SQL - Delete row based on number of rows in another table

Question

How would I go about deleting a row from the table 'subjects' that has a primary id 'subject_id' based on the number of rows in another table named 'replies' that uses a 'subject_id' column as a reference.

Example in pseudo code: If ('subject' has less than 1 reply){ delete 'subject'}

I don't know much about SQL triggers so I have no clue if I would be able to incorporate this directly in the database or if I'd have to write some PHP code to handle this...

Answer 1

To delete any subjects that have had no replies, this query should do the trick:

DELETE s.* FROM subjects AS s
WHERE NOT EXISTS 
    (
    SELECT r.subject_id
    FROM replies AS r
    WHERE r.subject_id = s.subject_id
    );

Demo: DB Fiddle Example

Answer 2

One of the MySQL gurus will need to weigh in on whether or not you can do this directly, but in PHP you could...

$query = "SELECT subject_id FROM subjects WHERE subject='test'";
$return = mysqli_query($mysqli, $query);
$id = mysqli_fetch_assoc($return);

$query = "SELECT reply_id FROM replies WHERE subject_id='".$id[0]."'";
$return = mysqli_query($mysqli, $query);

if(mysqli_num_rows($return) < 1){
    $query = "DELETE FROM subjects WHERE subject_id='1'";
    $return = mysqli_query($mysqli, $query);
}

This example assumes the "subject" is unique. In other words, SELECTing WHERE subject='test' will only ever return one subject_id. If you were doing this as a periodic cleaning, you would grab all the subject_id values (no WHERE clause) and loop through them to remove them if no replies.

Answer 3

You can achieve this in one query by selecting all (unique) subject-ids from the replies table, and delete all subjects that doesn't have a reply in there. Using SELECT DISTINCT , you don't get the IDs more than once (if a subject has more than one reply), so you don't get unnecessary data.

DELETE FROM subjects 
WHERE subject_id NOT IN (SELECT DISTINCT subject_id FROM replies)

Any subject that doesn't have a reply should be deleted!

Answer 4

So you want to delete all subjects with no replies:

DELETE FROM subjects WHERE subject_id NOT IN 
(SELECT subject_id FROM replies);

I think this is what you want...