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No RDBMS is fully compliant with the standard, especially newer versions of the standard.

There was a website comparing the major RDBMSs to the standard with examples. I can not find it now but I found this:

https://en.wikipedia.org/wiki/SQL_compliance

Edit: this was it https://troels.arvin.dk/db/rdbms/

It's not updated though.

And it has some mistakes: for example, in Postgres TIMESTAMP WITH TIME ZONE doesn't store the time zone. It's more like Oracle's TIMESTAMP WITH LOCAL TIME ZONE: it converts the timestamp to UTC when storing and converts it to the session's time zone when retrieving.

I would argue that at least at the time that webpage was written and maintained, none, of them (including the standard) got timestamps right. In the above case timezone is actually an offset.

Arguably, timestamp with timezone should store a timestamp plus a tz_database time zone. Storing offsets is easy and useful but not necessarily correct. Timestamp with Offset should be a different type.

Validating all of those rules would be a hairy mess. For example, what do you do with a timestamp in the future when changes are introduced (negative leap seconds, changes to DST, geopolitical changes affecting timezones).

https://modern-sql.com/ is another site with compatibility graphs.

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