Question

Im trying to do a query with the following. I have simplified the query as much as possible for purposes of the question. It is basically for an email schedular app I am writing.

Two tables. An email can belong to a particular schedular. A schedular comprises of several emails. Each email that has been sent is marked with a specific step number. step1, step2, step3, step4, step5

email_schedular (id, datecreated, recipient)
email(id, email_schedular_id, text, subject, sent, stepNumber)

Now what im trying to do is determine in a report if a step has been sent or not. I have had a go at it but im not sure if im going about it the right way?

SELECT
  ed.id,
  ed.datecreated
  ed.recipient
  if(e.stepNumber= 1, 'Y', 'N')  AS step_1_sent,
  if(e.stepNumber= 2, 'Y', 'N')  AS step_2_sent,
  if(e.stepNumber= 3, 'Y', 'N')  AS step_3_sent,
  if(e.stepNumber= 4, 'Y', 'N')  AS step_4_sent
FROM 
  email_echedular ed
JOIN email e ON (e.email_echedular_id = ed.id)
GROUP BY ed.id

Answer 1

Instead of using an N for not sent, use a NULL. Then when you group them, you can take the MAX() value for each and eliminate the NULLs down to one row. The end result should be one row per ed.id with a Yes for sent steps and NULL for unsent steps.

SELECT
  ed.id,
  ed.datecreated
  ed.recipient
  MAX(if(e.stepNumber= 1, 'Y', NULL)) AS step_1_sent,
  MAX(if(e.stepNumber= 2, 'Y', NULL)) AS step_2_sent,
  MAX(if(e.stepNumber= 3, 'Y', NULL)) AS step_3_sent,
  MAX(if(e.stepNumber= 4, 'Y', NULL)) AS step_4_sent
FROM 
  email_schedular ed
  LEFT JOIN email e ON (e.email_schedular_id = ed.id)
GROUP BY ed.id

If you really don’t want a NULL for the non-sent columns, you can wrap those in COALESCE() to replace the NULL with N:

SELECT
  ed.id,
  ed.datecreated
  ed.recipient
  COALESCE(MAX(if(e.stepNumber= 1, 'Y', NULL)), 'N') AS step_1_sent,
  COALESCE(MAX(if(e.stepNumber= 2, 'Y', NULL)), 'N') AS step_2_sent,
  COALESCE(MAX(if(e.stepNumber= 3, 'Y', NULL)), 'N') AS step_3_sent,
  COALESCE(MAX(if(e.stepNumber= 4, 'Y', NULL)), 'N') AS step_4_sent
FROM 
  email_schedular ed
  LEFT JOIN email e ON (e.email_schedular_id = ed.id)
GROUP BY ed.id

Edit Changed to LEFT JOIN and fixed join condition…