MySQL supports three forms of comment syntax. One of those forms has variants that allow special instructions to be passed through to the MySQL server.
C-style comments can contain embedded SQL text that's treated specially by the MySQL server, but ignored by other database engines. This is an aid to writing more portable SQL because it enables you to write comments that are treated as part of the surrounding statement if executed by MySQL and ignored if executed by other database servers. There are two ways to write embedded SQL in a C-style comment: