MySQL使用JSON编码查询PHP / PDO

Question

I know the query I need works, but I can't seem to execute it in a way that works via PHP with a PDO execute.

In short I have a table called type:

+-------------++----------------+
|   type_id   ||   type_name    |
+-------------++----------------+
      1               Car
      2               Bus
      3               Lorry

And a table called asset_details:

+-------------++----------------+
|   type_id   ||     cost       |
+-------------++----------------+
      1              10
      1              10
      1              20
      2              300
      3              600
      3              800
      1              70 

I want to output as JSON enconde so that I can sum all the type_ids from asset_details. ie, above we have 4 cars, 2 lorries, 1 bus. I only need the value so I've tried

$json = $pdo->query('SELECT COUNT(*) FROM asset_details GROUP BY type_id')->fetchAll(PDO::FETCH_UNIQUE);

echo json_encode($json);

But that gives me an echo of:

{"4":[],"2":[],"1":[]}

I need it to be:

["7" , "3"]

so that I can use that for data within chartJS. I'm sure it's the fetch method, but not sure how to solve it.