issues-118 Add SubQuery operator

[ikrivosheev]

PR Info

• Closes https://github.com/SeaQL/sea-query/issues/118
• https://github.com/SeaQL/sea-query/issues/216

Adds

• SubQuery operators: EXISTS, ALL, ANY, SOME

[ikrivosheev]

@billy1624 @tyt2y3 can you review?) I have done.

[ikrivosheev]

@billy1624 @tyt2y3 hello! I have small question. Expr::ne have signature: pub fn ne<V: Into<Value>>(self, v: V) -> SimpleExpr. If i rewrite this method to: pub fn ne<V: Into<SimpleExpr>>(self, v: V) -> SimpleExpr will not be so convenient to use for simple queries and it is breaking change. Should I add new method?

What is the best way to name new methods? For example:

1. ne_expr?
2. expr_ne?
3. Something else?)

[billy1624]

Hey @ikrivosheev, good suggestions!! I do think we need eq_expr and ne_expr inside Expr. That's definitely nice to have.

[ikrivosheev]

Hey @ikrivosheev, good suggestions!! I do think we need eq_expr and ne_expr inside Expr. That's definitely nice to have.

@billy1624 hello! Done

[ikrivosheev]

Hmm, @billy1624, I think about new method... Maybe is better impl From<T> for SimpleExpr, where T is all types for Value? And after that we don`t need new method. What do you think?

[ikrivosheev]

Hmm, @billy1624, I think about new method... Maybe is better impl From<T> for SimpleExpr, where T is all types for Value? And after that we don`t need new method. What do you think?

@tyt2y3 what do you think about it?

[ikrivosheev]

@billy1624 @tyt2y3 done! I add impl<T> From<T> for SimpleExpr and everything works without the new method!

[ikrivosheev]

@billy1624 @tyt2y3 can you review this PR?