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Re: using alias in where clause




On Mon, Jan 25, 2016 at 7:27 PM, Reindl Harald <h.reindl@xxxxxxxxxxxxx> wrote:
>
>
> Am 26.01.2016 um 01:16 schrieb Larry Martell:
>>
>> I know I cannot use an alias in a where clause, but I am trying to
>> figure out how to achieve what I need.
>>
>> If I could have an alias in a where clause my sql would look like this:
>>
>> SELECT IFNULL(f_tag_bottom,
>>                  IFNULL(f_tag_bottom_major_axis,
>>                   IFNULL(f_tag_bottom_minor_axis,
>>                    IFNULL(f_tag_ch_x_bottom,
>>                     IFNULL(f_tag_ch_y_bottom, NULL))))) as ftag,
>>                 STDDEV(ch_x_top)
>> FROM data_cst
>> WHERE ftag = 'E-CD7'
>> GROUP BY wafer_id, lot_id
>>
>> But I can't use ftag in the where clause. I can't put it in a having
>> clause, as that would exclude the already aggregated rows and I want
>> to filter then before the aggregation. Anyone have any idea how I can
>> do this?
>
>
> why would you want to do that?
> it would make the where clause not be able to use proper indexes

I do not know which of the 5 f_tag_* columns will be populated and I
need to filter on the one that is populated.

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