DatabaseCondition::exists in core/includes/database/query.inc

Sets a condition that the specified subquery returns values.

DatabaseCondition::isNotNull in core/includes/database/query.inc

Sets a condition that the specified field be NOT NULL.

DatabaseCondition::isNull in core/includes/database/query.inc

Sets a condition that the specified field be NULL.

DatabaseCondition::notExists in core/includes/database/query.inc

Sets a condition that the specified subquery returns no values.