Select record from table 1 where does not exist in table 2

I have a table with a list of surveys - tbl_survey

I have a table of campaigns - tbl_campaign

I have a table which links the surveys to campaigns - map_survey_to_campaign

What I need is to be able to retrieve a list of surveys from tbl_survey which have not been assigned to a campaign

Here is my current code:

$surveyList = Survey::find()

 ->leftJoin('map_survey_to_campaign','map_survey_to_campaign.fk_surveyID','tbl_survey.id')

 ->where(['map_survey_to_campaign.fk_campaignID']=>null)

 ->asArray()

 ->all();

The query is working if I set the null to a known campaign_id, so I know the syntax is correct. Somehow I need to replace the "where" with "Where campaign x does not exist in map_survey_to_campaign.fk_campaignID"

How can I do that?

Many thanks

In case anybody else needs to find the solution, here it is:

You need to perform a subquery as follows (copied from another website):

$subQuery = Follower::find()->select('foreignKey_Leader');

$query = Leader::find()->where(['not in', 'leaderID', $subQuery]);

$models = $query->all();

Key points:

[list=1]

[*]You do NOT add "->all()" to the initial subquery, instead you leave the query open

[*]You must select one column and only one column in the subquery, typically the foreign key column

[*]There is no need to define the association between the query and the subquery. The association is implied because there is only one column in the subquery.

[/list].

The result of the query above would return all leaders without a follower.

I hope this helps somebody else. It took me a whole day to figure this out!