Structured Query Language
From FreeRADIUS Wiki
SQL is an ANSI (American National Standards Institute) standard computer language for accessing and manipulating database systems. SQL statements are used to retrieve and update data in a database. Unfortunately, there are many different versions of the SQL language, but to be in compliance with the ANSI standard, they must support the same major keywords in a similar manner (such as SELECT, UPDATE, DELETE, INSERT, WHERE, and others).
FreeRADIUS Server has a master SQL abstraction module called rlm_sql which utilizes sub modules (or shims) which work with individual database program servers. These shims include DB2, ODBC, Oracle, PostgreSQL, MySQL and Firebird.
Whereever possible QUERIES in FreeRADIUS use SQL99 syntax although the config files for some shims have been optimized to use proprietary extensions.