Make `NOT EXISTS (subquery)` and `NOT IN (subquery)` more consistent in the AST

[andygrove]

The AST has one variant to support both IN (subquery) and NOT IN (subquery):

InSubquery {
    expr: Box<Expr>,
    subquery: Box<Query>,
    negated: bool,
},

However, the variant for EXISTS does not handle NOT EXISTS:

Exists(Box<Query>)

NOT EXISTS is represented in the AST by wrapping Exists in a UnaryOp with UnaryOperator::Not.

It would be nice if we could use a more consistent approach.

[alamb]

I would prefer to see

Exists {
      expr: Box<Expr>,
      subquery: Box<Query>,
}

[frolovdev]

I guess I can help 🚀

[frolovdev]

@alamb but what do you mean by expr in Exists, because your comment isn't clear for me right now

Maybe you mean smth like that

Exists {
      negated: bool,
      subquery: Box<Query>,
}

because we don't have any Expr::identifier in EXISTS and NOT EXISTS

[alamb]

Maybe you mean smth like that

Yes, that is what I mean -- thank you @frolovdev

[frolovdev]

Yes, that is what I mean -- thank you @frolovdev

Got your point, thanks for the clarification, here's the pr :d

#523