The table "webform_submission_data" uses the EAV model. To extract useful results you can use a static query.
Example SQL to the get sid meeting all the requirements:
SELECT DISTINCT sid
FROM webform_submission_data wsd
WHERE wsd.webform_id = 'ticket'
AND sid IN (SELECT sid FROM webform_submission_data wsd2
WHERE wsd2.name = 'ticket_for'
AND wsd2.value CONTAINS '$adnummer')
AND sid IN (SELECT sid FROM webform_submission_data wsd3
WHERE wsd3.name = 'entity_id'
AND wsd3.value = $eventID);
Example static query:
$query = $connection->query("SELECT DISTINCT sid
FROM webform_submission_data wsd
WHERE wsd.webform_id = 'ticket'
AND sid IN (SELECT sid FROM webform_submission_data wsd2
WHERE wsd2.name = 'ticket_for'
AND wsd2.value CONTAINS :adnummer)
AND sid IN (SELECT sid FROM webform_submission_data wsd3
WHERE wsd3.name = 'entity_id'
AND wsd3.value = :event_id)", [
':adnummer' => $adnummer,
':event_id' => $eventID
]);
Update 19 October 2018
I've created a module to make it easier to query webform submissions by submission field values. See: https://www.drupal.org/project/webform_query