在oracle SQL语句中结合order by子句使用rownum

Question

Which of the following two SQL statements will return the desired result set (ie the ten rows with Status=0 and the highest StartTimes)?

Will both statements always return the same result set (StartTime is unique)?

SELECT * 
FROM MyTable 
WHERE Status=0 
AND ROWNUM <= 10 
ORDER BY StartTime DESC

SELECT * 
FROM (
    SELECT * 
    FROM MyTable 
    WHERE Status=0 
    ORDER BY StartTime DESC
) 
WHERE ROWNUM <= 10

Background

My DBAdmin told me that the first statement will first limit the table to 10 rows and than order those random rows by StartTime, which is definitly not what I want. From what I learned in that answer , the order by clause of the second statement is redundant and could be removed by an optimizer, which is also not what I want.

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I asked a similar question concering the limit clause in a query to a SQLite database and am interested in understanding any differences to the above statements (using limit Vs rownum ) used with an oracle db.

Answer 1

Your Second Query will work

Because in the first ,the first ten rows with Status 0 are selected and then the order by is done in that case the first ten rows fetched need not be in the highest order