Using alias in mysql is not working -

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i confused usage of alias. example, query below works fine

select *  ( select row_number() on (partition prodid order quantity desc) 'rankin',prodid,quantity sales ) rankin=1

but when modify shown in snippet below error: "invalid column name 'rownumber'". 

select row_number() on (order quantity) 'rownumber' sales rownumber = 1

please explain difference.

this because select executed after where , from executed before where cannot used in second query can used in first query.


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