Re: AND or UNION

Wong Chung Yin wrote:
> Which one is more efficient?
>
> 1. select * from empdemo where empdemo = 0 and empdemo = 1;
> 2. select * from empdemo where empdemo = 0
> union
> select * from empdemo where empdemo = 1;

  It depends on the several factors such as the selectivity, the indexed columns or the optimizer_mode.

  In general, (1) seems to be more effective especially when you have set an index on empdemo. It can use index access. On the other hand, (2) requires sorting. It is time consuming.  

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Name   : Lun Wing San
Title  : Oracle Application Developer of Hong Kong Productivity Council
         Oracle Database Administrator and System Administrator of QRC
Phone  : (852)27885841

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