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Re: nested query problem

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On 09/06/2018 01:59 PM, David Gauthier wrote:
I'm having trouble with this query...

select sr.sqf_id, sr.status, fse.perl_sub_name, fse.end_datetime
   from
     sqf_runs sr,
    (select perl_sub_name, end_datetime from flow_step_events_view where sqf_id = sr.sqf_id order by 2 limit 1) fse
   where sr.userid='foo';

ERROR:  invalid reference to FROM-clause entry for table "sr"
LINE 4: ...atetime from flow_step_events_view where sqf_id = sr.sqf_id ...
                                                              ^
HINT:  There is an entry for table "sr", but it cannot be referenced from this part of the query.

This calls for a lateral join:

    SELECT  sr.sqf_id, sr.status, fse.perl_sub_name, fse.end_datetime
    FROM    sqf_runs sr
    LEFT OUTER JOIN LATERAL (
      SELECT  perl_sub_name, end_datetime
      FROM    flow_step_events_view fsev
      WHERE   fsev.sqf_id = sr.sqf_id
      ORDER BY 2
      LIMIT 1
    ) fse
    ON      true
    WHERE   sr.userid = 'foo'
    ;

It's nearly what you had already, but `LATERAL` lets the subquery reference columns in the other tables.

A lateral join is conceptually a lot like running your subquery in for loop, looping over all the rows produced by the rest of the query. It doesn't have to produce 1 row for each iteration, but saying `LIMIT 1` ensures that here.

The `ON true` is just pro forma because you can't have a join without an `ON` clause.

You might prefer an INNER JOIN LATERAL, depending on your needs.

--
Paul              ~{:-)
pj@xxxxxxxxxxxxxxxxxxxxxxxx




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