How to keep the uniqueness of the concurrent write database?

A, B two requests concurrent  registration of the same user name username= ' myname ' $is _user = "Select username from users WHERE username= ' myname '"; if (! $is _user) {echo ("INSERT into users (username) VALUES (' myname ')");}

If the field in the table does not have a unique index set, how does the program control uniqueness?
A, B query the table, the results can be registered, so all executed the INSERT, but the user name is the same, so that the data is not unique. Would that be so, how to avoid it?

Reply to discussion (solution)

Database lock operation

Database lock operation
Read the lock seems to be shared ah, everyone can read at the same time, so there are incorrect results.

Why not add a unique key?

The simplest method of database plus unique key

A little more complicated:
Single Web machine, using local file lock flock, a user name a local file
Multiple machines, using Memcache Inc to implement the locking mechanism http://ju.outofmemory.cn/entry/48924

I understand, thank you all.

I also just occasionally think, if not the only index, what method to solve the problem.

The simplest method of database plus unique key

A little more complicated:
Single Web machine, using local file lock flock, a user name a local file
Multiple machines, using Memcache Inc to implement the locking mechanism http://ju.outofmemory.cn/entry/48924
Please ask Memcache again.

$v = $memcache->get ($key);
if ($v = = = False) {
$memcache->set ($key, 0);
}
$index = $memcache->increment ($key, 1);

Two concurrent requests A/b will get ($key) at the same time
Then a set ($key), then B set ($key) is not covered by the latter

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