Add static Table.exists() (#88)

* Add static `Table.exists()`

* wip

* wip

* wip

Author: stephencelis

Sources/StructuredQueriesCore/Operators.swift

@@ -913,12 +913,21 @@ extension Statement where QueryValue: QueryBindable {
   }
 }
 
-extension Statement {
+extension PartialSelectStatement {
   /// Returns a predicate expression indicating whether this subquery contains any element.
   ///
   /// - Returns: A predicate expression indicating whether this subquery contains any element.
   public func exists() -> some QueryExpression<Bool> {
-    SQLQueryExpression("EXISTS \(self)")
+    SQLQueryExpression("EXISTS \(self.queryFragment)")
+  }
+}
+
+extension Table {
+  /// Returns a predicate expression indicating whether this table contains any element.
+  ///
+  /// - Returns: A predicate expression indicating whether this subquery contains any element.
+  public static func exists() -> some QueryExpression<Bool> {
+    all.exists()
   }
 }
 

Tests/StructuredQueriesTests/OperatorsTests.swift

@@ -568,6 +568,20 @@ extension SnapshotTests {
     }
 
     @Test func exists() {
+      assertQuery(Values(Reminder.exists())) {
+        """
+        SELECT EXISTS (
+          SELECT "reminders"."id", "reminders"."assignedUserID", "reminders"."dueDate", "reminders"."isCompleted", "reminders"."isFlagged", "reminders"."notes", "reminders"."priority", "reminders"."remindersListID", "reminders"."title"
+          FROM "reminders"
+        )
+        """
+      } results: {
+        """
+        ┌──────┐
+        │ true │
+        └──────┘
+        """
+      }
       assertQuery(Values(Reminder.where { $0.id == 1 }.exists())) {
         """
         SELECT EXISTS (