至少5.1已经有DUAL You are allowed to specify DUAL as a dummy table name in situations where no tables are referenced: mysql> SELECT 1 + 1 FROM DUAL; -> 2DUAL 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
3.mysql的dual一般在什么清下使用?DUAL is purely for the convenience of people who require that all SELECT statements should have FROM and possibly other clauses
You are allowed to specify DUAL as a dummy table name in situations where no tables are referenced: mysql> SELECT 1 + 1 FROM DUAL;
-> 2DUAL 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