SELECT DISTINCT in Scala slick

问题

I am using Slick 1, and I have to be able to apply a filter in a query to lookup all entities that match a condition in a related table.

This example using the Slick documentation shows what I am trying to do (this is a contrived example that is close to my situation).

Here, I want all coffees that are provided by suppliers on the west coast. I want the Coffee only, I am only interested in navigating to Suppliers to apply the filter:

val westCoast = Seq("CA", "OR", "WA")
val implicitInnerJoin = for {
  c <- Coffees
  s <- Suppliers if c.supID === s.id && s.state inSet westCoast
} yield c

This works ok, but it will duplicate Coffees if there is more than one match in the Suppliers table.

The obvious workaround is in normal SQL to do a SELECT DISTINCT; however, I cannot find a way to do that here.

You could in theory do a:

query.list.distinct

After the results are already returned; however, I have also implemented PAGING support, so you wouldn't want to process the results once the already come back from the database. Here is the paging support:

query.drop(offset).take(limit).list

So, in a nutshell, I need a way to specify SELECT DISTINCT in my query that goes out.

Anyone have any ideas?

回答1:

As a work around you can try to use groupBy:

query.groupBy(x=>x).map(_._1)

It should have the same semantics as distinct, but I'm not sure about performance.

回答2:

With slick 3.1.0 you can use distinct and distinctOn functions (Slick 3.1.0 release notes). For example:

val westCoast = Seq("CA", "OR", "WA")
val implicitInnerJoin = for {
  c <- Coffees
  s <- Suppliers if c.supID === s.id && s.state inSet westCoast
} yield c

db.run(implicitInnerJoin.distinctOn(_.name).result)

回答3:

I don't think this implemented yet. See https://github.com/slick/slick/issues/96

回答4:

For distinct on multiple columns coffee.name and coffee.price:

val westCoast = Seq("CA", "OR", "WA")
val implicitInnerJoin = for {
  c <- Coffees
  s <- Suppliers if c.supID === s.id && s.state inSet westCoast
} yield c

db.run(implicitInnerJoin.map(f => (f.name, f.price, f.state)).distinctOn(p => (p._1, p._2)).result)

来源：https://stackoverflow.com/questions/18256768/select-distinct-in-scala-slick

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