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Re: SQL question re aggregates & joins

From: "Dann Corbit" <DCorbit(at)connx(dot)com>
To: "Scott Ribe" <scott_ribe(at)killerbytes(dot)com>,"Thom Brown" <thombrown(at)gmail(dot)com>
Cc: "pgsql-general" <pgsql-general(at)postgresql(dot)org>
Subject: Re: SQL question re aggregates & joins
Date: 2010-01-28 22:29:52
Message-ID: D425483C2C5C9F49B5B7A41F8944154702962754@postal.corporate.connx.com (view raw or flat)
Thread:
Lists: pgsql-general
> -----Original Message-----
> From: pgsql-general-owner(at)postgresql(dot)org [mailto:pgsql-general-
> owner(at)postgresql(dot)org] On Behalf Of Scott Ribe
> Sent: Thursday, January 28, 2010 2:10 PM
> To: Thom Brown
> Cc: pgsql-general
> Subject: Re: [GENERAL] SQL question re aggregates & joins
> 
> > You can't include an aggregate in the select if you don't group by
> > non-aggregates, so it should be:
> >
> > select max(t1."When"), t1."Pt_Id", t2."DateOfBirth"
> > from "PtStaffAccess" t1, "Person" t2
> > where t1."Pt_Id" = t2.id
> > group by t1."Pt_Id", t2."DateOfBirth";
> 
> I was aware that I could alternatively group by all the columns, but
> that
> actually just highlights the redundancy even more--consider your second
> example:
> 
> select max(t1."When"), t2.id , t2."DateOfBirth"
> from "PtStaffAccess" t1, "Person" t2
> where t1."Pt_Id" = t2.id
> group by t2.id, t2."DateOfBirth";
> 
> Given that t2.id is the primary key, grouping by any other column of t2
> is
> really redundant. I know *what* SQL won't allow me to do, I'm
> interested in
> knowing if there's some reason *why* other than historical...

Having a group by on a primary key in a single table is of no value.  After all, the group by accomplishes nothing at all.

Having a group by on the primary key of one table in a join does not guarantee uniqueness of the join result.

Consider the simplified schema
Table Customers:
Custname varchar(80)
Custaddr varchar(80)
Custid int primary key
Data:
{'Joe', 'Paris France', 1}
{'Fred', 'Seattle USA', 2}

Table Orders:
OrderID int
Custid int
OrderItem int
OrderQty int
OrderPrice Numeric 12.2
Primary key: OrderID, CustID, OrderItem
Foreign key: CustiID references Customers(CustID)
Data:
{1, 1, 17, 2, 12.92},
{1, 1, 12, 5, 14.75},
{2, 2, 9, 1, 27.45},
{2, 2, 2, 8, 11.23},
{2, 2, 1, 1, 123.45}

SELECT c.Custid, c.CustName, c.Custaddr, o.OrderID, o.OrderItem, sum(o.OrderQty * o.OrderPrice) as dollars
>From Customers c, Orders o 
WHERE c.Custid = o.Custid
GROUP BY c.Custid

Now, c.Custid is the primary key for Customers and also a foreign key for Orders and yet the query is utter nonsense.
Had all of the primary key columns for the second table been included, then the group by is superfluous.  Having one or more of the primary key columns left out from the child table but added to the group by list changes the meaning and value of the result.

I have always thought this way for aggregate queries:
1. Add an aggregate function to each column that you are aggregating in some way.
2. Add a 'group by' for every column that is not being aggregated.
Anything else means that the query has not been thought through carefully.

IMO-YMMV


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