Is it possible to create this mysql Function? - mysql

Question

I'm a newbie in mysql programming, and i would create a mysql function myFunction with a string parametre; this function query myTable and return a string from the query-result like this example :
myTable
---------------
|id | value |
---------------
| id1 |value1 |
| id2 |value2 |
| id3 |value3 |
| id4 |value4 |
---------------
Calling this function is like this
myFunction('value2#value1#value4')
and must return
'id2#id1#id4'
Thank you very much

Answer 1

This is a demo of how you can do it, of course you can put all together but I find it cleaner to split the functions.
Check the SQL Fiddle
My table
-- the table def
create table myTable (id char(3), value char(6));
insert into myTable values( 'id1', 'value1');
insert into myTable values( 'id2', 'value2');
insert into myTable values( 'id3', 'value3');
insert into myTable values( 'id4', 'value4');
Get a specific id
-- get Id by Value
CREATE function getIdByValue( theValue TEXT )
RETURNS TEXT READS SQL DATA
BEGIN
DECLARE theId TEXT;
DECLARE ok INT DEFAULT FALSE;
DECLARE crs CURSOR FOR
SELECT id FROM myTable where value = theValue;
DECLARE CONTINUE HANDLER FOR NOT FOUND SET ok = TRUE;
SET theId = '';
OPEN crs;
read_loop: LOOP
FETCH crs INTO theId;
IF ok THEN
LEAVE read_loop;
END IF;
END LOOP;
CLOSE crs;
RETURN theId;
END//
MyFunction as you describe
-- the myFunction, usage: myFunction('value2#value1#value4')
CREATE function myFunction( v TEXT )
RETURNS TEXT READS SQL DATA
BEGIN
DECLARE theId TEXT;
DECLARE theIds TEXT;
DECLARE theValue TEXT;
DECLARE vInstr INT;
SET theId = '';
SET theIds = '';
v_loop: LOOP
SET vInstr = INSTR(v,'#');
IF vInstr = 0 THEN
SET theValue = v;
SET theId = getIdByValue(theValue);
ELSE
SET theValue = SUBSTRING(v, 1, vInstr-1);
SET v = SUBSTRING(v, vInstr+1);
SET theId = concat( getIdByValue(theValue), '#');
END IF;
SET theIds = CONCAT(theIds, theId);
IF vInstr = 0 THEN
LEAVE v_loop;
END IF;
END LOOP;
RETURN theIds;
END//
The call
SELECT myFunction( 'value2' );
SELECT myFunction( 'value2#value4' );
SELECT myFunction( 'value2#value4#value1' );
SELECT myFunction( 'value2#value4#value1#value3' );
The results
id2
id2#id4
id2#id4#id1
id2#id4#id1#id3

Answer 2

you can use something like,
CREATE FUNCTION myFunction (param1) RETURNS datatype
[NOT] DETERMINISTIC
statements
check this link for more information
http://www.mysqltutorial.org/mysql-stored-function/

Answer 3

Any reason you couldn't just run this kind of query?
SELECT GROUP_CONCAT(id ORDER BY FIELD(`value`, 'value2', 'value1', 'value4'))
FROM myTable
WHERE `value` IN ('value2','value1','value4')
;
Edited: Added value as first argument to FIELD() function to make it return the ids in the proper order.