What are the two kinds of identifiers used in SQL statements?

In SQL statements, identifiers are used to name database objects such as tables, columns, and databases. There are two kinds of identifiers: delimited and undelimited.

Undelimited identifiers are the simpler forms and are typically used without quotation marks. They can consist of letters, numbers, and underscore characters but cannot start with a number. For example, a column named 'employee_id' is an undelimited identifier. They must follow certain rules laid out by the SQL standard, such as being case-insensitive in many database systems.

On the other hand, delimited identifiers are enclosed in double quotes, allowing for more flexibility in naming conventions. This enables the use of spaces, special characters, and reserved keywords as identifiers. For instance, "Employee ID" would be a valid delimited identifier, complete with a space.

Understanding these two types of identifiers is critical for correctly writing SQL statements and ensuring compatibility across different database systems.