使用php安全快速地选择和插入mysql

Question

I have a simple table, the logic is that before inserting a new row, I should fetch one column from that table. Let me explain:

table

id    key     groupId      Note
1    00001      1        abd
2    00002      1        aasdas
3    00003      1        aasdas
4    00001      2        q2eqwd
5    00002      2        qwdvvd
6    00003      2        qwrqw
7    00004      2        qwdqdqw

You see, key increases like Auto Increment for each groupId. When group with id 2, wants to add a new note, he should know last key. After finding it, php addes +1 to last key and inserts a new row. I do it like below:

$groupId = 2; //for example
$note = $_POST['note'];

$select = $db -> prepare("SELECT key FROM table where groupId = :groupId ORDER BY id DESC limit 1");
$select -> bindValue(":groupId", $groupId, PDO::PARAM_INT);
$select -> execute();
$fetch = $select -> fetch(PDO::FETCH_ASSOC);

$lastKey = $fetch['key']+1;

$insert = "INSERT INTO table (key, groupId, note) VALUES(:key, :groupId, :note)";

$ins = $db -> prepare($insert);
$insert -> bindValue(":key", $lastKey, PDO::PARAM_INT);
$insert -> bindValue(":groupId", $groupId, PDO::PARAM_INT);
$insert -> bindValue(":note", $note, PDO::PARAM_STR);

This method works good for me, but I am afraid of is here will be any conflict while fetching last key from table? Because, at the same time, 10 user with same groupId can add a new row. May php fetch same key to 3 users with group ID 2 at the same time?

Is there any quickly and safely way?

Answer 1

You can do this with AUTO_INCREMENT using MyISAM.

From MySQL Docs :

For MyISAM and BDB tables you can specify AUTO_INCREMENT on a secondary column in a multiple-column index. ... This is useful when you want to put data into ordered groups.

Otherwise, you should set the value in your insert query with a subquery like SELECT MAX(key) + 1 FROM table WHERE groupID = 1 and read back the value.