The SQL standard knows an interesting feature where you can project any functional dependencies of a primary (or unique) key that is listed in the GROUP BY clause without having to add that functional dependency to the GROUP BY clause explicitly. What does this mean? Consider this simple schema: In order to count the number … Continue reading Functional Dependencies…
#sql standard
6 posts
29 Oct 2021
24 Apr 2019
I found a very interesting SQL question on Twitter recently: Hi @sfonplsql we have some scenario, Let us 01Jan Mkt Value 100, 02Jan 120, next entry available 25th Jan 125, from 3rd Jan 24 Jan, our value should be 120. How to arrive ? Thanks @oraclebase — Vikki (@vikkiarul) April 23, 2019 Rephrasing the question: … Continue reading Using IGNORE…
25 Feb 2015
The Modern SQL Twitter account (by Markus Winand) published a hint about how to extract a date part in SQL: The right way to get a part of a date/time is: EXTRACT(YEAR FROM CURRENT_DATE) = 2015http://t.co/UNLyUoQdVb Retweet to spread the word! — Modern SQL (@ModernSQL) February 24, 2015 Is it true? Yes it is, in … Continue reading How to…
20 Nov 2014
MySQL is a database that has been bending the SQL standard in ways that make it hard to move off MySQL. What may appear to be a clever technique for vendor lockin (or maybe just oversight of the standard) can be quite annoying in understanding the real meaning of the SQL language. One such example … Continue reading Use MySQL’s…
24 Jun 2014
Infrequent SQL developers often get confused about when to put parentheses and/or aliases on derived tables. There has been this recent Reddit discussion about the subject, where user Elmhurstlol was wondering why they needed to provide an alias to the derived table (the subselect with the UNION) in the following query: The question really was … Continue reading Should I…
15 Apr 2014
Haven’t we all been wondering: How can I do this? I have these data in Excel and I want to group / sort / assign / combine … While you could probably pull up a Visual Basic script doing the work or export the data to Java or any other procedural language of choice, why … Continue reading How can…