如何在数据库中查询空表

Question

In my system there are two tables called 'faculty' and 'faculty_assets'.

I need to delete faculty by faculty from the 'faculty' table. But system should allow deleting the faculty when only faculty_assets table empty. Otherwise It should not allows to delete the faculties from 'faculty' table.

Can anyone modify below sql query? ('fac_id' is the primary key of 'faculty' table )

function removeFaculty($fac_id){
    $conn=new connection();
    $sql="delete from faculty where fac_id='$fac_id'";
    $result=$conn->query($sql);
    return $result;
}

Answer 1

You can do this with a not exists clause:

delete from faculty
    where fac_id = '$fac_id' and
          not exists (select 1 from faculty_assets);

Answer 2

尝试这样做：

DELETE t1 FROM faculty t1 LEFT JOIN faculty_assets t2 ON t1.fac_id=t2.fac_id  WHERE t2.fac_id IS NULL;