Question

I have a table with lots of different columns, like

resolution, brightness, type, width, camera_type, projection_type etc 

I need to get the names of only those columns, that are relevant to, say LED tv’s to build filters, – which means I need to somehow

SHOW COLUMNS FROM Goods WHERE category_id = 5 AND [there is at least one value in the column for a row that meets the category_id = 5 condition]

Is it even possible?

Answer 1

I’m guessing what you want is the column names that have a non-null value where category_id = 5. The difficulty here is finding out which columns have a non-null value where category_id = 5. This will have to be accomplished on a per column basis but can be done.

To be honest… I don’t work in MySql but I’m certain you can follow along and make any adjustments needed.

-- create a temp table to hold the results
CREATE TEMPORARY TABLE TempTable (columnName varchar(100));  

-- create a bunch of dynamic SQL to insert its name into the temp table when category_id = 5 and its value is not null

DECLARE column_name VARCHAR(100);

DECLARE no_more_rows BOOLEAN;
DECLARE loop_cntr INT DEFAULT 0;
DECLARE num_rows INT DEFAULT 0;

-- Declare the cursor
DECLARE columns_cur CURSOR FOR
  SELECT
      column_name
  FROM information_schema.columns
  WHERE TABLE_NAME = 'Goods';

-- Declare 'handlers' for exceptions
DECLARE CONTINUE HANDLER FOR NOT FOUND
  SET no_more_rows = TRUE;

OPEN columns_cur;
  select FOUND_ROWS() into num_rows;

the_loop: LOOP

  FETCH  columns_cur 
  INTO   column_name;

  IF no_more_rows THEN
      CLOSE columns_cur;
      LEAVE the_loop;
  END IF;

  PREPARE stmt FROM 'IF(EXISTS(SELECT * FROM Goods WHERE ? IS NOT NULL AND catetory_id = 5)) INSERT INTO TempTable (column_name) VALUES ('?');'

  EXECUTE stmt USING @column_name;
  DEALLOCATE PREPARE stmt;

END LOOP the_loop;