Use string from select to select a specific column in mysql - mysql

Question

I'm trying to select a column based on a string I get from another select:
SELECT id, (
SELECT COLUMN_NAME
FROM information_schema.COLUMNS
WHERE TABLE_SCHEMA = 'database_name'
AND TABLE_NAME = 'a'
AND DATA_TYPE = 'varchar'
ORDER BY `COLUMNS`.`ORDINAL_POSITION` ASC LIMIT 1) AS `name`
FROM `a`
The problem is like this I'm getting only the string from the sub select not the column content of the main query.
(what I want to do in this case, is to return the value of the first column that is varchar)

Answer 1

You can do this with a dynamic query constructed in a stored procedure which you then call with the appropriate parameters.
I'd put the example in a sqlfiddle if I could find a way of querying the information_schema but you can just copy/paste this into your normal client and it should work.
I've gone a little further than your example by allowing you to select the id and first varchar column from any table but you can easily adjust the procedure and hardcode the database and table names if that's all you require.
CREATE DATABASE IF NOT EXISTS `mydb`;
USE `mydb`;
CREATE TABLE IF NOT EXISTS `mydb`.`tableA` (
id INT auto_increment primary key,
char_col char(3),
varchar_col varchar(25)
);
-- clear out any existing records
TRUNCATE `mydb`.`tableA`;
INSERT INTO `mydb`.`tableA` VALUES (null,'abc','varchar value abc');
INSERT INTO `mydb`.`tableA` VALUES (null,'def','varchar value def');
INSERT INTO `mydb`.`tableA` VALUES (null,'ghi','varchar value ghi');
INSERT INTO `mydb`.`tableA` VALUES (null,'klm','varchar value klm');
DELIMITER //
DROP PROCEDURE IF EXISTS `mydb`.`dyntest` //
CREATE PROCEDURE `mydb`.`dyntest` (IN dbname VARCHAR(64), IN tname VARCHAR(64))
BEGIN
DECLARE colname VARCHAR(64);
-- get the column name (as your example)
SELECT `COLUMN_NAME` INTO colname
FROM `information_schema`.`COLUMNS`
WHERE `TABLE_SCHEMA` = dbname
AND `TABLE_NAME` = tname
AND `DATA_TYPE` = 'VARCHAR'
ORDER BY `COLUMNS`.`ORDINAL_POSITION` ASC LIMIT 1;
-- construct the query
SET #sqlquery = CONCAT('SELECT `id`,`', colname , '` FROM `' , dbname, '`.`', tname, '`');
-- Prepare and execute
PREPARE stmt FROM #sqlquery;
EXECUTE stmt;
DEALLOCATE PREPARE stmt;
END //
DELIMITER ;
This returns
mysql> CALL `mydb`.`dyntest`('mydb','tableA');
+----+-------------------+
| id | varchar_col |
+----+-------------------+
| 1 | varchar value abc |
| 2 | varchar value def |
| 3 | varchar value ghi |
| 4 | varchar value klm |
+----+-------------------+
4 rows in set (0.06 sec)