Tokens such as SELECT, UPDATE, or VALUES in the example above are examples of key words, that is, words that have a fixed meaning in the SQL language. The tokens MY_TABLE and A are examples of identifiers. They identify names of tables, columns, or other database objects, depending on the command they are used in. Therefore they are sometimes simply called, “names”. Key words and identifiers have the same lexical structure, meaning that one cannot know whether a token is an identifier or a key word without knowing the language.SQL identifiers and key words must begin with a letter (a-z or A-Z). Subsequent characters in an identifier or key word can be letters, underscores, digits (0-9), dollar signs ($), or number signs (#).There is a second kind of identifier: the delimited identifier or quoted identifier. It is formed by enclosing an arbitrary sequence of characters in double-quotes ("). A delimited identifier is always an identifier, never a key word. So "select" could be used to refer to a column or table named "select", whereas an unquoted select would be taken as a key word and would therefore provoke a parse error when used where a table or column name is expected. The example can be written with quoted identifiers like this:To include a double quote, use two double quotes. This allows you to construct table or column names that would otherwise not be possible (such as ones containing spaces or ampersands). The length limitation still applies.Quoting an identifier also makes it case-sensitive, whereas unquoted names are always folded to lower case. For example, the identifiers FOO, foo, and "foo" are considered the same by Advanced Server, but "Foo" and "FOO" are different from these three and each other. The folding of unquoted names to lower case is not compatible with Oracle databases. In Oracle syntax, unquoted names are folded to upper case: for example, foo is equivalent to "FOO" not "foo". If you want to write portable applications you are advised to always quote a particular name or never quote it.