Possible feature: Add support for conditional joins over ConstantExpression

Currently, there is no way how to do conditional join.

I can use trick with multi-column (tuples):

let query = 
    select {
        for l in table<MultiJoinLeft> do
        leftJoin r in table<MultiJoinRight> on ((123, l.Key2) = (r.Key1, r.Key2)) 
        selectAll
    }

…but this fails with NotImplementedException, because of ConstantExpression.

Need to investigate how difficult this could be to add…

cc: @JordanMarr : Any thoughts on this? 🤔

[JordanMarr]

I think the tuple on syntax would map to LeftJoinOnMany which would probably be the easiest approach that would avoid having to change the InnerJoin and LeftJoin DUs.

But if I am understanding you correctly, implementing the “ideal syntax” example would involve changing to something like this:

type Join =
    | InnerJoin of table:string * onClause:Where
    | LeftJoin of table:string * onClause:Where
    | InnerJoinOnMany of table:string * List<string * string>
    | LeftJoinOnMany of table:string * List<string * string>

Which seems like that might be a good way to go. In fact that might even eliminate the need for InnerJoinOnMany and LeftJoinOnMany, which could actually be a nice consolidation:

type Join =
    | InnerJoin of table:string * onClause:Where
    | LeftJoin of table:string * onClause:Where

[JordanMarr]

That’s exactly what I did for the builder/expressions part. I will start a draft PR with what I have done so far.