CODEWITHSUNDEEP
f#computation-expressionquery-expressions
Jason Kleban
Jason Kleban

Reputation: 20778

query expression self-join

How can I change this query expression's join clause so that I don't have to wrap parent.ID in an option just to join on a candidate child.ParentID which might be None?

query { for parent in d.People do
        join child in d.People on (Some parent.ID = child.ParentID)
        exists (child.Birthdate <= parent.Birthdate) }

Thanks!

Upvotes: 1

Views: 195

Answers (1)

phoog
phoog

Reputation: 43046

You might like this. There may be a more elegant syntax for this approach, and I expect in the end it is not going to perform much differently, if at all, but here it is:

query { for child in d.People do
        where (child.ParentID.IsSome)
        join parent in d.People on (child.ParentID.Value = parent.ID)
        exists (child.Birthdate <= parent.Birthdate) }

I first came up with this, but I wondered whether it implies calling Value before filtering out the None values:

query { for parent in d.People do
        join child in d.People on (parent.ID = child.ParentID.Value)
        where (child.ParentID.IsSome)
        exists (child.Birthdate <= parent.Birthdate) }

To keep parent first and also put the child filter before the join condition, you can do a subquery, but that seemed a worse solution, not better. If someone knows a way of doing that without the subquery, please add a comment.

Upvotes: 3

Related Questions