CTEs are implemented by most (all?) major RDBMS platforms and were introduced in the SQL:1999 standard revision.

- SQL:1999 https://en.wikipedia.org/wiki/SQL:1999#Common_table_expressi...

- SQL standardization https://en.wikipedia.org/wiki/SQL#Interoperability_and_stand...

The WITH clause, otherwise known as Common Table Expressions, is in ANSI SQL99. Common table expressions are supported by all of the major databases: PostgreSQL, Microsoft, Oracle, MySQL, MariaDB, and SQLite. They are also in certain minor ones, like Teradata, DB2, Firebird, and HyperSQL. Recursive WITH clauses are especially useful.

Not anymore. Other implementations, like MariaDB and SQLite, have adopted common table expressions. They're basically syntactic sugar for subqueries in most implementations, but they can make some queries much more readable

the WITH clause is now in SQL Server and Oracle