Comments newline, and so on). This syntax differs slightly from standard SQL comment syntax, as discussed in Section 1.7.2.4, “’--’ as the Start of a Comment” . The following example demonstrates all three comment styles: Nested comments are not supported, and are deprecated; expect them to be removed in a future MySQL release. (Under some conditions, nested comments might be permitted, but usually are not, and users should avoid them.) MySQL Server supports certain variants of C-style comments. These enable you to write code that includes MySQL extensions, but is still portable, by using comments of the following form: In this case, MySQL Server parses and executes the code within the comment as it would any other SQL statement, but other SQL servers should ignore the extensions. For example, MySQL Server recognizes the  STRAIGHT_JOIN  keyword in the following statement, but other servers should not: If you add a version number after the  !  character, the syntax within the comment is executed only if the MySQL version is greater than or equal to the specified version number. The  KEY_BLOCK_SIZE  keyword in the following comment is executed only by servers from MySQL 5.1.10 or higher: The comment syntax just described applies to how the  mysqld  server parses SQL statements. The  mysql client program also performs some parsing of statements before sending them to the server. (It does this to determine statement boundaries within a multiple-statement input line.) For information about differences between the server and  mysql  client parsers, see Section 4.5.1.6, “mysql Client Tips” . Comments in  /*!12345 ... */  format are not stored on the server. If this format is used to comment stored programs, the comments are not retained in the program body. Another variant of C-style comment syntax is used to specify optimizer hints. Hint comments include a  + character following the  /*  comment opening sequence. Example: For more information, see Section 8.9.3, “Optimizer Hints” . The use of short-form  mysql  commands such as  \C  within multiple-line  /* ... */  comments is not supported. Short-form commands do work within single-line  /*! ... */  version comments, as do  / *+ ... */  optimizer-hint comments, which are stored in object definitions. If there is a concern that optimizer-hint comments may be stored in object definitions so that dump files when reloaded with  mysql •From a  /*  sequence to the following  */  sequence, as in the C programming language. This syntax enables a comment to extend over multiple lines because the beginning and closing sequences need not be on the same line. mysql>  SELECT 1+1; # This comment continues to the end of line mysql>  SELECT 1+1; -- This comment continues to the end of line mysql>  SELECT 1 /* this is an in-line comment */ + 1; mysql>  SELECT 1+ /* this is a multiple-line comment */ 1; /*!  MySQL-specific code  */ SELECT /*! STRAIGHT_JOIN */ col1 FROM table1,table2 WHERE ... CREATE TABLE t1(a INT, KEY (a)) /*!50110 KEY_BLOCK_SIZE=1024 */; SELECT /*+ BKA(t1) */ FROM ... ; 1923