[DaiPlusPlus]

> Flipside, allegedly it is nearly undecipherable

It's readable - though obviously you're going to need experience in RDBMS theory (Codd's paper, etc) to grok it.

It is very verbose, though.

Fortunately, the only people who need the ISO SQL spec are the vendors: the people writing SQL engines and tooling: it's next-to-useless for people who are designing databases or writing queries against them, chiefly because no RDBMS implementation, ever, has come close to implementing the full specification, and everyone has their own extensions to SQL - so all the vendors put out their own documentation and the world's largely been happy with that for the past ~40 years that SQL's been relevant.

It's not too hard to find the standards in PDF form if you know where to look - and as someone who does a fair bit of SQL, I'll say I've only ever referred to the standards to back-up my more controversial posts on StackOverflow.

[_a_a_a_]

I'm heading in the direction which is engines/tooling and I'm amazed at the number of corners of SQL that I have discovered that I don't know properly. You can use it fine for decades but when you need to know precisely what is acceptable to the GROUP BY/HAVING clauseS... then you suddenly realise you don't. It's quite surprising.

it would be useful to have that standard. Thanks, will have a hunt.

[DaiPlusPlus]

Are you at-least familiar with the concept of relational-division?

[_a_a_a_]

Very. Why?

[DaiPlusPlus]

In that case I'm curious what it is about GROUP BY and HAVING that you recently learned - everyone I know that groks division knows their RDBMS-of-choice's SQL dialect inside out - it's like it's impossible to learn one without the other.

[_a_a_a_]

I use both in SQL regularly and without any problem, done so for decades. The problem is if you're actually implementing something then those rules you informally understand suddenly start to look less obvious when you try to write them down. Even SQLite got it wrong:

.

6. Aggregate Queries Can Contain Non-Aggregate Result Columns That Are Not In The GROUP BY Clause

For example to find the highest paid employee:

SELECT max(salary), first_name, last_name FROM employee;

.

https://www.sqlite.org/quirks.html

[DaiPlusPlus]

ISO SQL permits aggregate queries returning (“plain ol’ data”) non-aggregate/non-grouping-key columns - but only when the engine can prove a functional-dependency from the grouping-keys - this isn’t exactly the same thing as what SQLite supports (and MySQL 4-5’s weird behaviour was disabled by default in v8) - but Postgres supports it now too.

I don’t know exactly where in the spec it’s defined, though - but I’ve seen people say it was introduced in SQL99.

[barrkel]

SQLite would describe this as a feature, because it's a PITA do that type of query otherwise - window functions with rank() or row_number(), or a self-join. (Obviously this specific query could be done with ORDER BY salary DESC LIMIT 1 but if you want the highest paid employee per team or department etc...)