Question

In my table I need to search specific columns in a specific row to find which column contains the value I define.

For example in my table

ID | Col1 | Col2 | Col3 | Col4 | Col5 | Col6
--------------------------------------------
1  | A    | B    | C    | C    | B    | A
2  | C    | B    | A    | A    | C    | B
3  | B    | A    | C    | B    | A    | C

I need a query to search columns 4, 5 and 6 in Row 2 and tell me in which column value B occurs (Col6).

Answer 1

Using a series of CASE conditions to match 'B' against each column will do the job. Each must supply the string literal name of the column as its THEN value. You cannot dynamically determine the column names – they must be hard-coded.

SELECT
  CASE
    WHEN Col2 = 'B' THEN 'Col2' 
    WHEN Col4 = 'B' THEN 'Col4'
    WHEN Col6 = 'B' THEN 'Col6'
    ELSE 'Not found' 
  END AS which_column
FROM 
  yourtable
WHERE id = 2

To return more than one, you can wrap the CASE statements in a CONCAT() that returns either the column name or an empty string. This means using several separate CASE statements though, instead of multiple conditions for the same case.

SELECT
  CONCAT(
    CASE WHEN col2 = 'B' THEN 'Col2 ' ELSE '' END,
    CASE WHEN col4 = 'B' THEN 'Col4 ' ELSE '' END,
    CASE WHEN col6 = 'B' THEN 'Col6' ELSE '' END
  ) AS which_columns
FROM yourtable
WHERE id = 2