Add support for the SET command to assign values to variables

[lukaseder]

In databases supporting procedural languages, there is always a way to assign a value to a variable.

• In SQL Server, we can generate the SET command for this. It can even take SELECT statements as argument expressions
• In Oracle, a SET command translates either to an assignment var := value; or ...
• ... to a SELECT .. INTO statement

There may be other syntaxes

---

For variable declarations, see #7305 (this depends on declarations)

[sainthDE]

Maybe it's also possible to support the SET LOCAL of PostgreSQL. So it will be possible to support RLS shown like here.

[lukaseder]

@sainth- Thanks for your comment. That's a different type of SET command. This issue is about assigning a value to a previously declared local variable, within a statement batch, anonymous block, or stored procedure - i.e. part of the procedural languages. Yours is part of the SQL language.

[lukaseder]

(by which I mean to say: I sense a completely separate use-case in your message, which I'm more than happy to discuss in a new github issue!)

[sainthDE]

@lukaseder Sorry, my mistake. I will create a new issue.