I\'ve been searching for half an hour and can\'t find any resources stating what level of the SQL ANSI standard is supported on various database engines.
It looks li
these might help a little:
Comparison of different SQL implementations
wikipedia - Comparison of relational database management systems
wikipedia - SQL - Standardization
According to the HSQLDB manual, it is the most standards compliant RDBMS.
Firebird 3 sql standard conformance page
The question of "which level of the standard" is too simple.
There are portions of the standard that are implemented by no engine at all, e.g. CREATE ASSERTION.
For that reason, it might be better to speak of "percentage of standard features supported" for each individual engine. But the vendors will be very reluctant to publish such measurements themselves.
The PostgreSQL manual has for all SQL statements a compatibility section.