MySQL Stored Procedures.

Hello. Often there is a need for recursive data processing, building chains, tying products to categories and so on.

If you use PHP + MySQL (while, foreach, for + mysql_query), processing a huge number of requests, the execution time of the script increases to 10-15 seconds, because There are a lot of requests, and reducing the number of requests is almost impossible.

Therefore, I would like to ask MySQL gurus how to build procedures (functions) in MySQL, maybe they will help me to reduce the time it takes to execute the script.
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Hence the request:

Please tell me the links, and preferably the book, which describes the topic.