Transaction secure MYSQL code question

[Pompiuses]

When executing a specific query, I need to lock the db to ensure that no data is altered while the query executes.

My question is: Do I need to take a ROLLBACK on the db to release the lock, or is it enought just to set autocommit=1 again?

Simplified example of how I do it now:
SET AUTOCOMMIT=0
BEGIN
SELECT .......bla bla bla <execute some php code based on the result>
ROLLBACK
SET AUTOCOMMIT=1

Example of how I want it to be:
SET AUTOCOMMIT=0
BEGIN
SELECT .......bla bla bla <execute some php code based on the result>
SET AUTOCOMMIT=1

Are there any downsides by not using ROLLBACK in the second example? The database has not been altered in any way.

[Pompiuses]

Found the solution to my original question 🙂

I've got another question:

Do you know if it's possible to start a transaction within another transaction? Couldn't find anything about it in the documentation.

Like this:
SET AUTOCOMMIT=0
BEGIN

SET AUTOCOMMIT=0
BEGIN
COMMIT or ROLLBACK
SET AUTOCOMMIT=1

COMMIT or ROLLBACK
SET AUTOCOMMIT=1

Will it work properly?