https://ru.wikipedia.org/wiki/%D0%A0%D0%B5%D0%BB%D1%8F%D1%86%D0%B8%D0%BE%D0%BD%D0%BD%D0%B0%D1%8F_%D0%B0%D0%BB%D0%B3%D0%B5%D0%B1%D1%80%D0%B0
https://wikicsu.ru/wiki/relational_algebra
    https://wikicsu.ru/wiki/Selection_(relational_algebra)
https://hmong.ru/wiki/Relational_algebra

2022
KGeorgiy - DB Intro of 14 parts
    https://www.youtube.com/playlist?list=PLBAwg87wLtfLY7WCevPxuT0Uig_NHTHF1
2021
KGeorgiy - DB Intro of 14 parts
    https://www.youtube.com/playlist?list=PLBAwg87wLtfKvZSBL2yg5WynnTcdeVc8s
2020
KGeorgiy - DB Intro of 28 parts
    https://www.youtube.com/playlist?list=PLBAwg87wLtfLrQv5yPi9AVQD3HnVRnFje
2019
https://aliev.me/osnovy-relyacionnoj-algebry/
https://coderlessons.com/tutorials/bazy-dannykh/osnovy-subd/6-reliatsionnaia-algebra-v-subd
2012
https://habr.com/ru/post/145381/