SQL:如何限制在第一个找到的行上的联接? [英] SQL: how to limit a join on the first found row?

问题描述

如何在两个表之间建立联接，但仅限于满足联接条件的第一行?

How to make a join between two tables but limiting to the first row that meets the join condition ?

在这个简单的示例中，我想为table_A中的每一行获取table_B中满足条件的第一行:

In this simple example, I would like to get for every row in table_A the first row from table_B that satisfies the condition :

select table_A.id, table_A.name, table_B.city 
from table_A join table_B 
on table_A.id = table_B.id2
where ..

table_A (id, name)
1, John
2, Marc

table_B (id2, city)
1, New York
1, Toronto
2, Boston

The output would be:
1, John, New York
2, Marc, Boston

可能是Oracle提供了这样的功能(性能是一个问题).

May be Oracle provides such a function (performance is a concern).

推荐答案

此处的关键字是 FIRST .您可以使用解析函数FIRST_VALUE或聚合构造FIRST.
对于FIRST或LAST，性能永远不会比同等的FIRST_VALUE或LAST_VALUE构造差，并且经常会更好，因为我们没有多余的窗口排序，因此执行成本更低:

The key word here is FIRST. You can use analytic function FIRST_VALUE or aggregate construct FIRST.
For FIRST or LAST the performance is never worse and frequently better than the equivalent FIRST_VALUE or LAST_VALUE construct because we don't have a superfluous window sort and as a consequence a lower execution cost:

select table_A.id, table_A.name, firstFromB.city 
from table_A 
join (
    select table_B.id2, max(table_B.city) keep (dense_rank first order by table_B.city) city
    from table_b
    group by table_B.id2
    ) firstFromB on firstFromB.id2 = table_A.id 
where 1=1 /* some conditions here */
;

由于12c引入了运算符LATERAL以及CROSS/OUTER APPLY联接，因此可以在JOIN子句的右侧使用相关子查询:

Since 12c introduced operator LATERAL, as well as CROSS/OUTER APPLY joins, make it possible to use a correlated subquery on right side of JOIN clause:

select table_A.id, table_A.name, firstFromB.city 
from table_A 
cross apply (
    select max(table_B.city) keep (dense_rank first order by table_B.city) city
    from table_b
    where table_B.id2 = table_A.id 
    ) firstFromB
where 1=1 /* some conditions here */
;

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