21

I have made a draft remote application on top of libpq for PostrgreSQL. It behaves well, but I have profiled the general functioning of the application. For each final business result that I produce, it happens that I call something like 40 select clause (over tcpip).

I have reminiscences from SQL-Server reminding me to minimize the number of interactions between my remote application and the database. Having analyzed my selects, I do think I could reduce this number to 3 SELECT clauses, using joins. But I don't remember the syntax for using the result of a SELECT in another SELECT.

E.g.:

SELECT * FROM individual
INNER JOIN publisher
ON individual.individual_id = publisher.individual_id
WHERE individual.individual_id = 'here I would like to use the results of a another select'

This other SELECT would be simply of the kind:

SELECT identifier FROM another_table WHERE something='something'

Here is the simplified tables layout, declined a number of times for different item_types ... (3 totally different types, hence the 3 SQL queries if optimized).

table passage
  id_passage PK
  business_field_passage bytea

table item
  id_item PK
  id_passage FK
  business_field_item text

table item_detail
  id_item_detail PK
  id_item FK
  business_field_item_detail text
  image_content bytea

There are several id_item for one id_passage.
There are several id_item_detail for one id_item.

How would you write that?
What is the name for describing the action of redirecting one select into another (if any)?

30

Is this what you're aiming for? Make sure the fields that are being compared are comparable (i.e. both fields are numeric, text, boolean, etc).

SELECT * FROM Individual
INNER JOIN Publisher
ON Individual.IndividualId = Publisher.IndividualId
WHERE Individual.IndividualId = (SELECT someID FROM table WHERE blahblahblah)

If you wish to select based on multiple values:

SELECT * FROM Individual
INNER JOIN Publisher
ON Individual.IndividualId = Publisher.IndividualId
WHERE Individual.IndividualId IN (SELECT someID FROM table WHERE blahblahblah)
  • can it be that straight forward ?? does it still work if SELECT someID FROM table WHERE blahblahblah has multiple records ? I'm gonna check that right now. – Stephane Rolland Jan 25 '13 at 16:38
  • Which query is selecting multiple records? It can work if you're selecting multiple records, but if you could show us your table layouts that would help us refine the answer. – Angry Spartan Jan 25 '13 at 16:40
  • 1
    WHERE Individual.IndividualId IN... looks good. – Stephane Rolland Jan 25 '13 at 17:00
10

You can just rewrite that as another JOIN. This is normally simplest and fastest:

SELECT i.*, p.*
FROM   individual    i
JOIN   publisher     p USING (individualid)
JOIN   another_table a ON a.identifier = i.individualid
WHERE  a.something = 'something'

I also simplified somewhat and did away with the gratuitous CamelCase spelling of identifiers.

  • 1
    Yes, this. I die a little inside whenever I see IN (SELECT ..) syntax. – Mark Storey-Smith Jan 26 '13 at 12:56
  • @MarkStorey-Smith Do you mean that it's more than simpler and faster: this a standard of sql coding to use another join instead of a in ( select...) In such case I should also attribute the good answer to Erwin. – Stephane Rolland Jan 26 '13 at 19:27
  • 1
    @StephaneRolland Whether its faster or not will be platform and version dependent. SQL Server 2008+ for example will generate identical execution plans for INNER JOIN and IN (SELECT ...) syntax. No idea on whether same applies to PostgreSql. Performance aside, the IN (SELECT ...) style leaves me wondering if the author has fully grasped the semantics and concepts of SQL. AngrySpartan has answered your original question correctly. ErwinBrandstetter has shown you the way you should do it :). – Mark Storey-Smith Jan 26 '13 at 19:41
  • 6
    @MarkStorey-Smith: a JOIN is not always equivalent to an IN condition. The question is not which one is faster, the question is which one is correct. – a_horse_with_no_name Dec 11 '15 at 11:48

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