Add static Table.exists()

Author: stephencelis

Sources/StructuredQueriesCore/Operators.swift

@@ -921,6 +921,15 @@ extension Statement {
   }
 }
 
+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> {
+    SQLQueryExpression("EXISTS \(all)")
+  }
+}
+
 private struct UnaryOperator<QueryValue>: QueryExpression {
   let `operator`: QueryFragment
   let base: QueryFragment

Tests/StructuredQueriesTests/OperatorsTests.swift

@@ -568,6 +568,16 @@ 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: {
+        """
+        near "SELECT": syntax error
+        """
+      }
       assertQuery(Values(Reminder.where { $0.id == 1 }.exists())) {
         """
         SELECT EXISTS (