MySQL Stored Functions - Dynamic/Variable Table & Column Names - mysql

Question

I'm learning stored procedures/functions/triggers in MySQL this morning and I'm having some problems trying to use variable table and column names in queries.
DELIMITER |
USE R2R |
DROP FUNCTION IF EXISTS getCategoryName |
CREATE FUNCTION getCategoryName(LMID INT, levelNum INT)
RETURNS VARCHAR(255)
BEGIN
DECLARE levelName VARCHAR(255) DEFAULT makeLevelName(levelNum);
DECLARE levelID INT;
DECLARE levelNameID VARCHAR(255) DEFAULT CONCAT(levelName, 'ID');
DECLARE ret VARCHAR(255);
SELECT #levelNameID INTO levelID FROM LevelMaster WHERE id=LMID;
SELECT description INTO ret FROM #levelName WHERE id=levelID;
RETURN ret;
END;
|
DROP FUNCTION IF EXISTS makeLevelName |
CREATE FUNCTION makeLevelName(levelNum INT)
RETURNS VARCHAR(255)
BEGIN
DECLARE word VARCHAR(255);
DECLARE ret VARCHAR(255);
IF levelNum=2 THEN SET word='Two';
ELSEIF levelNum=3 THEN SET word='Three';
ELSEIF levelNum=4 THEN SET word='Four';
ELSEIF levelNum=5 THEN SET word='Five';
END IF;
SET ret=CONCAT('Level', word);
RETURN ret;
END;
|
SELECT getCategoryName(347, 2) |
It's the first function (getCategoryName) that's causing me the problems, I need the two variables marked with # to be the table/column names - these two lines:
SELECT #levelNameID INTO levelID FROM LevelMaster WHERE id=LMID;
SELECT description INTO ret FROM #levelName WHERE id=levelID;
I want to keep this function as a function rather than a procedure if possible, but would accept answers for a procedure if it's the only way.
Thanks for you help,
Richard

Answer 1

Use User/Global Vars for this along with PREPARE & EXECUTE:
SET #columnName='myColumn';
SET #tableName='myTable';
SET #whatEver='requiredValue';
SET #query=CONCAT('SELECT ', #columnName, ' FROM ', #tableName, ' WHERE Column=', #whatEver);
PREPARE QUERY FROM #QUERY;
EXECUTE QUERY;
Haven't tested this EXACT code but something along these lines will work. Also has to be inside a Procedure, cannot be used with a function or trigger, if anyone has a soloution for that then please post.