合并 SQL 个表以保留最新的行

Question

我有一个现有的 SQL table 有点像这样

Question	Answer	AnsWhen	AnsWho
Q1	Red	2022-03-01	Rod
Q1	Green	2022-03-20	Freddie
Q2	Blue	2022-04-05	Jane
Q2	Yellow	2022-04-01	Rod
Q3	Green	2022-04-05	Jane
Q3	Green	2022-04-04	Rod
Q3	Green	2022-04-06	Freddie

我需要删除重复的问题，只保留最新的数据，所以完成的table看起来像

Question	Answer	AnsWhen	AnsWho
Q1	Green	2022-03-20	Freddie
Q2	Blue	2022-04-05	Jane
Q3	Green	2022-04-06	Freddie

Answer 1

如果你的数据库支持，ROW_NUMBER可以在这里使用：

WITH cte AS (
    SELECT t.*, ROW_NUMBER() OVER (PARTITION BY Question ORDER BY AnsWhen DESC) rn
    FROM yourTable t
)

SELECT Question, Answer, AnsWhen, AnsWho
FROM cte
WHERE rn = 1;

Answer 2

我没有测试过，大概是这样。

with cteQuestions
as
(
select a.question,max(a.AnsWhen) as MAX_DATE
from tblAnswers as a
group by a.question
)
select q.question,a.answer,q.MAX_DATE,a.AnsWho
from cteQuestions as q
left join tblAnswers as a on q.question=a.question and q.max_date=a.AnsWhen

合并 SQL 个表以保留最新的行

Merging SQL tables keeping the latest rows

sql

greatest-n-per-group