如何从具有 JOINed 记录的 SELECT 中获取唯一结果？

Question

我有这 2 个表，我想查询其中的单个唯一记录：

create table active_pairs
(
    id                    integer,
    pair                  text,
    exchange_id           integer
);


create table exchanges
(                        
    exchange_id         integer,
    exchange_full_name  text
);

INSERT INTO active_pairs (pair, exchange_id)

VALUES ('London/Berlin', 2),
       ('London/Berlin', 3),
       ('Paris/Berlin', 4),
       ('Paris/Berlin', 3),
       ('Oslo/Berlin', 2),
       ('Oslo/Berlin', 6),
       ('Huston/Berlin', 2);
       
INSERT INTO exchanges (exchange_id, exchange_full_name)

VALUES (2, 'Exchange 1'),
       (3, 'Exchange 2'),
       (4, 'Exchange 3'),
       (3, 'Exchange 21'),
       (2, 'Exchange 12'),
       (6, 'Exchange 11'),
       (2, 'Exchange 31');

查询列出只有一个 pair 记录的项目：

SELECT *  FROM active_pairs ap
    INNER JOIN exchanges ce on ap.exchange_id = ce.exchange_id
WHERE ap.exchange_id = :exchangeId
GROUP BY pair, ap.exchange_id, ce.exchange_id, ap.id
HAVING COUNT(ap.pair) = 1
ORDER BY :sort
LIMIT :limit
OFFSET :offset

当我运行查询时，我没有得到正确的结果。我只需要获取 Huston/Berlin，因为这是唯一的记录（请注意，我们还有另一条 exchange_id = 2 的记录）。现在我得到结果 Huston/Berlin 和 'London/Berlin' 与 exchange_id = 2 这是不正确的。

另一个例子：当我查询 exchange_id=4 时，我需要得到空结果，因为如您所见，我有 Paris/Berlin 用于 exchange_id 3 和4.

你能告诉我如何解决这个问题吗？

Answer 1

没有更多的样本来检查结果，解决方案可以是这样的：

SELECT ap.pair, ap.exchange_id, ce.exchange_id, ap.id  FROM active_pairs ap
    INNER JOIN exchanges ce on ap.exchange_id = ce.exchange_id
    INNER JOIN (SELECT pair FROM active_pairs GROUP BY pair HAVING COUNT(pair) = 1) p on p.pair = ap.pair
WHERE ap.exchange_id = :exchangeId
GROUP BY pair, ap.exchange_id, ce.exchange_id, ap.id
ORDER BY :sort
LIMIT :limit
OFFSET :offset

我猜你只想要你给出的小例子中唯一的活动对名称。

Answer 2

如果我理解正确，这就是你想要做的：

select * from (
   select *, count(*) over (partition by pair) as cc from active_pairs 
   ) t join exchanges e on t.exchange_id = e.exchange_id and t.cc=1

db<>fiddle here

Answer 3

SQL

SELECT ap.*, ce.* FROM active_pairs ap
INNER JOIN
  (SELECT pair
   FROM active_pairs
   GROUP BY pair
   HAVING COUNT(*) = 1) subq
  ON ap.pair = subq.pair
INNER JOIN exchanges ce
  ON ap.exchange_id = ce.exchange_id
WHERE ap.exchange_id = :exchangeId
ORDER BY :sort
LIMIT :limit
OFFSET :offset;

说明

子查询 (subq) 过滤以仅包含出现一次的对名称。然后将其加入 active_pairs table 以获取 exchange_id，然后根据您的原始查询加入交易所 table。

演示

https://dbfiddle.uk/?rdbms=mysql_8.0&fiddle=b3e2dcc5d09401e5bed1f42fdde82a6b

如何从具有 JOINed 记录的 SELECT 中获取唯一结果？

How to obtain unique results from a SELECT with JOINed records?

sql

hibernate

SQL

说明

演示