I don't know if it's obvious but I'm not entirely sure how to delete joined table records? For example. I have a project which also has associated contracts, files etc in other tables. They are all linked using the project foreign key.

How do I construct a query to that I delete the actual project record in the projects table AS WELL AS the other entries in the other tables?

Deleting from multiple tables have constraints. you can do something like this.

Here`s a code that I`ve used

function delete_employee(){
        //connect to use database
	$dbc=connect();
	
       //enable transactions
 	mysqli_autocommit($dbc, FALSE);
	
   
	$emp_id = $_POST['emp_id'];
		
       //default success = true
	$success = true;
	
        //delete from the first table 
	$query = "delete from employee
				where id = '".$emp_id."'";
	
		if($result = mysqli_query($dbc, $query) or die(mysqil_error()))
		{
			if(mysqli_affected_rows()>0)
			{
				//if everything went through delete from second table
				$query = "delete from training_date
							where employee_id = '".$emp_id."'";
				
				$result = mysqli_query($dbc, $query) or die(mysqli_error());
				    
					if (($result == false) && (mysqli_affected_rows() == 0))
					{
						$success = false;
						
					}
					else {$success = true;}
					
			}#end if(mysql_affected_rows()>0) 1
			
			if (($result == false) && (mysqli_affected_rows() == 0))
			{
				
				$success = false;	
				
			}#end if (($result == false) && (mysql_affected_rows() == 0)) 2
		
		}#end 1st if
		
		else
		{
		$success = false;	
		}
		
		if($success==true){
			//if everything went fine. Confirm the delete
			mysqli_commit($dbc);

			mysqli_close($dbc); 
			
		}
	
		else{
                        //If something went wrong, cancel everything
			mysqli_rollback($dbc);
			
			mysqli_close($dbc);
			echo `failed to delete`;
		}
}

I hope this helps.

Note: if you are not using mysqli, then to begin transactions you have to use this

$query  = "SET AUTOCOMMIT=0";
    $result = mysql_query($query) or die(mysql_error());

    $query  = "BEGIN";
    $result = mysql_query($query) or die(mysql_error());

//Instead of this

mysqli_autocommit($dbc, FALSE);

Thank you for this code. It's very clear and helpful.