Parameter MARKER: an application may wish to call the same query multiple times, only changing specific values each time (ex: parameters sent to a stored procedure, or a value in a WHERE clause).

参数标志(PARAMETER MARKER)：应用程序可能希望多次调用同一个查询，每次调用时只改变特定的值(例如，发送给存储过程的参数，或者where子句中的值)。

youdao

You cannot use a column with RAND() values in an ORDER BY clause, because ORDER BY would evaluate the column multiple times.

查看官方手册，也说rand（）放在ORDER BY 子句中会被执行多次，自然效率及很低。

youdao