Welcome to the Forum, Kevin.
I think that when you say "sql" you actually mean MySQL database server.
Here are the things to remember:
SQL, as language, is defined by standards.
The original SQL standard was completed as a USA ANSI (American National Standards Institute) standard in 1986, and adopted as an ISO (International Standards Organization) standard in 1987. Expansions and revisions were released in 1989 and 1992.
Then there was SQL 1999, which was developed by the INCITS Technical Committee H2 on Database. This committee develops standards for the syntax and semantics of database languages.
Since then the SQL standard kept evolving, I think the latest draft (or standard?) is SQL 2003.
f you are really interested run a Qoogle serach for "sql standards" and you'll have enough papers to read for a very long time!
Any major database vendor provides SQL which for most parts is compliant with the SQL standard (one of them, anyway), but typically has something that's called "vendor extensions" - non-standard commands that they add for convenience.
Sometimes they also choose not to comply with the SQL standards, I think a good example would be how Oracle treats the NULL values.
I did not check, but I'm pretty sure that things like "show columns from tablename" are not coming from the SQL standards, these are vendor provided extensions, and as such can be different from one database to another.
The easiest way to display the table schema in PostgreSQL is to use the metacommand \d in psql.
You may also want to look at
psql man page and at
this posting for more info.