Add Table.none (#119)

* Add `Table.none`

Acts as a "black hole" query, rendering as an empty SQL string and not
executed by the database.

* wip

* wip

Author: stephencelis

Package.resolved

@@ -22,6 +22,9 @@
 
 ### Transforming queries
 
+- ``unscoped``
+- ``all``
+- ``none``
 - ``map(_:)``
 - ``subscript(dynamicMember:)``
 - ``StructuredQueriesCore/+(_:_:)``

Sources/StructuredQueriesCore/Documentation.docc/Extensions/Select.md

@@ -0,0 +1,5 @@
+enum Scope {
+  case unscoped
+  case `default`
+  case empty
+}

Sources/StructuredQueriesCore/Internal/Scope.swift

@@ -29,20 +29,28 @@ public struct With<QueryValue>: Statement {
   }
 
   public var query: QueryFragment {
+    guard !statement.isEmpty else { return "" }
+    let cteFragments = ctes.compactMap(\.queryFragment.presence)
+    guard !cteFragments.isEmpty else { return "" }
     var query: QueryFragment = "WITH "
     query.append(
-      "\(ctes.map(\.queryFragment).joined(separator: ", "))\(.newlineOrSpace)\(statement)"
+      "\(cteFragments.joined(separator: ", "))\(.newlineOrSpace)\(statement)"
     )
     return query
   }
 }
 
+extension QueryFragment {
+  fileprivate var presence: Self? { isEmpty ? nil : self }
+}
+
 public struct CommonTableExpressionClause: QueryExpression {
   public typealias QueryValue = ()
   let tableName: QueryFragment
   let select: QueryFragment
   public var queryFragment: QueryFragment {
-    "\(tableName) AS (\(.newline)\(select.indented())\(.newline))"
+    guard !select.isEmpty else { return "" }
+    return "\(tableName) AS (\(.newline)\(select.indented())\(.newline))"
   }
 }
 

Sources/StructuredQueriesCore/Statements/CommonTableExpression.swift

@@ -66,6 +66,8 @@ private struct CompoundSelect<QueryValue>: PartialSelectStatement {
   }
 
   var query: QueryFragment {
-    "\(lhs)\(.newlineOrSpace)\(`operator`.indented())\(.newlineOrSpace)\(rhs)"
+    guard !lhs.isEmpty else { return rhs }
+    guard !rhs.isEmpty else { return lhs }
+    return "\(lhs)\(.newlineOrSpace)\(`operator`.indented())\(.newlineOrSpace)\(rhs)"
   }
 }

Sources/StructuredQueriesCore/Statements/CompoundSelect.swift

@@ -23,7 +23,7 @@ extension PrimaryKeyedTable {
   /// - Parameter row: A row to delete.
   /// - Returns: A delete statement.
   public static func delete(_ row: Self) -> DeleteOf<Self> {
-    Delete()
+    delete()
       .where {
         $0.primaryKey.eq(TableColumns.PrimaryKey(queryOutput: row[keyPath: $0.primaryKey.keyPath]))
       }
@@ -36,6 +36,7 @@ extension PrimaryKeyedTable {
 ///
 /// To learn more, see <doc:DeleteStatements>.
 public struct Delete<From: Table, Returning> {
+  var isEmpty: Bool
   var `where`: [QueryFragment] = []
   var returning: [QueryFragment] = []
 
@@ -107,6 +108,7 @@ public struct Delete<From: Table, Returning> {
       returning.append("\(quote: resultColumn.name)")
     }
     return Delete<From, (repeat each QueryValue)>(
+      isEmpty: isEmpty,
       where: `where`,
       returning: Array(repeat each selection(From.columns))
     )
@@ -126,6 +128,7 @@ public struct Delete<From: Table, Returning> {
       returning.append("\(quote: resultColumn.name)")
     }
     return Delete<From, From>(
+      isEmpty: isEmpty,
       where: `where`,
       returning: returning
     )
@@ -139,6 +142,7 @@ extension Delete: Statement {
   public typealias QueryValue = Returning
 
   public var query: QueryFragment {
+    guard !isEmpty else { return "" }
     var query: QueryFragment = "DELETE FROM "
     if let schemaName = From.schemaName {
       query.append("\(quote: schemaName).")

Sources/StructuredQueriesCore/Statements/Delete.swift

@@ -297,6 +297,7 @@ extension Table {
 }
 
 public struct _SelectClauses: Sendable {
+  var isEmpty = false
   var distinct = false
   var columns: [any QueryExpression] = []
   var joins: [_JoinClause] = []
@@ -320,6 +321,11 @@ public struct Select<Columns, From: Table, Joins> {
   // NB: A parameter pack compiler crash forces us to heap-allocate this storage.
   @CopyOnWrite var clauses = _SelectClauses()
 
+  fileprivate var isEmpty: Bool {
+    get { clauses.isEmpty }
+    set { clauses.isEmpty = newValue }
+    _modify { yield &clauses.isEmpty }
+  }
   fileprivate var distinct: Bool {
     get { clauses.distinct }
     set { clauses.distinct = newValue }
@@ -362,6 +368,7 @@ public struct Select<Columns, From: Table, Joins> {
   }
 
   fileprivate init(
+    isEmpty: Bool,
     distinct: Bool,
     columns: [any QueryExpression],
     joins: [_JoinClause],
@@ -371,6 +378,7 @@ public struct Select<Columns, From: Table, Joins> {
     order: [QueryFragment],
     limit: _LimitClause?
   ) {
+    self.isEmpty = isEmpty
     self.columns = columns
     self.distinct = distinct
     self.joins = joins
@@ -387,7 +395,8 @@ public struct Select<Columns, From: Table, Joins> {
 }
 
 extension Select {
-  init(where: [QueryFragment] = []) {
+  init(isEmpty: Bool = false, where: [QueryFragment] = []) {
+    self.isEmpty = isEmpty
     self.where = `where`
   }
 
@@ -558,6 +567,7 @@ extension Select {
     Joins == (repeat each J)
   {
     Select<(repeat each C1, repeat (each C2).QueryValue), From, (repeat each J)>(
+      isEmpty: isEmpty,
       distinct: distinct,
       columns: columns + Array(repeat each selection((From.columns, repeat (each J).columns))),
       joins: joins,
@@ -612,6 +622,7 @@ extension Select {
       )
     )
     return Select<(repeat each C1, repeat each C2), From, (repeat each J1, F, repeat each J2)>(
+      isEmpty: isEmpty || other.isEmpty,
       distinct: distinct || other.distinct,
       columns: columns + other.columns,
       joins: joins + [join] + other.joins,
@@ -651,6 +662,7 @@ extension Select {
       )
     )
     return Select<(repeat each C1, repeat each C2), From, (repeat each J, F)>(
+      isEmpty: isEmpty || other.isEmpty,
       distinct: distinct || other.distinct,
       columns: columns + other.columns,
       joins: joins + [join] + other.joins,
@@ -686,6 +698,7 @@ extension Select {
       )
     )
     return Select<QueryValue, From, (F, repeat each J)>(
+      isEmpty: isEmpty || other.isEmpty,
       distinct: distinct || other.distinct,
       columns: columns + other.columns,
       joins: joins + [join] + other.joins,
@@ -738,6 +751,7 @@ extension Select {
       From,
       (repeat each J1, F._Optionalized, repeat (each J2)._Optionalized)
     >(
+      isEmpty: isEmpty || other.isEmpty,
       distinct: distinct || other.distinct,
       columns: columns + other.columns,
       joins: joins + [join] + other.joins,
@@ -785,6 +799,7 @@ extension Select {
       From,
       (repeat each J, F._Optionalized)
     >(
+      isEmpty: isEmpty || other.isEmpty,
       distinct: distinct || other.distinct,
       columns: columns + other.columns,
       joins: joins + [join] + other.joins,
@@ -822,6 +837,7 @@ extension Select {
       )
     )
     return Select<QueryValue, From, (F._Optionalized, repeat (each J)._Optionalized)>(
+      isEmpty: isEmpty || other.isEmpty,
       distinct: distinct || other.distinct,
       columns: columns + other.columns,
       joins: joins + [join] + other.joins,
@@ -874,6 +890,7 @@ extension Select {
       From._Optionalized,
       (repeat (each J1)._Optionalized, F, repeat each J2)
     >(
+      isEmpty: isEmpty || other.isEmpty,
       distinct: distinct || other.distinct,
       columns: columns + other.columns,
       joins: joins + [join] + other.joins,
@@ -921,6 +938,7 @@ extension Select {
       From._Optionalized,
       (repeat (each J)._Optionalized, F)
     >(
+      isEmpty: isEmpty || other.isEmpty,
       distinct: distinct || other.distinct,
       columns: columns + other.columns,
       joins: joins + [join] + other.joins,
@@ -958,6 +976,7 @@ extension Select {
       )
     )
     return Select<QueryValue, From._Optionalized, (F, repeat each J)>(
+      isEmpty: isEmpty || other.isEmpty,
       distinct: distinct || other.distinct,
       columns: columns + other.columns,
       joins: joins + [join] + other.joins,
@@ -1010,6 +1029,7 @@ extension Select {
       From._Optionalized,
       (repeat (each J1)._Optionalized, F._Optionalized, repeat (each J2)._Optionalized)
     >(
+      isEmpty: isEmpty || other.isEmpty,
       distinct: distinct || other.distinct,
       columns: columns + other.columns,
       joins: joins + [join] + other.joins,
@@ -1057,6 +1077,7 @@ extension Select {
       From._Optionalized,
       (repeat (each J)._Optionalized, F._Optionalized)
     >(
+      isEmpty: isEmpty || other.isEmpty,
       distinct: distinct || other.distinct,
       columns: columns + other.columns,
       joins: joins + [join] + other.joins,
@@ -1094,6 +1115,7 @@ extension Select {
       )
     )
     return Select<QueryValue, From._Optionalized, (F._Optionalized, repeat (each J)._Optionalized)>(
+      isEmpty: isEmpty || other.isEmpty,
       distinct: distinct || other.distinct,
       columns: columns + other.columns,
       joins: joins + [join] + other.joins,
@@ -1348,6 +1370,7 @@ extension Select {
       SQLQueryExpression(iterator.next()!.queryFragment)
     }
     return Select<(repeat (each C2).QueryValue), From, Joins>(
+      isEmpty: isEmpty,
       distinct: distinct,
       columns: Array(repeat each transform(repeat { _ in next() }((each C1).self))),
       joins: joins,
@@ -1358,6 +1381,23 @@ extension Select {
       limit: limit
     )
   }
+
+  /// Returns a fully unscoped version of this select statement.
+  public var unscoped: Where<From> {
+    From.unscoped
+  }
+
+  /// Returns this select statement unchanged.
+  public var all: Self {
+    self
+  }
+
+  /// Returns an empty select statement.
+  public var none: Self {
+    var select = self
+    select.isEmpty = true
+    return select
+  }
 }
 
 /// Combines two select statements of the same table type together.
@@ -1387,6 +1427,7 @@ public func + <
   return Select<
     (repeat each C1, repeat each C2), From, (repeat each J1, repeat each J2)
   >(
+    isEmpty: lhs.isEmpty || rhs.isEmpty,
     distinct: lhs.distinct || rhs.distinct,
     columns: lhs.columns + rhs.columns,
     joins: lhs.joins + rhs.joins,
@@ -1406,6 +1447,7 @@ extension Select: SelectStatement {
   }
 
   public var query: QueryFragment {
+    guard !isEmpty else { return "" }
     var query: QueryFragment = "SELECT"
     let columns =
       columns.isEmpty

Sources/StructuredQueriesCore/Statements/Select.swift

@@ -91,6 +91,7 @@ extension PrimaryKeyedTable {
 ///
 /// To learn more, see <doc:UpdateStatements>.
 public struct Update<From: Table, Returning> {
+  var isEmpty: Bool
   var conflictResolution: ConflictResolution?
   var updates: Updates<From>
   var `where`: [QueryFragment] = []
@@ -159,6 +160,7 @@ public struct Update<From: Table, Returning> {
       returning.append("\(quote: resultColumn.name)")
     }
     return Update<From, (repeat each QueryValue)>(
+      isEmpty: false,
       conflictResolution: conflictResolution,
       updates: updates,
       where: `where`,
@@ -180,6 +182,7 @@ public struct Update<From: Table, Returning> {
       returning.append("\(quote: resultColumn.name)")
     }
     return Update<From, From>(
+      isEmpty: isEmpty,
       conflictResolution: conflictResolution,
       updates: updates,
       where: `where`,
@@ -195,7 +198,7 @@ extension Update: Statement {
   public typealias QueryValue = Returning
 
   public var query: QueryFragment {
-    guard !updates.isEmpty
+    guard !isEmpty, !updates.isEmpty
     else { return "" }
 
     var query: QueryFragment = "UPDATE "