SQL Inner Join only one row based on filtered records

Question

我有以下 tables

用户

id	name
1	John
2	Jack
3	George

作文

id	date_submitted	user_id	project_id
1	2020-12-10	1	1
2	2020-05-01	2	2
3	2020-04-10	3	2

project_phase

id	project_id	date	phase
1	1	2020-01-01	early
2	1	2020-05-31	mid
3	1	2020-11-30	late
4	2	2020-01-01	early
5	2	2020-03-31	mid
6	2	2020-04-30	late

1 位用户在一个项目中只提交了 1 篇论文我想要一个组合 table 来显示用户和提交论文的阶段。

我正在尝试从 project_phase 获取最大日期低于 date_submitted

的记录

所以使用上面的数据我想得到

user	date_submitted	phase
John	2020-12-10	late
Jack	2020-05-01	late
George	2020-04-10	mid

到目前为止，我将 tables 与 INNER JOIN 组合在一起，但以下查询 returns 重复记录。我也尝试了 ROW_NUMBER() 和 MAX() 但我没有正确构建它来获得我正在寻找的答案

SELECT
    U.name,
    E.date_submitted,
    P.phase
FROM
    essay E
INNER JOIN user U ON U.id = E.user_id
INNER JOIN project_phase P ON P.project_id = E.project_id and E.date <= P.date_submitted

感谢任何帮助，谢谢。

Answer 1

我建议使用相关子查询：

SELECT U.name, E.date_submitted,
       (SELECT P.phase
        FROM project_phase P
        WHERE P.project_id = E.project_id AND
              P.date <= E.date_submitted 
        ORDER BY p.date DESC
        LIMIT 1
       ) as phase
FROM essay E JOIN
     user U
     ON U.id = E.user_id;

SQL Inner Join only one row based on filtered records

SQL Inner Join only one row based on filtered records

mysql

sql

inner-join