Just let MySQL do its work :) The memory management for each request isn't your task, it's the task of database engine.

the syntax of the select command to read by primary key or another index key?

I did not understand your question. Maybe you want to get the data corresponding to a key? In this case you need to specify the WHERE clause to read only certain data from the table, e.g.

SELECT id, name FROM mytable WHERE id=1
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I repeat: RTFM! MySQL documentation contains the full info on SQL syntax. 10 minutes of reading the docs will save for you hours of asking questions :)