Re: using alias in where clause

[Johnny Withers <johnny@xxxxxxxxxxxxx>]

You should probably turn this into a UNION and put an index on each column:

SELECT f_tag_ch_y_bottom AS ftag FROM data_cst WHERE f_tag_ch_y_bottom =
'E-CD7'
UNION ALL
SELECT f_tag_ch_x_bottom AS ftag FROM data_cst WHERE f_tag_ch_x_bottom =
'E-CD7'
UNION ALL
SELECT f_tag_bottom_minor_axis AS ftag FROM data_cst WHERE
f_tag_bottom_minor_axis = 'E-CD7'
UNION ALL
SELECT f_tag_bottom_major_axis AS ftag FROM data_cst WHERE
f_tag_bottom_major_axis = 'E-CD7'
UNION ALL
SELECT f_tag_bottom FROM data_cst AS ftag WHERE f_tag_bottom = 'E-CD7'
;

Doing this any other way will prevent index usage and a full table scan
will be required.

Is there a possibility of more than one column matching? How would you
handle that?


On Mon, Jan 25, 2016 at 6:16 PM, Larry Martell <larry.martell@xxxxxxxxx>
wrote:

> 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?
>
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-- 
-----------------------------
Johnny Withers
601.209.4985
johnny@xxxxxxxxxxxxx