mysql中的dual表

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Table - `dual`：a dummy table in mysql
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.
   


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