PHP从MySQL表中选择最大的`id`值

Question

I have a table with more than 300 000 rows and I need to select the highest value for the column 'id'. Usually, I will do like this:

SELECT id FROM my_table ORDER BY id DESC

... but this will cause slow queries and I don't want to use it. Is there a different way to solve this situation? id is auto increment and primary key.

Later Edit: It seems my full code is quite bad written, as I deduct from your comments. Below I posted a sample of the code I'm working and the tables. Can you suggest a proper way to insert the last ID+1 of table_x in two tables (including table_x itself). I have to mention that the script will be running more than once.

TABLE_X          TABLE_Y
------------     ----------
id_x | value     id_y | id_x
------------     ----------
   1 | A            1 | 3
   2 | B            
   3 | C

<?php
for($i=0; $i<10; $i++){
    $result_x = mysql_query('SELECT id_x FROM table_x ORDER BY id_x DESC');
    $row_x = mysql_fetch_array($result_x);

    $next = $row_x['id_x'] + 1;
    mysql_query('INSERT INTO table_x(id_x) VALUES("'.$next.'")');
    mysql_query('INSERT INTO table_y(id_x) VALUES("'.$next.'")');
}
?>

Answer 1

Slightly better:

SELECT id FROM my_table ORDER BY id DESC LIMIT 1

Significantly better:

SELECT MAX(id) FROM my_table

Answer 2

Here is the right code you have to use.

mysql_query('INSERT INTO table_x(id_x) VALUES(NULL)');
$id = mysql_insert_id();
mysql_query("INSERT INTO table_y(id_x) VALUES($id)");

Answer 3

Depending on the context either

SELECT id FROM my_table ORDER BY id DESC LIMIT 1

or mysql_insert_id() in PHP or ( SELECT LAST_INSERT_ID() ) in MySQL

Answer 4

As other said, you should use the MAX operator:

SELECT MAX( id ) FROM my_table ORDER BY id DESC

As a general rule of thumb, always reduce the amount of records returned from the database . The database always is faster than your application program when operating on result sets.

In case of slow queries, please give EXPLAIN a try:

EXPLAIN  SELECT id FROM my_table ORDER BY id DESC

vs.

EXPLAIN  SELECT MAX( id ) FROM my_table ORDER BY id DESC

EXPLAIN ask MySQL's query optimizer how it sees the query. Look in the documentation, to learn how to read its output.

PS: I really wonder, why you need MAX(id) . Even if your application gets the value back from the database, it is useless: Another process might just during the next CPU cycle have inserted a new record - and MAX(id) isn't valid any more.

Answer 5

I guess it is slow because you retrieve all 300 000 rows. Add LIMIT 1 to the query.

SELECT id FROM my_table ORDER BY id DESC LIMIT 1

Or use the MAX() operator.