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Mysql 中的dual表
0.参看mysql refman
SELECT can also be used to retrieve rows computed without reference to any table.
SELECT Syntax
2099
For example:
mysql> SELECT 1 + 1;
-> 2
You are permitted 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.
1.
简言之，from dual完全是一个可有可无的东西。只是为了方便使用select 语句中喜欢带上from的开发者。