PHP & amp; SQL: The subquery does not work

This question already has an answer here:

• Warning: mysql_fetch_array() expects parameter 1 to be resource, boolean given in [duplicate] 5 answers

I have the following query with a subquery, this is the php code:

$staticsQuery = "SELECT * FROM statics WHERE mmsi = (SELECT mmsi FROM positions,active WHERE active.mmsi = positions.position_ID)";
$staticsResult = mysql_query($staticsQuery);
        while($rows = mysql_fetch_array($staticsResult)){
            echo $rows['time'];
            echo $rows['mmsi'];
            echo $rows['name'];
            echo $rows['imo'];
        }

The subquery part is normally working, which prints out results. I am not sure if this outer query part is correct: SELECT * FROM statics WHERE mmsi =

This is the error I am getting,

Warning: mysql_fetch_array() expects parameter 1 to be resource, boolean given in C:\xampp\htdocs\retrieve.php on line 44

The error obviously comes from mysql_fetch_array because the query does not return a result. Please don't mark this question as a duplicate, because there can be a lot of different type issues with this error message.

Any help is welcome!

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if you will ever have more than one result, you need IN not =