Support 'exists' on tables.

Sources/StructuredQueriesCore/Statements/Select.swift

@@ -294,6 +294,13 @@ extension Table {
   ) -> Select<Int, Self, ()> {
     Where().count(filter: filter)
   }
+
+  /// A select statement for if this table contains any rows.
+  ///
+  /// - Returns: A select statement that selects `exists()`.
+  public static func exists() -> some PartialSelectStatement<Bool> {
+    ExistsSelect<Self, _>(statement: all)
+  }
 }
 
 public struct _SelectClauses: Sendable {
@@ -1525,3 +1532,11 @@ private struct CopyOnWrite<Value> {
 extension CopyOnWrite: Sendable where Value: Sendable {}
 
 extension CopyOnWrite.Storage: @unchecked Sendable where Value: Sendable {}
+
+private struct ExistsSelect<From: Table, S: Statement>: SelectStatement {
+  typealias QueryValue = Bool
+  let statement: S
+  var query: QueryFragment {
+    "SELECT EXISTS\(statement)"
+  }
+}

Tests/StructuredQueriesTests/SelectTests.swift

@@ -1214,6 +1214,23 @@ extension SnapshotTests {
         }
       }
     }
+
+    @Test func exists() {
+      assertQuery(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 │
+        └──────┘
+        """
+      }
+    }
   }
 }