Question

I have a query

SELECT 
        cd.signoffdate,
        min(cmp.dsignoff) as dsignoff
      FROM clients AS c 
      LEFT JOIN campaigns AS cmp ORDER BY dsignoff;

If I want to have something like this built into the postgres query will it work and how do I do it

if the cd.signoffdate is empty it should take min(cmp.dsignoff) as dsignoff as the value and then order by this column, so in other words it should order by dsignoff and cd.signoffdate and tread it as one column, is this possible and how?

Answer 1

Your query could look like this:

SELECT c.client_id, COALESCE(c.signoffdate, min(cmp.dsignoff)) AS signoff
FROM   clients c 
LEFT   JOIN campaigns cmp ON cmp.client_id = c.client_id  -- join condition!
GROUP  BY c.client_id, cd.signoffdate                     -- group by!
ORDER  BY COALESCE(c.signoffdate, min(cmp.dsignoff));

Or, with simplified syntax:

SELECT c.client_id, COALESCE(c.signoffdate, min(cmp.dsignoff)) AS signoff
FROM   clients c 
LEFT   JOIN campaigns cmp USING (client_id)
GROUP  BY 1, cd.signoffdate
ORDER  BY 2;

Major points:

• Used alias c, but referenced as cd.
• No join condition leads to cross join, probably not intended.
• Missing GROUP BY.
• I assume that you want to group by the primary key column of clients and call it client_id.
• I also assume that client_id links the two tables together.
• COALESCE() serves as fallback in case signoffdate IS NULL.