Get variables using SQL

Hello good, I'm currently working on a project that are continually being introduced by data users, this data is not stored just want to have them, I would like to deal with them to calculate certain values. For example, I would like to store in a variable number of documents created, (SELECT (*) COUNT), or the sum of a field integer (SELECT SUM ..). Anyone know how I can make these judgments in drupal directly, thanks.