有人可以解释一下这个查询...... [英] Can some body explain this query ...

问题描述

其实我从这个链接找到了这个查询

http://use-the-index-luke.com/sql/where-clause/obfuscation/smart-logic [ ^ ]

Actually i found this query from this link
http://use-the-index-luke.com/sql/where-clause/obfuscation/smart-logic[^]

SELECT first_name, last_name, subsidiary_id, employee_id
  FROM employees
 WHERE( subsidiary_id    = NULL     OR NULL IS NULL )
   AND( employee_id      = NULL     OR NULL IS NULL )
   AND( UPPER(last_name) = 'WINAND' OR 'WINAND' IS NULL )

解释这是什么意思NULL IS NULL及其功能在哪里？谢谢

Explain what does this mean " NULL IS NULL" and what is its functionality here ?Thanks

推荐答案

NULL IS NULL将只返回true，因此where子句的那一部分完全没有意义，因为无论topic_id字段发生什么，OR的结果总是为真。

NULL IS NULL will simply return true, hence that part of the where clause is completely pointless, as regardless what happens with the subsidiary_id field, the result of the OR will always be true.

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