Quasi Believable Quantifier Laws
with occasional pedestrian SQL topics
Recent Comments
Vadim Tropashko on Formula for Formatting Mikołaj on Formula for Formatting Vadim Tropashko on Formula for Formatting Mikołaj on Formula for Formatting Vadim Tropashko on Formula for Formatting
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July 3, 2010 at 3:20 pm
I disagree that Relational Division comes once in a lifetime. I use it all the time for reports that look for “shopping basket” patterns in sales data.
February 8, 2014 at 10:14 am
Вадим, огромное спасибо за отличную книгу!!!
С уважением, Михаил.
February 8, 2014 at 10:15 am
в ссылке на вторую главу есть маленькая опечатка (pdff вместо pdf)
…book_sql_chap2_v2.pdff
еще раз спасибо за книгу!!!!!!
February 10, 2014 at 8:13 pm
Hi Vadim, a thread on another forum has mentioned an exotic operator you don’t include:
SQL’s UNION CORRESPONDING seems to be the same as the RL’s inner union.
(Hugh Darwen says that this operator was available in ISBL/BS12, called plain UNION. So not the same as SQL’s plain UNION.)
I don’t see any evidence that anybody realised this operation is the inverse of Nat Join.
February 11, 2014 at 3:43 am
I was not aware of UNION CORRESPONDING, thank you!
May 17, 2015 at 3:38 pm
Hi Vadim
Just bought your book and reading chapter 1 counting. I have a minor quibble about the scalar subquery corresponding to the predecessor example
I ran your examples on the northwind sample database using the web interface on w3 schools http://www.w3schools.com/sql/trysql.asp?filename=trysql_select_all
and replaced their query versions of your example queries on their database. It worked a treat. But my version of the scalar subquery
SELECT t.ContactName t, t.CustomerID id,
(SELECT count(*) from Customers tt WHERE tt.CustomerID < t.CustomerId) seq
FROM Customers t
GROUP BY t.CustomerID
;
is not exactly the same query as you gave – no #, and <= is replaced with <
But it also return zero as a result which did not occur in my version of the predecessor query
SELECT t.ContactName t, t.CustomerID id, Max(tt.CustomerID) predecessor FROM Customers t, Customers tt WHERE tt.CustomerID < t.CustomerID
GROUP BY t.CustomerID
;
thanks
Dave