SQL Variables inside SELECT query in a function

Question

MariaDB 10.3.14

I have a function that generates the root id in a hierarchy:

The function is this:

CREATE FUNCTION `getRootId`(GivenID INT) RETURNS int(11)
DETERMINISTIC
BEGIN
DECLARE ch INT;
DECLARE root INT;
SET ch = GivenID;

WHILE ch >= 0 DO
    SELECT id, parent_id INTO root, ch  FROM
    (SELECT id, parent_id FROM pctable WHERE id = ch) A;
END WHILE;
RETURN root;
END

I want to be able to provide the id, parent_id and table name as variables to this function, so i can easily be able to use it in different hierarchy tables easily.

something like this:

CREATE FUNCTION `getRootId`(GivenID INT,id_column_name varchar(45),parent_id_column_name varchar(45),target_table_name varchar(45))

One function to be used for both Topics, Indicators ....

I'm new to advanced SQL coding, so apologies if this is a newbie question.

Many thanks.

Updates:

I switched to procedure to use prepared statements:

CREATE DEFINER=`root`@`localhost` PROCEDURE `getRootIdFlex`(IN GivenID INT,IN id_name varchar(45),IN parent_name varchar(45),IN tbl_name varchar(45) , OUT root varchar(1000))
DETERMINISTIC
BEGIN
DECLARE id_or_parent INT;

SET @tbl_name = tbl_name;
SET @id_name = id_name;
SET @parent_name = parent_name;

SET id_or_parent = GivenID;

SET @s := CONCAT('SELECT ?, ? INTO root, id_or_parent FROM (SELECT ?, ? FROM ? WHERE ? = id_or_parent) A');
PREPARE stmt FROM @s;
WHILE id_or_parent >= 0 DO
    EXECUTE stmt USING @id_name, @parent_name, @id_name, @parent_name, @tbl_name, @id_name;
END WHILE;
END

But when i execute the procedure this way:

SET @root := NULL;
CALL getRootIdFlex(11, 'Topic_id', 'Topic_Topic_id','Topic', @root);
SELECT @root;

I get:

Error Code: 1327. Undeclared variable: root

The root is declared as OUT variable. Why this error ?

Answer 1

I fixed it, here is the original function with some edits to make it readable. I changed the while condition to stop at NULL, because that's the endpoint in my hierarchy.

CREATE FUNCTION `getRootId`(GivenID INT) RETURNS int(11)
DETERMINISTIC
BEGIN
SET @root = NULL;
SET @tmp_parent = GivenID;
WHILE @tmp_parent IS NOT NULL DO
    SELECT id, parent_id INTO @root, @tmp_parent  FROM
    (SELECT id, parent_id FROM pctable WHERE id = @tmp_parent) A;
END WHILE;
RETURN @root;
END

In order to have variables inside a prepared statement you need to use PROCEDURES so functions will not do it:

CREATE PROCEDURE `getRootIdFlex`(IN GivenID INT,IN id_name varchar(45),IN parent_name varchar(45),IN tbl_name varchar(45) , OUT root varchar(1000))
DETERMINISTIC
BEGIN
SET @tmp_parent = GivenID;
SET @tmp_root := 111;

SET @s := CONCAT('SELECT ',id_name,',',parent_name, ' INTO @tmp_root, @tmp_parent FROM (SELECT ',id_name,',',parent_name, ' FROM ',tbl_name,' WHERE ',id_name,' = @tmp_parent) A');

PREPARE stmt FROM @s;
WHILE @tmp_parent IS NOT NULL DO
    EXECUTE stmt;
END WHILE;
SET root := @tmp_root;
END

Key point here is: Make sure to add '@' symbol in your variables everywhere to avoid confusion with table columns in your SQL queries.

Thanks.