Can't execute a MySQL SELECT statement when inside CONCAT

Question

I have been trying to create a simple loop of SELECT statements in MySQL to reduce code. I have started this using CONCAT() however this causes the procedure to stop/fail. For example (where k is a loop counter):

CONCAT('SELECT (Child_', k, ' INTO @Age_Child_', k, ' FROM lookup_childage WHERE ModYear = ModYear_var LIMIT 1)');

To diagnose the issue, I simply tried to place the SELECT statement (without concatenated loop variables) inside a string to then be executed. While I could get this to work for simple statements it would not work for the following:

SET @queryString = CONCAT('SELECT Child_1 INTO @Age_Child_1 FROM lookup_childage WHERE ModYear = ModYear_var LIMIT 1');    
PREPARE stmt FROM @queryString;
EXECUTE stmt;

Does anyone know why the @queryString containing the CONCAT() statement will not be executed/cause the procedure to fail?

Answer 1

tl;dr The statement you're trying to write has the form SELECT(rest of statement) LIMIT 1 . It should have the form SELECT rest of statement LIMIT 1 .

It looks like you want to create variable column names, ummm, because your lookup_childage table is denormalized. I guess that table has these columns.

  Child_1   INT
  Child_2   INT
  Child_3   INT
  Child_4   INT

It looks like you hope to get a @queryString value containing this sort of thing:

SELECT Child_4 INTO @Age_Child_4 FROM lookup_childage WHERE ModYear = ModYear_var LIMIT 1

Only the 4 s are variable.

So to get that string you want

 SELECT CONCAT('SELECT Child_', k,
               ' INTO @Age_Child_', k,
               ' FROM lookup_childage WHERE ModYear = ModYear_var LIMIT 1'
              )
    INTO @queryString;