How to combine CONCAT() and WHERE in MariaDB?

Question

I like to use the result from another query to address the column name. For this I like to use CONCAT() . But somehow it don't work; when I run this line I get 0 rows back:

SELECT * FROM cover WHERE CONCAT('c','9') = 1;

When I don't make use of CONCAT() it work perfectly:

SELECT * FROM cover WHERE c9 = 1;

And also CONCAT() seems to work. With this I get a result:

SELECT CONCAT('c','9');

I tried all solution from this question: MySQL select with CONCAT condition

like this one, but i always got 0rows back:

SELECT * FROM (
  SELECT id, CONCAT('c', '9') as target 
  FROM cover) base 
WHERE target = "1"

My MySQL Version is; 10.1.16-MariaDB

Answer 1

It is bad schema design to splay an array across a bunch of columns. And, as you are discovering, it is hard to use the columns. Build another table for the c values.

Or...

With lots of 0/1 "columns", consider SET or BIGINT UNSIGNED ; either will hold up to 64 boolean flags in a tiny fraction of the space. And, with different code, BLOB could be used.

To extract bit 22 from a BIGINT , ((col >> 22) & 1) will give you 0 or 1.

Answer 2

Consider using a case when , since the number of options is known beforehand (you can only access columns that exist):

SELECT id 
FROM   cover 
WHERE  case ? 
           when 1 then c1
           when 2 then c2
           when 9 then c9
       end = 1

... where the question mark would be the provided value, like 9 in your example.