How to select DISTINCT results in an SQL JOIN query - sql

Question

this is my query so please check it and tell me. in this query is execute successfully but distinct is not working:
SELECT
DISTINCT(ticket_message.ticket_id),
support_ticket.user_id,
support_ticket.priority,
support_ticket.subject,
support_ticket.status,
ticket_message.message
FROM
support_ticket
LEFT OUTER JOIN ticket_message ON support_ticket.ticket_id = ticket_message.ticket_id
LEFT OUTER JOIN assign_ticket ON ticket_message.ticket_id = assign_ticket.ticket_id

Answer 1

The word distinct is a modifier to the keyword SELECT. So you need to think of it as SELECT DISTINCT and it ALWAYS operates across the entire row. It simply ignores the parentheses seen in the following:
select distinct(ticket_message.ticket_id)
because distinct is NOT a function.
So. What we appear to have is a support ticket with associated messages. There are usually multiple messages per support ticket, so I suspect what you want is more complex. For example you might want just the most recent message for each support ticket.
To achieve most recent we need a timestamp (or "datetime") column and we also need to know if your database supports "window functions". Let's assume you have a timestamp column called message_at and you database does support window functions, then this would reduce the number of rows:
SELECT
support_ticket.ticket_id
, support_ticket.user_id
, support_ticket.support_section
, support_ticket.priority
, support_ticket.subject
, support_ticket.status
, tm.file
, tm.message
, assign_ticket.section_id
, assign_ticket.section_admin_id
FROM support_ticket
LEFT OUTER JOIN (
SELECT
ticket_id
, file
, message
, ROW_NUMBER() OVER (PARTITION BY ticket_id ORDER BY message_at DESC) AS row_num
FROM ticket_message
) tm ON support_ticket.ticket_id = tm.ticket_id
AND tm.row_num = 1
LEFT OUTER JOIN assign_ticket ON tm.ticket_id = assign_ticket.ticket_id
ROW_NUMBER() OVER (PARTITION BY ticket_id ORDER BY message_at DESC) assigns the number 1 to the most recent message, and later we ignore all rows that are > 1 thus removing unwanted repetition in the results.
So.
We really need to know much more about your actual data, the database (and version) you are using and your real needs. It is almost certain that select distinct is NOT the right technique for what you are trying to achieve.
I suggest you read these: Provide a Minimal Complete Verifiable Example (MCVE)
and Why should I provide a MCVE

Answer 2

Use this statement:
SELECT DISTINCT
ticket_message.ticket_id
FROM
support_ticket
LEFT OUTER JOIN ticket_message ON
support_ticket.ticket_id = ticket_message.ticket_id
LEFT OUTER JOIN assign_ticket ON
ticket_message.ticket_id = assign_ticket.ticket_id
As soon as you add more columns to your query, DISTINCT also takes them into account as well.