data2) FROM table"

4. Writing the UDF

Now let's get started on writing our first UDF in steps:

1. Create a new shared-library project (in the example I used VC++ 6.0 with a standard DLL)

2. First we need some headers. These headers are either standard library headers or from the MySQL Server's include directory

   #ifdef STANDARD
   
   /* STANDARD is defined, don't use any mysql functions */
   
   #include <stdlib.h>
   
   #include <stdio.h>
   
   #include <string.h>
   
   #ifdef __WIN__
   
   typedef unsigned __int64 ulonglong;     /* Microsofts 64 bit types */
   
   typedef __int64 longlong;
   
   #else
   
   typedef unsigned long long ulonglong;
   
   typedef long long longlong;
   
   #endif /*__WIN__*/
   
   #else
   
   #include <my_global.h>
   
   #include <my_sys.h>
   
   #endif
   
   #include <mysql.h>
   
   #include <ctype.h>
   
   static pthread_mutex_t LOCK_hostname;

3. Now we have to decide what kind of function we want. There are essentially two choices to be made:

   • is the function an aggregate function or not? (we will learn more about aggregate functions later)

   • which type of return value should the function return? Here we have 4 options:

     Type	Description
     STRING	A string literal. Translates to char* in C
     INTEGER	A normal integer. Translates to a 64 bit integer in C
     REAL	A floating point number. Translates to double in C
     DECIAML	This one isn't really finished at this time. MySQL treats it as STRING

4. Let's talk about non-aggregate functions first. Now we have to declare and implement some functions the MySQL server needs to use our UDF. But first some structs we'll need for that:

   • UDF_INIT:

     Type	Name	Description
     <code> my_bool	maybe_null	1 if function can return NULL
     unsigned int	decimals	for REAL functions
     unsigned long	max_length	For string functions
     char *	ptr	free pointer for function data
     my_bool	const_item	0 if result is independent of argument

   • UDF_ARGS:

     Type	Name	Description
     unsigned int	arg_count	Number of argument
     enum Item_result *	arg_type	Array containing the types of the arguments
     char **	args	Array of pointer to the arguments
     unsigned long *	lengths	Array of the argument's lengths (only needed for strings)
     char *	maybe_null	Array of "maybe_null" flags (1 if argument maybe null)
     char **	attributes	Array of pointers to the arguments' attributes (see chapter x for details)
     unsigned long *	attribute_lengths	Array of attributes lengths

   now let's take a look at the functions:

   De-/Initialization:

   extern "C" my_bool MyTest_init(UDF_INIT *initid, UDF_ARGS *args,
   
                                  char *message)
   
   {
   
           // The most important thing to do here is setting up the memory
   
           // you need...
   
           // Lets say we need a lonlong type variable to keep a checksum
   
           // Although we do not need one in this case
   
           longlong* i = new longlong; // create the variable
   
           *i = ;                     // set it to a value
   
           // store it as a char pointer in the pointer variable
   
           // Make sure that you don`t run in typecasting troubles later!!
   
           initid->ptr = (char*)i;
   
           // check the arguments format
   
           if (args->arg_count != )
   
           {
   
               strcpy(message,"MyTest() requires one arguments");
   
               return ;
   
           }
   
           if (args->arg_type[] != INT_RESULT)
   
           {
   
               strcpy(message,"MyTest() requires an integer");
   
               return ;
   
           }
   
           return ;
   
   }
   
   extern "C" void MyTest_deinit(UDF_INIT *initid)
   
   {
   
           // Here you have to free the memory you allocated in the
   
           // initialization function
   
           delete (longlong*)initid->ptr;
   
   }

   The actual function:

   extern "C" longlong MyTest(UDF_INIT *initid, UDF_ARGS *args,
   
                              char *is_null, char *error)
   
   {
   
           /* So finally this is the part were we do the real work. This
   
           function is called for every record and the current value(s)
   
           or better pointers to the current values are stroed in the
   
           UDF_ARGS variable. We have to get the values, do our calculation
   
           and return the result. NOTE: You can access the memory
   
           allocated in MyTest_init through the UDF_INIT variable.
   
           In this example we will just add 5 to every value...*/
   
           return *((longlong*)args->args[])+;
   
   }

5. All done! Now we have to compile the library and copy it to a directory where our OS can find it. On Windows that would be anywhere the PATH System variable says. Personally I use the MySQL servers bin directory. You have to make sure that the library is in one of those directories otherwise MySQL can't use it! And also make sure to export all the functions MySQL needs!

6. And at last we have to tell MySQL about it. This is really straightforward: Just execute the following SQL command:

   CREATE [AGGREGATE] FUNCTION MyTest
   
   RETURNS [INTEGER|STRING|REAL|DECIMAL] SONAME the_libraries_exact_name

   Now you can use it like any other function.

5. Aggregate functions

Now some words to aggregate functions. When your UDF is an aggregate function you have to add some more functions and some functions are used in a different way. The calling sequence is:

1. Call MyTest_init to allocate memory (just like a normal UDF)
2. MySQL sorts the table according to the GROUP BY statement
3. Call MyTest_clear for the first row in each group
4. Call MyTest_add for each row that belongs to the same group
5. Call MyTest to get the result when the group changes or the last row has been processed
6. Repeat 3 to 5 until all rows have been processed
7. Call MyTest_deinit to free any used memory

Now let's look at the new functions needed for the aggregate function. In this example we'll simply add up all the values. (like the native SUM function)

void MyTest_clear(UDF_INIT *initid, char *is_null, char *error)

{

        /* The clear function resets the sum to 0 for each new group

        Of course you have to allocate a longlong variable in the init

        function and assign it to the pointer as seen above */

        *((longlong*)initid->ptr) = ;

}

void MyTest_add(UDF_INIT *initid, UDF_ARGS *args, char *is_null, char *error)

{

        // For each row the current value is added to the sum

        *((longlong*)initid->ptr) = *((longlong*)initid->ptr) +

                                    *((longlong*)args->args[]);

}

longlong MyTest(UDF_INIT *initid, UDF_ARGS *args, char *is_null, char *error)

{

        // And in the end the sum is returned

        return *((longlong*)initid->ptr);

}

6. Advanced topics

Here are some things you should know when you write more complex UDFs:

• A string function should return a pointer to the result and set *result and *length to the contents and length of the return value. For example:

  memcpy(result, "result string", );
  
  *length = ;

  The result buffer that is passed to the MyTest function is 255 bytes long. If your result fits in this, you don't have to worry about memory allocation for results.

  If your string function needs to return a string longer than 255 bytes, you must allocate the space for it with malloc or new in your MyTest_init function or your MyTest function and free it in yourMyTest_deinit function. You can store the allocated memory in the ptr slot in the UDF_INIT structure for reuse by future MyTest calls.

• To indicate an error return in the main function, set *error to 1: If MyTest() sets *error to 1 for any row, the function value is NULL for the current row and for any subsequent rows processed by the statement in which MyTest() was invoked.

• For more information see the MySQL Online Manual

7. Some guidelines

Here are some guidelines that you should follow if you want to make sure your UDF runs smoothly 

• Do not call any other applications or processes inside an UDF!

• Do not store any information locally! (This goes for shared libraries in general)

• Do not allocate any global or static variables!

• Always check the type of your arguments. As you can see MySQL converts everything to char pointers. This can lead to major troubles if you convert a string literal to a integer pointer and so on.

• Be extra careful with the memory allocation! If you have memory leaks you can bring down the sever in no time.

8. Debugging UDFs

Debugging an UDF can be pretty nerve wracking because every time your UDF crashes it takes down the whole MySQL server along with it. So I wrote a little command line tool to work around that problem. Just execute it after compilation and it does the rest. Meaning, it emulates a call to the function by calling an "SELECT" command and then passing the results to the library and printing out the result on the command line . So when the UDF produces some serious errors only the little helper goes down and not the whole server. It is still in "beta" so don't expect to much....

9. Sources

MySQL Online Manual

摘自：研究如何编写和使用MySQL UDF 2008/06/30

1. 简介

MySQL的UDF(User Defined Function)类似于一种API, 用户根据一定的规范用C/C++(或采用C调用规范的语言)编写一组函数(UDF)，然后编译成动态链接库，通过CREATE FUNCTION和DROP FUNCTION语句来加载和卸载UDF。UDF被加载后可以像调用MySQL的内置函数一样来调用它，并且服务器在启动时会自动加载原来存在的UDF。

2. 特性

• UDF的参数和返回值的类型可以为字符串，整数或实型。
• UDF分为简单UDF和聚合UDF。
• 服务器会向UDF提供相应的信息以便UDF检查实参的数量、类型和名称。
• UDF也可以强制服务器在调用UDF前检查实参类型。
• UDF可以指明返回值为NULL或发生了错误。

3. CREATE FUNCTION的语法

CREATE [AGGREGATE] FUNCTION function_name RETURNS {STRING|INTEGER|REAL|DECIMAL} SONAME shared_library_name

function_name 为函数名 ，RETURNS子句表明UDF的返回值类型，目前DECIMAL和STRING类型是等价的 。

shared_library_name是UDF所在的动态链接库的名称（不含路径），该文件需放在可被系统搜索到的目录中。

在MySQL 5.1中，动态链接库需要放在由plugin_dir变量所指定的插件目录中。

4. DROP FUNCTION的语法

DROP FUNCTION function_name

5. 编写UDF

对于每个UDF，需要编写若干相关C/C++函数，下面用XXX()代表在SQL语句中调用UDF，xxx()代表相应的C/C++函数调用，实际使用时，两者应相同。

• xxx()  ——  必须，主函数，该函数的返回值即为UDF的返回值。返回值类型与SQL数据类型的对应关系如下：

• xxx_init()  ——  可选，xxx()的初始化函数，它可被用来
  • 检查XXX()的实参个数和类型，或强制由服务器来检查。
  • 为主函数分配内存。
  • 指明UDF返回值的最大长度或精度，以及返回值是否可能为NULL.
• xxx_deinit() ——  可选，xxx()的清理函数，用来释放在xxx_init()中分配的内存

对于聚合UDF, 还需要另外两个函数。

• xxx_clear()  ——  必须(MySQL5.1)，重置当前聚合值。
• xxx_add() ——  必须，将参数加到当前聚合值中去。
• xxx_reset() —— 必须(MySQL 5.1以下版本)，重置当前聚合值，并将参数作为初始聚合值。

所以函数必须是线程安全的。可以参考《MySQL参考手册》第30章第3节以获得更详细的说明。此外，在MySQL源代码包中有编写UDF的示例，位置为sql/udf_example.c.

6. 简单UDF的调用顺序

当SQL语句调用XXX()时，服务器首先调用xxx_init()进行参数检查或内存分配，若xxx_init()出错，则SQL语句被终止，主函数和清理函数将不会被调用。然后，服务器对每一行记录调用一次主函数，最后清理函数xxx_deinit()被调用。

7. 聚合UDF的调用顺序

1. 调用xxx_init()进行初始化。
2. 根据GROUP BY表达式对表进行排序。
3. 对每个新组调用xxx_clear()重置聚合值。
4. 对组中每一行记录调用xxx_add().
5. 当该组数据处理完后，调用xxx()计算聚合值.
6. 重复步骤3-5直到所有数据处理完毕。
7. 调用xxx_deinit()进行清理。

8. 编译和安装UDF

用类似的命令来编译UDF：

shell> gcc -shared -o udf_example.so udf_example.c

将经过编译得到的动态链接库(.so文件)放到系统可以找到并加载该文件的目录中。然后通过CREATE FUNCTION语句让服务器加载动态链接库中的函数。

9. 示例

udf_test.c

#include <mysql.h>

#include <time.h>

#include <string.h>

my_bool datetime_now_init( UDF_INIT *initid, UDF_ARGS *args, char *message )

{

    if ( args->arg_count )

    {

        strcpy( message, "datetime_now() does not need any argument." );

        return();

    }

    return();

}

char *datetime_now( UDF_INIT *initid __attribute__( (unused) ), UDF_ARGS *args __attribute__( (unused) ),

            char *result, unsigned long *length, char *is_null, char *error __attribute__( (unused) ) )

{

    const time_t now_time = time( NULL );

    strftime( result, , "%Y/%m/%d %I:%M:%S %p %Z %z", localtime( &now_time ) );

    *length = strlen( result );

    return(result);

}

编译

shell> gcc -fPIC -Wall -O3 -I/usr/local/mysql/include -shared -o udf_test.so udf_test.c

安装

mysql> CREATE FUNCTION datetime_now RETURNS STRING SONAME 'udf_test.so';

Query OK, 0 rows affected (0.00 sec)

使用

mysql> SELECT datetime_now();

+----------------------------------+

| datetime_now()                   |

+----------------------------------+

| 2008/06/27 05:27:03 PM CST +0800 |

+----------------------------------+

1 row in set (0.00 sec)

10. 与存储过程函数的区别

存储过程中的函数是一组SQL语句的集合，其能力受限于SQL的表达能力，它还要受限于MySQL自身的一些限制。它主要用于较高层次的数据处理。

UDF的限制相对来说要小很多，与编写服务器代码时的限制相同，只要符合相应的约定即可。它的定位偏向于底层，主要用于提供一些基础的数据处理功能。另外要注意的是，UDF一旦出错，会导致服务器崩溃！

11. 参考资料

• "MySQL Reference Manual"  Chapter 30 Section 3
• http://www.xaprb.com/blog/2007/10/30/how-i-built-the-now_usec-udf-for-mysql/

红黑联盟上文章

MySQL:如何编写UDF

void name_add(UDF_INIT *initid, UDF_ARGS *args, char *is_null,char *error)

void name_clear(UDF_INIT *initid, char *is_null, char *error)

/* 初始化 */

my_booludf_int_init( UDF_INIT * initid, UDF_ARGS * args, char *message )

{

    if ( args->arg_count !=  )             /* 检查参数个数 */

    {

        strcpy( message,

            "udf_intexample() can onlyaccept one argument" );

        return();

    }

    if ( args->arg_type[] != INT_RESULT )  /* 检查参数类型 */

    {

        strcpy( message,

            "udf_intexample() argumenthas to be an integer" );

        return();

    }

    return();

}

/* 清理操作 */

voidudf_int_deinit( UDF_INIT * initid )

{

}

/* 主函数 */

long long udf_int( UDF_INIT *initid, UDF_ARGS *args, char *is_null, char *error )

{

    long long num = (*(long long *) args->args[]); /* 获取第一个参数值 */

    return(num);

}

/* 初始化 */

my_booludf_floatsum_init( UDF_INIT * initid, UDF_ARGS * args, char *message )

{

    double *total = (double *) malloc( sizeof(double) );

    if ( total == NULL )             /* 内存分配失败 */

    {

        strcpy( message, "udf_floatsum:alloc mem failed!" );

        return();

    }

    *total = ;

    initid->ptr = (char *) total;

    if ( args->arg_count !=  )       /* 检查参数个数 */

    {

        strcpy( message, "too moreargs,only one!" );

        return();

    }

    if ( args->arg_type[] != REAL_RESULT ) /* 检查参数类型 */

    {

        strcpy( message, "wrongtype" );

        return();

    }

    initid->decimals = ;     /* 设置返回值精度 */

    return();

}

/* 清理、释放在init函数中分配的内存 */

voidudf_floatsum_deinit( UDF_INIT * initid )

{

    free( initid->ptr );

}

/* 每一行都会调用到该函数 */

voidudf_floatsum_add( UDF_INIT * initid, UDF_ARGS * args, char *is_null, char *error )

{

    double* float_total;

    float_total = (double *) initid->ptr;

    if ( args->args[] )

        *float_total += *(double *) args->args[];

}

/* 每个分组完成后，返回结果 */

doubleudf_floatsum( UDF_INIT * initid, UDF_ARGS * args, char *is_null, char *error )

{

    double* float_total;

    float_total = (double *) initid->ptr;

    return(*float_total);

}

/* 在进行下一个分组前调用，设置initid->ptr指向的值为0，以便下一次分组统计 */

voidudf_floatsum_clear( UDF_INIT * initid, char *is_null, char *error )

{

    double *float_total;

    float_total = (double *) initid->ptr;

    *float_total = ;

}

• MySQL 5.0 Reference Manual :: 21 Extending MySQL :: 21.2 Adding New Functions to MySQL :: 21.2.2 Adding a New User-Defined Function