为什么此MySQL语句不起作用？

Question

I have a table Seats with the columns, SeatID , Date , RouteID and Seats . The primary key is SeatsID . The value of seats is 50, and I wish to subtract whatever the user has entered into $tickettotal from 50 and then insert the new value.

The user also has to enter which RouteID and Date they wish to travel. I want these to be inserted into the Seats table along with the new value of Seats .

If another user enters the same Date and RouteID then the Seats should be updated. Otherwise a new record should be inserted with SeatID , Date , RouteID , and the reduced value for Seats .

I thought this statement should do it but I keep getting errors back such as

Can't find string terminator '"' anywhere before EOF, although when i put " at the end of the statement i just get another syntax error line 108, near ""') ON DUPLICATE KEY UPDATE Seats=Seats-'"$tickettotal" .

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Update

I now have it inserting fine, although on a duplicate entry of both Date and RouteID it doesn't update Seats : it still just inserts a new value. The Seats - $tickettotal isn't working either.

$dbh->do("INSERT INTO $Stable(Date,RouteID) 
VALUES ('$Tdate','$Rid') ON DUPLICATE KEY UPDATE Seats=Seats-'$tickettotal'");

Update Answer

Because i didn't have a unique column, I created one using both date and RouteID added together so.

$Tdate =~ s/-//gi;
$dateandrid = $Tdate . $Rid;

The first line removes the hyphens and the second puts them together. Then using these statements get the desired effect i wanted.

$dbh->do("INSERT INTO $Stable(RouteAndDate,Date,RouteID) VALUES    ('$dateandrid','$Tdate','$Rid') ON DUPLICATE KEY UPDATE Seats=Seats");
$dbh->do("INSERT INTO $Stable(RouteAndDate,Date,RouteID) VALUES ('$dateandrid','$Tdate','$Rid') ON DUPLICATE KEY UPDATE Seats=Seats-'$tickettotal'");

Answer 1

You must avoid SQL Injection with parameter binding:

$dbh->do( qq{
     INSERT INTO $Stable(Date, RouteID) 
     VALUES (?, ?)
     ON DUPLICATE KEY UPDATE Seats=Seats-?
     },
     undef,
     $Tdate, $Rid, $tickettotal
);