Common Table Expressions (CTEs), introduced in the SQL Standard 1999, are similar to subroutines in programming languages: they can be referenced from multiple places in a query and may refer to themselves, providing recursive queries. Many RDBMSs implemented this feature, yet not in full. Until recently MariaDB did not have this feature either. This article describes CTE overall and looks through some interesting cases that are implemented in MariaDB only, like non-linear recursion and mutual recursion. It also compares optimizations for non-recursive CTEs across different RDBMSs. Finally, the results of experiments comparing computation of recursive CTEs for MariaDB and PostgreSQL are presented.