How to interpret syntax diagrams in the Database Performance and Query Optimization book

Throughout this book, syntax is described using the structure defined as follows:

• Read the syntax diagrams from left to right, from top to bottom, following the path of the line.

  The >>--- symbol indicates the beginning of a statement.

  The ---> symbol indicates that the statement syntax is continued on the next line.

  The >--- symbol indicates that a statement is continued from the previous line.

  The --->< symbol indicates the end of a statement.

  Diagrams of syntactical units other than complete statements start with the >--- symbol and end with the ---> symbol.

• Required items appear on the horizontal line (the main path).

  >>-required_item-----------------------------------------------><
   
  

• Optional items appear below the main path.

  >>-required_item---+---------------+---------------------------><
                     '-optional_item-'
   
  

  If an optional item appears above the main path, that item has no effect on the execution of the statement and is used only for readability.

                     .-optional_item-.
  >>-required_item---+---------------+---------------------------><
   
  

• If you can choose from two or more items, they appear vertically, in a stack.

  If you must choose one of the items, one item of the stack appears on the main path.

  >>-required_item----+-required_choice1-+-----------------------><
                      '-required_choice2-'
   
  

  If choosing one of the items is optional, the entire stack appears below the main path.

  >>-required_item----+------------------+-----------------------><
                      +-optional_choice1-+
                      '-optional_choice2-'
   
  

  If one of the items is the default, it will appear above the main path and the remaining choices will be shown below.

                      .-default_choice--.
  >>-required_item----+-----------------+------------------------><
                      +-optional_choice-+
                      '-optional_choice-'
   
  

• An arrow returning to the left, above the main line, indicates an item that can be repeated.

                      .-------------------.
                      V                   |
  >>-required_item-------repeatable_item--+----------------------><
   
  

  If the repeat arrow contains a comma, you must separate repeated items with a comma.

                      .-,-----------------.
                      V                   |
  >>-required_item-------repeatable_item--+----------------------><
   
  

  A repeat arrow above a stack indicates that you can repeat the items in the stack.

• Keywords appear in uppercase (for example, FROM). They must be spelled exactly as shown. Variables appear in all lowercase letters (for example, column-name). They represent user-supplied names or values.
• If punctuation marks, parentheses, arithmetic operators, or other such symbols are shown, you must enter them as part of the syntax.