What should I know about SQL?

Chris Saxon from the Developer Advocates group asked the following question on Twitter.

What three things do you think every developer must know about #SQL ?

Reply and share yours!

— Chris Saxon (@chrisrsaxon) July 5, 2016

My immediate thought regarded features I'd be disappointed to live without. Looking at some other responses I realise that the answer matches Tom Kyte's creed: "it depends".

Here is a collection of responses that I think sums(sql) really well, syntax pun = intended.

What feature should I know, to be productive & efficient?
(me)
1. analytics
2. scalar subqueries
2. with clause

What fundamental principles should I understand?
(Stew Ashton)
1. DDL: data design is key to good SQL
2. ACID
3. Bind variables

What frequent mechanics/behaviour should I be consistently aware of?
(me)
1. null behaviour
2. date arithmetic
3. data type comparison issues, for lack of better desc

All the while remembering architecture is so important.
(Dani Schnider)
1. SQL is a set-based language
2. Clean data model is important
3. Constraints are not just for fun

and so is performance
(Draft A Center? Or Kyle Hailey posting from wrong account?)
1, Bitmap Indexes
2. Reading Explain Plans
3. SQL Tracing

Why there are often exceptions to rules:
(That Jeff Smith)
4. getting results back doesn't mean your code is right.
5. learning never stops

For the creative out there.
(David Fitzjarrell)
6. There is usually more than one way to write a query.
7. Sometimes you need to think 'different'

And there's always the "comedian".
(Connor McDonald)
select, from, where :-)

That's quite the collection!