"Error" in creating trigger code in MySQL Must Know

　　In MySQL 5.7.20, the code for creating triggers in the chapter using triggers in "MySQL Must Know and Know" reports an error: ERROR 1415 (0A000): Not allowed to return a result set from a trigger.

　　Here is the original quote from the book:

 

Error: Trigger cannot return result set

Looking up information online, you can assign strings to variables