为什么VARCHAR变量无法在内部查询中用作mysql存储过程中的列名？

Question

Is there a solution for using variable filter in the select statement for a column name without concatenating it with the query ?.
Not working

BEGIN 
    Declare StartRow int;
    set StartRow = ((PageNumber - 1) * PageSize);



  SELECT 
t.qid,
t.title,
t.vote,
t.answer_count,
t.creation_date,
t.uid,
t.username,
MAX(CASE WHEN t.constantTagNumber = 1 THEN t.tag END) AS tag1,
MAX(CASE WHEN t.constantTagNumber = 1 THEN t.tid END) AS tid1,
MAX(CASE WHEN t.constantTagNumber = 2 THEN t.tag END) AS tag2,
MAX(CASE WHEN t.constantTagNumber = 2 THEN t.tid END) AS tid2,
MAX(CASE WHEN t.constantTagNumber = 3 THEN t.tag END) AS tag3,
MAX(CASE WHEN t.constantTagNumber = 3 THEN t.tid END) AS tid3,
MAX(CASE WHEN t.constantTagNumber = 4 THEN t.tag END) AS tag4,
MAX(CASE WHEN t.constantTagNumber = 4 THEN t.tid END) AS tid4,
MAX(CASE WHEN t.constantTagNumber = 5 THEN t.tag END) AS tag5,
MAX(CASE WHEN t.constantTagNumber = 5 THEN t.tid END) AS tid5
FROM
(
    SELECT
        questions.id as qid,
        title,
        creation_date,
        vote,
        answer_count,
        tags.id as tid,
        tag,            
        username,
        users.id as uid,            
        IF (@prev = qid ,@c := @c + 1,@c := 1) constantTagNumber,
        @prev := qid
    FROM    (   SELECT @prev := 0 ,@c := 1) var,question_tags
    INNER JOIN tags ON question_tags.tid = tags.id
    RIGHT JOIN questions ON question_tags.qid = questions.id 
    INNER JOIN users ON users.id=owner_id
    ORDER BY    qid,tid
) t
GROUP BY t.qid ORDER BY filter desc LIMIT StartRow,PageSize;         
END

The problem is only with the last order by filter here filter is a varchar variable that I am using as a parameter.

Since filter is a variable ,the above code is not working.
Can anyone suggest an simple solution to make it work.

Make It work means ,I need to use the column name to use as order by from parameter.

Update

   BEGIN 
    Declare StartRow int;
    set StartRow = ((PageNumber - 1) * PageSize);

set @sql1=(
  SELECT 
t.qid,
t.title,
t.vote,
t.answer_count,
t.creation_date,
t.uid,
t.username,
MAX(CASE WHEN t.constantTagNumber = 1 THEN t.tag END) AS tag1,
MAX(CASE WHEN t.constantTagNumber = 1 THEN t.tid END) AS tid1,
MAX(CASE WHEN t.constantTagNumber = 2 THEN t.tag END) AS tag2,
MAX(CASE WHEN t.constantTagNumber = 2 THEN t.tid END) AS tid2,
MAX(CASE WHEN t.constantTagNumber = 3 THEN t.tag END) AS tag3,
MAX(CASE WHEN t.constantTagNumber = 3 THEN t.tid END) AS tid3,
MAX(CASE WHEN t.constantTagNumber = 4 THEN t.tag END) AS tag4,
MAX(CASE WHEN t.constantTagNumber = 4 THEN t.tid END) AS tid4,
MAX(CASE WHEN t.constantTagNumber = 5 THEN t.tag END) AS tag5,
MAX(CASE WHEN t.constantTagNumber = 5 THEN t.tid END) AS tid5
FROM
(
    SELECT
        questions.id as qid,
        title,
        creation_date,
        vote,
        answer_count,
        tags.id as tid,
        tag,            
        username,
        users.id as uid,            
        IF (@prev = qid ,@c := @c + 1,@c := 1) constantTagNumber,
        @prev := qid
    FROM    (   SELECT @prev := 0 ,@c := 1) var,question_tags
    INNER JOIN tags ON question_tags.tid = tags.id
    RIGHT JOIN questions ON question_tags.qid = questions.id 
    INNER JOIN users ON users.id=owner_id
    ORDER BY    qid,tid
) t
GROUP BY t.qid); 
set @sql=CONCAT(@sql1," ORDER BY ",filter," desc LIMIT", StartRow,",",PageSize);

PREPARE stmt FROM @sql;
EXECUTE stmt;
DEALLOCATE PREPARE stmt;
END

I Updated the above code ,what am I missing here .

Answer 1

You are getting close: Change your code to create a VARCHAR with your SQL in it, use IF/ELSE logic to concatenate your order by in there. Then pass this SQL to the prepared statement:

SET @sql = "Your SQL";
PREPARE stmt FROM @sql;
EXECUTE stmt;
DEALLOCATE PREPARE stmt;