MySQL Stored Functions - Dynamic/Variable Table & Column Names

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

Drahcir · Accepted Answer · 2011-07-08 15:45:01Z

3

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.

answered Jul 8, 2011 at 15:45

Drahcir

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Jeff Over a year ago

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