A # character begins a comment that extends to the end of the line.
A /* sequence begins a comment that ends with a */ sequence:
/* this is a comment */
/*
this
is a
comment
*/
A -- (double dash) sequence followed by a space begins a comment that extends to the end of the line.
If the comment begins with /*! rather than with /*, MySQL executes the body of the comment as part of the surrounding query. The following statement creates a table named t, but for MySQL creates it as a HEAP table:
CREATE TABLE t (i INT) /*! TYPE = HEAP */;