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 1.5.5.7 `--' as the Start of a Comment
 ......................................
 
 Some other SQL databases use `--' to start comments.  MySQL Server uses
 `#' as the start comment character. You can also use the C comment
 style `/* this is a comment */' with MySQL Server.   Comments.
 
 MySQL Server 3.23.3 and above support the `--' comment style, provided
 the comment is followed by a space (or by a control character such as a
 newline). The requirement for a space is to prevent problems with
 automatically generated SQL queries that have used something like the
 following code, where we automatically insert the value of the payment
 for `!payment!':
 
      UPDATE account SET credit=credit-!payment!
 
 Think about what happens if the value of `payment' is a negative value
 such as `-1':
 
      UPDATE account SET credit=credit--1
 
 `credit--1' is a legal expression in SQL, but if `--' is interpreted as
 the start of a comment, part of the expression is discarded. The result
 is a statement that has a completely different meaning than intended:
 
      UPDATE account SET credit=credit
 
 The statement produces no change in value at all!  This illustrates that
 allowing comments to start with `--' can have serious consequences.
 
 Using our implementation of this method of commenting in MySQL Server
 3.23.3 and up, `credit--1' is actually safe.
 
 Another safe feature is that the `mysql' command-line client removes
 all lines that start with `--'.
 
 The following information is relevant only if you are running a MySQL
 version earlier than 3.23.3:
 
 If you have an SQL program in a text file that contains `--' comments,
 you should use the `replace' utility as follows to convert the comments
 to use `#' characters:
 
      shell> replace " --" " #" < text-file-with-funny-comments.sql \
               | mysql DB_NAME
 
 instead of the usual:
 
      shell> mysql DB_NAME < text-file-with-funny-comments.sql
 
 You can also edit the command file "in place" to change the `--'
 comments to `#' comments:
 
      shell> replace " --" " #" -- text-file-with-funny-comments.sql
 
 Change them back with this command:
 
      shell> replace " #" " --" -- text-file-with-funny-comments.sql
 

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