cannt get a record by the row number

i get an error with this query

select ID, ROW_NUMBER() OVER(ORDER BY ID) as num from T_TASK where ROW_NUMBER() = 5

and this one

select ID, ROW_NUMBER() OVER(ORDER BY ID) as num from T_TASK where num = 4

whats w开发者_开发问答rong with the queries?

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SELECT ID
FROM
(select ID, ROW_NUMBER() OVER(ORDER BY ID) as rownum from T_TASK) dr
WHERE rownum = 5

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Use a subquery:

SELECT ID
FROM (
    SELECT ID, ROW_NUMBER() OVER(ORDER BY ID) AS num
    FROM T_TASK
) T1 WHERE num = 5

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1 You can't use windows functions directly on the WHERE clause

2 The same is applied to alias name too