如何提取sql语句的第n行？

Question

I am trying to extract the nth row of an SQL statement (not the nth row of a table). Is there an easy way to run a query and read values from specific rows.

I have tried something similar to this but it does not work since rownum tells me what nth record it is in the table.

SELECT * FROM (
    SELECT
        ROW_NUMBER() OVER(ORDER BY RewardID ASC) AS rownumber,
        RewardID,
        Name,
        Description,
        Image,
        RewardType,
        price
    FROM
        Reward
) AS num 
WHERE
    RewardType = 'Electronics' and rownum = 2

Answer 1

Are you using Postgres Or SQLServer? Both have a ROW_NUMBER() window function, but the syntax, especially around subqueries, may be slightly different. I'll assume Postgres.

You query looks good to me, except that the rownum = 2 in your WHERE should presumably be rownumber = 2 , since that's what you aliased the ROW_NUMBER() column as. You also presumably don't need the RewardType and rownumber columns in your result, since their values will always be 'Electronics' and 2 respectively. Corrected and formatted for readability, this looks ok to me:

SELECT RewardID, Name, Description, Image, RewardType, price
FROM (
    SELECT
        ROW_NUMBER() OVER(ORDER BY RewardID ASC) AS rownumber,
        RewardID,
        Name,
        Description,
        Image,
        RewardType,
        price
    FROM Reward) AS num
WHERE RewardType = 'Electronics' and rownumber = 2

The only questionable part of that is the WHERE RewardType = 'Electronics' . Should that actually be in the subquery rather than the outer query? The difference is that if it is in the subquery, the row counting will include only reward type 'Electronics', whereas in the outer query, all reward types will be counted. To only count the reward type 'Electronics', modify it as so:

SELECT RewardID, Name, Description, Image, RewardType, price
FROM (
    SELECT
        ROW_NUMBER() OVER(ORDER BY RewardID ASC) AS rownumber,
        RewardID,
        Name,
        Description,
        Image,
        RewardType,
        price
    FROM Reward
    WHERE RewardType = 'Electronics') AS num
WHERE rownumber = 2

Edit: Since the comment you just made clarifies what you're really trying to do, I'll add that you should NOT be making an individual query for each row of the data that you want. Whatever interface you are using to your database will have a way of iterating over a query result that uses multiple rows, and if that's what you really need, you should find out how to do that.

Answer 2

You can try using OFFSET/FETCH :

SELECT
    RewardID,
    Name,
    Description,
    Image,
    RewardType,
    price
FROM Reward
WHERE RewardType = 'Electronics'
OFFSET 1 ROWS FETCH NEXT 1 ROWS ONLY

Tell it to OFFSET (skip) row 1 then FETCH (select) 1 row after that, which would be row 2. This removes the need for a sub select.

Answer 3

The key is the order by, try using null in the order by. SELECT * FROM ( SELECT ROW_NUMBER() OVER(ORDER BY (select null)) AS rownumber, RewardID, Name, Description, Image, RewardType, price FROM Reward ) AS num WHERE RewardType = 'Electronics' and rownum = 2