如何仅从SQL Join中的第二个表中获得前1行

Question

我想加入两个表，其中table1仅包含1条记录，table2包含具有相同ID但序列号不同的多个记录。 加入是在wkid上，但我只想从table2获取最大序列号

SELECT
        t1.wkid
    ,   t1.ser
    ,   t2.nama
    ,   t1.comments
FROM
        table1 t1
    ,   table2 t2
WHERE
        t1.wkid = t2.wkid
    AND t2.ser = MAX(ser)

表创建和插入：

create table table1(wkid int, ser int, comments text);
insert into table1 values(721142, 1, 'asdfasd');
insert into table1 values(721142, 2, 'vnivnie');
insert into table1 values(721142, 3, 'el;eklke');
insert into table1 values(721142, 4, 'fefeo');
insert into table1 values(721142, 5, 'jijie');
insert into table1 values(721143, 1, 'my comments 1');
insert into table1 values(721143, 2, 'my comments 2');
insert into table1 values(721143, 3, 'my comments 3');

create table table2(wkid int, nama text);
insert into table2 values(721142, 'John');
insert into table2 values(721143, 'Andy');
insert into table2 values(721144, 'Khan');
insert into table2 values(721145, 'Jack');

预期结果：

721142 | 5 | John | jijie
721143 | 3 | Andy | my comments 3

Answer 1

您可以使用row_number()

演示

select * from 
(
select t1.wkid, t1.ser, t2.nama, t1.comments, row_number() over(partition by t1.wkid order by ser desc) as rn 
from table1 t1 inner join table2 t2 on t1.wkid = t2.wkid
)A where rn=1 

输出：

wkid    ser nama    comments       rn
721142  5   John    jijie           1
721143  3   Andy    my comments 3   1

Answer 2

隐式加入

SELECT A.wkid, A.ser, B.nama, A.comments
    FROM table1 A, table2 B
    WHERE B.wkid = A.wkid
        AND A.ser = (SELECT max(X.ser) FROM table1 X
                        WHERE X.wkid = A.wkid)

显式加入

SELECT A.wkid, A.ser, B.nama, A.comments
    FROM table1 A JOIN table2 B
    ON B.wkid = A.wkid
    WHERE A.ser = (SELECT max(X.ser) FROM table1 X
                        WHERE X.wkid = A.wkid)

Answer 3

请勿在FROM子句中使用逗号。 始终使用正确，明确， 标准的 JOIN语法。

在这种情况下，具有讽刺意味的是，最好的解决方案可能是使用APPLY而不是JOIN ：

SELECT t1.wkid, t2.ser, t2.nama, t1.comments
FROM table1 t1 CROSS APPLY
     (SELECT TOP (1) t2.*
      FROM table2 t2
      WHERE t1.wkid = t2.wkid
      ORDER BY t2.ser DESC
     ) t2;

这应该有效地使用table2(wkid, ser desc)上的索引。