Unquoted identifiers can use letters (a-z), digits (0-9), 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.

• MS SQL: @, $, \\#, and other Unicode letters (source)
• MySQL: $ (source)
• Oracle: $, \\#, and other letters from database character set (source)
• PostgreSQL: $, 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.