left join without duplicate values using MIN()

问题:

I have a table_1:

id  custno
1   1
2   2
3   3

and a table_2:

id  custno  qty  descr
1   1       10   a
2   1       7    b
3   2       4    c
4   3       7    d
5   1       5    e
6   1       5    f

When I run this query to show the minimum order quantities from every customer:

SELECT DISTINCT table_1.custno,table_2.qty,table_2.descr 
FROM table_1 
LEFT OUTER JOIN table_2 
    ON  table_1.custno = table_2.custno AND  qty = (SELECT MIN(qty) FROM table_2 
WHERE table_2.custno = table_1.custno   )

Then I get this result:

custno qty  descr
1      5    e 
1      5    f
2      4    c
3      7    d

Customer 1 appears twice each time with the same minimum qty (& a different description) but I only want to see customer 1 appear once. I don't care if that is the record with 'e' as a description or 'f' as a description.

回答1:

"Generic" SQL way:

SELECT table_1.custno,table_2.qty,table_2.descr 
FROM table_1, table_2 
WHERE table_2.id = (SELECT TOP 1 id 
                    FROM table_2 
                    WHERE custno = table_1.custno 
                    ORDER BY qty )

SQL 2008 way (probably faster):

SELECT custno, qty, descr 
FROM
(SELECT 
    custno, 
    qty, 
    descr,
    ROW_NUMBER() OVER (PARTITION BY custno ORDER BY qty) RowNum
    FROM table_2
) A
WHERE RowNum = 1

回答2:

First of all... I'm not sure why you need to include table_1 in the queries to begin with:

select  custno, min(qty) as min_qty 
from    table_2 
group   by custno;

But just in case there is other information that you need that wasn't included in the question:

select table_1.custno, ifnull(min(qty),0) as min_qty
from   table_1 
       left outer join table_2 
          on table_1.custno = table_2.custno 
group  by table_1.custno;

回答3:

If you use SQL-Server you could use ROW_NUMBER and a CTE:

WITH CTE AS
(
    SELECT table_1.custno,table_2.qty,table_2.descr,
       RN = ROW_NUMBER() OVER ( PARTITION BY table_1.custno
                                Order By table_2.qty ASC)
    FROM table_1 
    LEFT OUTER JOIN table_2 
        ON  table_1.custno = table_2.custno 
)
SELECT custno, qty,descr
FROM CTE
WHERE RN = 1

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