如何避免在mysql中同时选择第一行？

Question

I have three php scripts on server1, server2, server3. I have remote mysql server4 with database "watch" - table "mytable" with one column "data"

All these scripts connect to db "watch" table "watch" column "data" and all do the same operation:

1. Select first row of column "data"
2. Make operation with data
3. Delete first row of column "data"

 $connect = mysql_connect("ip","user","password") OR DIE("Can't connect "); mysql_select_db("watch") or die("Can't select database"); $xv=1 do { $select = mysql_query("SELECT data FROM mytable LIMIT 1") or die("wrong select"); if (!$select) { echo 'Could not run query: ' . mysql_error(); exit;} $data = mysql_fetch_row($select); ----------- operation with first row of "data" ------------- mysql_query("DELETE FROM mytable LIMIT 1") or die("wrong delete"); } while ($xv++<100); 

For example,

first row of "data" contains uniq value "Alex".

second row of "data" contains uniq value "Michael".

third row of "data" contains uniq value "George".

PHP scripts run in cron at the same time but from different servers.

Is it possible that two or all three scripts select the same value of first row "Alex"?

I would like to avoid that two or three scripts select simultaneously the same value of first row.

Answer 1

Use locking, unlocking of tables. LOCK TABLES explicitly acquires table locks for the current client session. UNLOCK TABLES explicitly releases any table locks held by the current session. Also read this link .

$connect = mysql_connect("ip","user","password") OR DIE("Can't connect ");
mysql_select_db("watch") or die("Can't select database");

$xv=1
do {
    mysql_query("LOCK TABLES mytable WRITE");
    $select = mysql_query("SELECT data FROM mytable LIMIT 1") or die("wrong select");
    if (!$select) {
         echo 'Could not run query: ' . mysql_error(); 
         exit;
    }
    $data = mysql_fetch_row($select);

-----------
operation with first row of "data" 
-------------

    mysql_query("DELETE FROM mytable LIMIT 1") or die("wrong delete");
    mysql_query("UNLOCK TABLES");

} while ($xv++<100);

Answer 2

I tnink that in mysql the only way to do it is locking table before reading or writing. here you have the link: http://dev.mysql.com/doc/refman/5.0/en/lock-tables.html

Please don't forget to unlock the table, try to catch every exception you can have or the table can be locked for a heck long time.