Concurrency in mysql database

i want to maintain concurrency in mysql. like when user is updating a particular table then no can access this row. how could i can do this. or give me some tips to maintain concurrency in mysql or any other database indetail

How would i get time in am or pm format by using curtime() function???????

I'm not totally sure you can do this just in MySQL. You might have to implement an external locking solution to handle this in your application.

If you use the engine InnoDB and transactions, you can do shared and exclusive locks on data.

http://dev.mysql.com/doc/refman/5.0/en/innodb-lock-modes.html

Look up date_format() in the manual for answers to your second question.