使用內部聯接為每個ID返回一行

Question

在SQL Server數據庫中，我有兩個表，一個表是另一個的父表。 子表與父表具有多對一關系。 我只需要為每個父項返回一個記錄，而其對應的子表記錄則是最上面的。 但是我不知道該怎么做。

以下是我現在擁有的查詢：

SELECT DISTINCT
    products.id,
    sku,
    name,
    mediaid
FROM
    products
INNER JOIN 
    productmedia ON products.id = productmedia.productid
WHERE
    sku IN ('HCFL 207 23 (A)','HCFL 207 22 (A)','HCFL 207 21 (A)','HCFL 207 20 (A)')
    AND productmedia.sort = 100 

我得到以下信息：

183816  HCFL 207 20 (A) HCFL SERIES- 2-BOLT FLANGE, ECCENTRIC LOCKING COLLAR, BORE DIA. 1 1/4", WIDE INNER RACE INSERT -ASIAN   2929082
183816  HCFL 207 20 (A) HCFL SERIES- 2-BOLT FLANGE, ECCENTRIC LOCKING COLLAR, BORE DIA. 1 1/4", WIDE INNER RACE INSERT -ASIAN   2929083
183817  HCFL 207 21 (A) HCFL SERIES-2-BOLT FLANGE, ECCENTRIC LOCKING COLLAR, BORE DIA. 1 5/16", WIDE INNER RACE INSERT-ASIAN    2929138
183817  HCFL 207 21 (A) HCFL SERIES-2-BOLT FLANGE, ECCENTRIC LOCKING COLLAR, BORE DIA. 1 5/16", WIDE INNER RACE INSERT-ASIAN    2929139
183818  HCFL 207 22 (A) HCFL SERIES- 2-BOLT FLANGE, ECCENTRIC LOCKING COLLAR, BORE DIA. 1 3/8", WIDE INNER RACE INSERT -ASIAN   2929086
183818  HCFL 207 22 (A) HCFL SERIES- 2-BOLT FLANGE, ECCENTRIC LOCKING COLLAR, BORE DIA. 1 3/8", WIDE INNER RACE INSERT -ASIAN   2929087
183819  HCFL 207 23 (A) HCFL SERIES- 2-BOLT FLANGE, ECCENTRIC LOCKING COLLAR, BORE DIA. 1 7/16", WIDE INNER RACE INSERT -ASIAN  2929084
183819  HCFL 207 23 (A) HCFL SERIES- 2-BOLT FLANGE, ECCENTRIC LOCKING COLLAR, BORE DIA. 1 7/16", WIDE INNER RACE INSERT -ASIAN  2929085

Answer 1

;with cte as(
select s.id studentId, s.name, c.class, 
ROW_NUMBER() over(partition by class order by s.id) rn 
from Student s
inner join class c on s.classId = c.id
)
select * from cte where rn = 1

您可以將“按s.id排序”更改為您認為對結果進行排序所基於的任何其他列。

Answer 2

CROSS APPLY是您的朋友：

SELECT DISTINCT
    products.id,
    sku,
    name,
    mediaid
FROM
    products
    CROSS APPLY (
        SELECT TOP(1)
            * -- List the required columns!
        FROM
            productmedia
        WHERE           
            products.id = productmedia.productid
            AND productmedia.sort = 100 
        ORDER BY
            YourOrderColumn -- Change it to your required order!
    ) AS X
WHERE
    sku IN ('HCFL 207 23 (A)','HCFL 207 22 (A)','HCFL 207 21 (A)','HCFL 207 20 (A)')