mysql中的dual表

mysql文档中对于dual表的解释：

You are allowed to specify ​​DUAL​​ as a dummy table name in situations where no tables are referenced:

mysql> SELECT 1 + 1 FROM DUAL;
        -> 2

​​DUAL​​​ is purely for the convenience of people who require that all ​​​SELECT​​​​ statements should have ​​FROM​​​ and possibly other clauses. MySQL may ignore the clauses. MySQL does not require ​​FROM DUAL​​ if no tables are referenced.