Mysql Triggers, declare a variable and use it as column name - mysql

Question

I have the table "mytable" that contains the "columnname" field wich is the name of a column in mytable2.
I use this one for the selection:
SET #DptScn = (SELECT columnname FROM mytable WHERE tablename = 'CustomTableName' AND fieldlabel = 'CustomField');
SET #identifiedid=144;
but, when I try:
SELECT #DptScn FROM mytable2 WHERE identifiedid = #identifiedid;
this give me NOT the content of the field but the name containted into variable #DptScn...
Any advice?
I can't use Prepared Statement because I'm in a Trigger...
UPDATE:
As suggested by spencer7593 I'm creating a procedure:
DROP PROCEDURE IF EXISTS p_t;
DELIMITER $$
CREATE PROCEDURE p_t (IN DptTcn VARCHAR(255), IN tid INT, OUT tT INT)
BEGIN
SET #DptTcn = DptTcn;
SET #tid = tid;
SET #sql = CONCAT('SELECT #DptTcn FROM mytable3 WHERE tid = #tid');
PREPARE stmt FROM #sql;
EXECUTE stmt;
END$$
DELIMITER ;
Then I try it:
SET #DptTcn = (SELECT columnname mytable WHERE tablename = 'CustomTableName' AND fieldlabel = 'CustomField');
SET #identifiedid=145;
CALL proc_ticket(#DptTcn, #identifiedid, #DptT);
But I receive a:
#2014 - Commands out of sync; you can't run this command now

Answer 1

One option to consider is creating a PROCEDURE that makes use of prepared statements, and then calling the the stored procedure from the trigger.
The SQL statement you execute to get the value from a particular column MUST have the column_name specified in the SQL text; this can't be derived "dynamically" in the execution of the statement.
To achieve something like this, you'll need to run two separate statements; one to get the column_name; the second to "SELECT column_name FROM". And the MySQL provided mechanism for executing that second query is a prepared statement.
Followup
Here's an example. I tried to get this built in SQLFiddle, but wasn't able to get it working (it just hung. So, here's the output from a mysql command line client instead.
(All of the statements below use the same delimiter // because we can't use a semicolon as a delimiter for the stored procedure. In SQLFiddle, we have to use the same delimiter on all statements, and the // just happens to be one of the options in SQLFiddle.)
mysql> DELIMITER //
mysql> CREATE PROCEDURE foo(IN colname VARCHAR(255), IN id INT, OUT val VARCHAR(255))
-> BEGIN
-> -- handler for "Unknown column" and "No data" exceptions
-> DECLARE EXIT HANDLER FOR 1054, 1329 BEGIN SET val = NULL; END;
-> SET #sql = CONCAT('SELECT ',colname,' INTO #val FROM t WHERE id = ',id,' LIMIT 1');
-> PREPARE stmt FROM #sql;
-> EXECUTE stmt;
-> SET val = #val;
-> END//
Query OK, 0 rows affected (0.00 sec)
mysql> CREATE TABLE t (id INT, attr VARCHAR(4), ball VARCHAR(4))//
Query OK, 0 rows affected (0.11 sec)
mysql> INSERT INTO t VALUES (1, 'abcd','efgh'),(2,'ijkl','mnop')//
Query OK, 2 rows affected (0.00 sec)
Records: 2 Duplicates: 0 Warnings: 0
mysql> CALL foo('attr',1,#attr_1)//
Query OK, 0 rows affected (0.00 sec)
mysql> CALL foo('attr',2,#attr_2)//
Query OK, 0 rows affected (0.00 sec)
mysql> CALL foo('ball',1,#ball_1)//
Query OK, 0 rows affected (0.00 sec)
mysql> CALL foo('ball',2,#ball_2)//
Query OK, 0 rows affected (0.00 sec)
mysql> CALL foo('attr',777,#err_bad_id)//
Query OK, 0 rows affected (0.00 sec)
mysql> CALL foo('badcol',1,#err_badcol)//
Query OK, 0 rows affected (0.00 sec)
mysql> SELECT #attr_1
-> , #attr_2
-> , #ball_1
-> , #ball_2
-> , #err_bad_id
-> , #err_badcol//
+---------+---------+---------+---------+-------------+-------------+
| #attr_1 | #attr_2 | #ball_1 | #ball_2 | #err_bad_id | #err_badcol |
+---------+---------+---------+---------+-------------+-------------+
| abcd | ijkl | efgh | mnop | NULL | NULL |
+---------+---------+---------+---------+-------------+-------------+
1 row in set (0.00 sec)
mysql> DELIMITER ;

Answer 2

you should create a SP and give the column name.
create proc dbo.TestGetData(#DptScn nvarchar(256))
as
begin
set nocount on
DECLARE #SQL NVARCHAR(MAX)
SET #SQL = 'SELECT #DptScn FROM mytable2 WHERE identifiedid = 144'
exec sp_executesql #SQL, N'#DptScn nvarchar(256)', #DptScn =#DptScn
end
Then
exec dbo.TestGetData 'Column1'