Question

I would like to add a column to a result set based on whether a field field_value in the table is NULL.

Here is my query that works:

SELECT field_id, (
   SELECT field_value
   FROM employee_field_info
   WHERE employee_id = '1'
   AND field_id = persona_fields.field_id
) AS field_value, ( 
   SELECT field
   FROM FIELDS WHERE id = persona_fields.field_id
) AS field_name
FROM persona_fields
WHERE persona_id = '1'

What i want to be able to do is to add a tinyint (for a bool) column if the field_value is NULL.

I’ve tried this:

SELECT field_id, (
   SELECT field_value
   FROM employee_field_info
   WHERE employee_id = '1'
   AND field_id = persona_fields.field_id
) AS field_value, ( 
   SELECT field
   FROM FIELDS WHERE id = persona_fields.field_id
) AS field_name,
IF(field_value IS NULL, "0","1") AS value_bool
FROM persona_fields
WHERE persona_id = '1'

But that just comes up with the “Unknown column ‘field_value’ in ‘field list.’
Is there anyway i can accomplish what I’m trying to do, or am I pretty much out of luck?

Answer 1

You can’t reference columns in the parent SELECT from a sub-select. The sub-select is basically run in isolation. What you need to do is re-write this as a JOIN. Your query is quite convoluted and hard to follow, and I think incomplete, so I can only approximate a solution.

Here’s an example:

SELECT field_id,
  employee_field_info.field_value,
  IF(employee_field_info.field_value IS NULL, "0","1") AS value_bool
  FROM persona_fields
  LEFT JOIN employee_field_info ON employee_field_info.field_id=persona_fields.field_id
  WHERE employee_id=1