Add CTE support to createTemporaryView (pointfreeco#176)

* Add CTE support to `createTemporaryView`

* wip

Author: stephencelis

Package.resolved

@@ -1,6 +1,7 @@
 /// Creates a common table expression that can be used to factor subqueries, or create hierarchical
 /// or recursive queries of trees and graphs.
-public struct With<QueryValue>: Statement, Sendable {
+public struct With<Base: Statement>: Statement, Sendable {
+  public typealias QueryValue = Base.QueryValue
   public typealias From = Never
 
   var ctes: [CommonTableExpressionClause]
@@ -9,7 +10,7 @@ public struct With<QueryValue>: Statement, Sendable {
   @_disfavoredOverload
   public init(
     @CommonTableExpressionBuilder _ ctes: () -> [CommonTableExpressionClause],
-    query statement: () -> some Statement<QueryValue>
+    query statement: () -> Base
   ) {
     self.ctes = ctes()
     self.statement = statement().query
@@ -22,7 +23,21 @@ public struct With<QueryValue>: Statement, Sendable {
   where
     S.QueryValue == (),
     S.Joins == (repeat each J),
-    QueryValue == (S.From, repeat each J)
+    Base == Select<(S.From, repeat each J), S.From, (repeat each J)>
+  {
+    self.ctes = ctes()
+    self.statement = statement().query
+  }
+
+  @_disfavoredOverload
+  public init<S: SelectStatement>(
+    @CommonTableExpressionBuilder _ ctes: () -> [CommonTableExpressionClause],
+    query statement: () -> S
+  )
+  where
+    S.QueryValue == (),
+    S.Joins == (),
+    Base == Select<S.From, S.From, ()>
   {
     self.ctes = ctes()
     self.statement = statement().query
@@ -40,6 +55,8 @@ public struct With<QueryValue>: Statement, Sendable {
   }
 }
 
+extension With: PartialSelectStatement where Base: PartialSelectStatement {}
+
 extension QueryFragment {
   fileprivate var presence: Self? { isEmpty ? nil : self }
 }

Sources/StructuredQueriesCore/Statements/CommonTableExpression.swift

@@ -7,7 +7,7 @@ extension Table where Self: _Selection {
   ///   - ifNotExists: Adds an `IF NOT EXISTS` clause to the `CREATE VIEW` statement.
   ///   - select: A statement describing the contents of the view.
   /// - Returns: A temporary trigger.
-  public static func createTemporaryView<Selection: SelectStatement>(
+  public static func createTemporaryView<Selection: PartialSelectStatement>(
     ifNotExists: Bool = false,
     as select: Selection
   ) -> TemporaryView<Self, Selection>
@@ -21,7 +21,7 @@ extension Table where Self: _Selection {
 /// This type of statement is returned from ``Table/createTemporaryView(ifNotExists:as:)``.
 ///
 /// To learn more, see <doc:Views>.
-public struct TemporaryView<View: Table & _Selection, Selection: SelectStatement>: Statement
+public struct TemporaryView<View: Table & _Selection, Selection: PartialSelectStatement>: Statement
 where Selection.QueryValue == View {
   public typealias QueryValue = ()
   public typealias From = Never

Sources/StructuredQueriesSQLiteCore/Views.swift

@@ -62,6 +62,34 @@ extension SnapshotTests {
         """
       }
     }
+
+    @Test func ctes() {
+      assertQuery(
+        CompletedReminder.createTemporaryView(
+          as: With {
+            Reminder
+              .where(\.isCompleted)
+              .select { CompletedReminder.Columns(reminderID: $0.id, title: $0.title) }
+          } query: {
+            CompletedReminder.all
+          }
+        )
+      ) {
+        """
+        CREATE TEMPORARY VIEW
+        "completedReminders"
+        ("reminderID", "title")
+        AS
+        WITH "completedReminders" AS (
+          SELECT "reminders"."id" AS "reminderID", "reminders"."title" AS "title"
+          FROM "reminders"
+          WHERE "reminders"."isCompleted"
+        )
+        SELECT "completedReminders"."reminderID", "completedReminders"."title"
+        FROM "completedReminders"
+        """
+      }
+    }
   }
 }
 
@@ -70,7 +98,3 @@ private struct CompletedReminder {
   let reminderID: Reminder.ID
   let title: String
 }
-
-extension Table where Self: _Selection {
-  static func foo() {}
-}