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.