Unquoted identifiers can use letters (
z), digits (
9), and underscore (
_), and must start with a letter.
Depending on SQL implementation, and/or database settings, other characters may be allowed, some even as the first character, e.g.
#, and other Unicode letters (source)
#, and other letters from database character set (source)
$, and other Unicode letters (source)
Unquoted identifiers are case-insensitive. How this is handled depends greatly on SQL implementation:
MS SQL: Case-preserving, sensitivity defined by database character set, so can be case-sensitive.
MySQL: Case-preserving, sensitivity depends on database setting and underlying file system.
Oracle: Converted to uppercase, then handled like quoted identifier.
PostgreSQL: Converted to lowercase, then handled like quoted identifier.
SQLite: Case-preserving; case insensitivity only for ASCII characters.