2.1.1 Lexical StructureSQL input consists of a sequence of commands. A command is composed of a sequence of tokens, terminated by a semicolon (;). The end of the input stream also terminates a command. Which tokens are valid depends on the syntax of the particular command.A token can be a key word, an identifier, a quoted identifier, a literal (or constant), or a special character symbol. Tokens are normally separated by whitespace (space, tab, new line), but need not be if there is no ambiguity (which is generally only the case if a special character is adjacent to some other token type).Additionally, comments can occur in SQL input. They are not tokens - they are effectively equivalent to whitespace.This is a sequence of three commands, one per line (although this is not required; more than one command can be on a line, and commands can usually be split across lines).The SQL syntax is not very consistent regarding what tokens identify commands and which are operands or parameters. The first few tokens are generally the command name, so in the above example we would usually speak of a SELECT, an UPDATE, and an INSERT command. But for instance the UPDATE command always requires a SET token to appear in a certain position, and this particular variation of INSERT also requires a VALUES token in order to be complete. The precise syntax rules for each command are described in Section 2.3.