Add Expression.equalsNullable

drift/lib/src/runtime/query_builder/expressions/expression.dart

@@ -49,9 +49,28 @@ abstract class Expression<D extends Object> implements FunctionParameter {
   /// type. The [compare] value will be written
   /// as a variable using prepared statements, so there is no risk of
   /// an SQL-injection.
+  ///
+  /// This method only supports comparing the value of the column to non-
+  /// nullable values and translates to a direct `=` comparison in SQL.
+  /// To compare this column to `null`, use [equalsNullable].
   Expression<bool> equals(D compare) =>
       _Comparison.equal(this, Variable<D>(compare));
 
+  /// Compares the value of this column to [compare] or `null`.
+  ///
+  /// When [compare] is null, this generates an `IS NULL` expression in SQL.
+  /// For non-null values, an [equals] expression is generated.
+  /// This means that, for this method, two null values are considered equal.
+  /// This deviates from the usual notion in SQL that doesn't allow comparing
+  /// `NULL` values with equals.
+  Expression<bool> equalsNullable(D? compare) {
+    if (compare == null) {
+      return this.isNull();
+    } else {
+      return equals(compare);
+    }
+  }
+
   /// Casts this expression to an expression of [D].
   ///
   /// Calling [dartCast] will not affect the generated sql. In particular, it

drift/test/database/expressions/expression_test.dart

@@ -111,4 +111,14 @@ void main() {
           .runSelect(argThat(contains('ON categories._rowid_ = ?')), [3]));
     });
   });
+
+  test('equals', () {
+    const a = CustomExpression<int>('a', precedence: Precedence.primary);
+    const b = CustomExpression<int>('b', precedence: Precedence.primary);
+
+    expect(a.equals(3), generates('a = ?', [3]));
+    expect(a.equalsNullable(3), generates('a = ?', [3]));
+    expect(a.equalsNullable(null), generates('a IS NULL'));
+    expect(a.equalsExp(b), generates('a = b'));
+  });
 }