如何从输入类别多次的表中选择每个类别的一项

Question

Product table

_________________________________

|id |name |category |date |

---------------------------------

|1 |p1 |1 |13586486|

---------------------------------

|2 |p2 |2 |13586445|

---------------------------------

|3 |p3 |1 |13567456|

---------------------------------

|5 |p5 |3 |13586422|

--------------------------------

|6 |p6 |3 |13586678|

--------------------------------

|7 |p7 |1 |13586495|

-------------------------------- this somehow my table looks, now my question is how to select one product of each category which is the latest in that category, is this possible in a single query? i tried so many things but its too lengthy... help please

Answer 1

Maybe something like this (if you want it per the highest date):

SELECT
    *
FROM
    Product AS t1
    JOIN 
    (
    SELECT 
        MAX(t.date) AS date,
        t.category
    FROM 
        Product AS t
    GROUP BY
        t.category
    )
    AS maxCat 
    ON maxCat.date=t1.date
    AND maxCat.category=t1.category

If you want it by the highest product id then you can do this:

SELECT
    *
FROM
    Product AS t1
    JOIN 
    (
    SELECT 
        MAX(t.id) AS id,
        t.category
    FROM 
        Product AS t
    GROUP BY
        t.category
    )
    AS maxCat 
    on maxCat.id=t1.id

Answer 2

select p1.* from Product p1 left join Product p2 on p1.category = p2.category and p1.id < p2.id where p2.id is null

This will give you latest record based on latest id as there is no other field by which I can find time of added record.

Edit changed select * from to select p1.* from , it will not return extra empty columns from table p2.

Answer 3

In Sql server you may use this:

with productWithRows as(
select row_number() over (partition by category order by date desc) as rownb,*
from product
) as productWithRows,
select * from productWithRows where rownb=1