Question

我有这张＆＃34; Letter_Senders＆＃34;：

的表格

> LETTER_ID | SENDER | RECIPIENT  
> 123       | John   | David  
> 234       | Josh   | Andrew  
> 345       | David  | John  
> 456       | Matt   | David 
> 567       | David  | Matt  
> 678       | John   | David

我非常感谢帮助总结每对人民之间发送的信件数量，如下所示＆＃34; Counting_Letters＆＃34;表：

Person_1  | Person_2 | Num_Of_Letters
John      | David    | 3
Josh      | Andrew   | 1
Matt      | David    | 2

试图弄清楚但不知道，需要考虑从两个人方只计算一次。

Answer 1

如果您的dbms支持greatest和least等功能，请使用

SELECT greatest(person_1,person_2) AS person_1,
       least(person_1,person_2) AS person_2,
       count(*) AS num_letters
FROM letter_senders
GROUP BY greatest(person_1,person_2),
         least(person_1,person_2)

也可以使用case表达式来实现。

SELECT CASE
           WHEN person_1>person_2 THEN person_1
           ELSE person_2
       END AS person_1,
       CASE
           WHEN person_1<person_2 THEN person_1
           ELSE person_2
       END AS person_2,
       count(*) AS num_letters
FROM letter_senders
GROUP BY CASE
             WHEN person_1>person_2 THEN person_1
             ELSE person_2
         END,
         CASE
             WHEN person_1<person_2 THEN person_1
             ELSE person_2
         END

Answer 2

我想出了与vkp相同的答案（在相同的假设下工作）：

IF OBJECT_ID('tempdb..#Test') IS NOT NULL
    DROP TABLE #Test

CREATE TABLE #Test
(
    LETTER_ID INT
    ,SENDER VARCHAR(20)
    ,RECIPIENT VARCHAR(20)
)

INSERT #Test
VALUES
(123, 'John', 'David'),
(234, 'Josh', 'Andrew'),
(345, 'David', 'John'),
(456, 'Matt', 'David'),
(567, 'David', 'Matt'),
(678, 'John', 'David')

SELECT
CASE
    WHEN SENDER < RECIPIENT
        THEN SENDER
    ELSE RECIPIENT
END [Person_1]
,CASE
    WHEN SENDER > RECIPIENT
        THEN SENDER
    ELSE RECIPIENT
END [Person_2]
,COUNT(*) [Num_Of_Leters]
FROM #Test
GROUP BY CASE
    WHEN SENDER < RECIPIENT
        THEN SENDER
    ELSE RECIPIENT
END
,CASE
    WHEN SENDER > RECIPIENT
        THEN SENDER
    ELSE RECIPIENT
END

Answer 3

WITH
    letters (sender, recipient, num_of_letters) AS
    (
        SELECT
            sender
            , recipient
            , COUNT(*) num_of_letters
        FROM
            letter_senders
        GROUP BY
            sender
            , recipient
    )
SELECT
    person_1            sent.sender
    , person_2          sent.recipient
    , num_of_letters    sent.num_of_letters + received.num_of_letters
FROM        letters_sent sent
INNER JOIN  letters_sent received 
ON 
    (sent.sender = received.sender AND sent.recipient = received.recipient)
    OR (sent.recipient = received.sender AND sent.sender = received.recipient)

计算sql上双方之间的出现次数

3 个答案: