Question

lastReports子查询本身返回10行。 “包装器”记录确实存在，但是当我加入它们时，我得到0 rows returned in 12ms from:在SQLite上。

我的加入很好，我没有犯选择错误（SQL LEFT JOIN Subquery Alias）……我想念的是什么，还是不明白？

SELECT
    name
FROM
    teachers
INNER JOIN (SELECT
            teacher_id,
            reportingPeriod,
            ReportingType,
            date('now') - 3 as AgeOfReport,
            count(id) as NumberOf
        FROM
            reports
        GROUP BY
            teacher_id
        ORDER BY
            teacher_id ASC,
            reportingPeriod asc
    ) AS lastReports 
ON teachers.id = lastReports.teacher_id;


SAMPLE DATA

TEACHERS
ID NAME 
-----------------
1  Mr John Smith
2  Ms Janet Smith

REPORTS
---------------------------------------------------
ID   TEACHER_ID   REPORTINGPERIOD REPORTINGTYPE
1   1             Jan 2017         Draft
2   1             Feb 2017         Draft
3   2             Jun 2018         Draft
4   2             Jul 2018         Draft
5   1             Mar 2017         Final


DESIRED RESULTS
------------------
Mr John Smith      Final    Mar 2017
Ms Janet Smith     Draft    Jul 2018

Answer 1

您可以尝试按row_number列创建TEACHER_ID并按REPORTINGPERIOD进行排序，这意味着在子查询中更接近日期，获得rn = 1每个{{1 }}最大日期和TEACHER_ID上的JOIN。

TEACHERS

查询1 ：

CREATE TABLE TEACHERS(
  ID INT,
  NAME VARCHAR(50)
);



INSERT INTO TEACHERS VALUES (1, 'Mr John Smith');
INSERT INTO TEACHERS VALUES (2, 'Ms Janet Smith');


CREATE TABLE REPORTS(
  ID INT,
  TEACHER_ID int,
   REPORTINGPERIOD DATE,
  REPORTINGTYPE varchar(100)
);



INSERT INTO REPORTS VALUES (1,1,'Jan 2017', 'Draft');
INSERT INTO REPORTS VALUES (2,1,'Feb 2017', 'Draft');
INSERT INTO REPORTS VALUES (3,2,'Jun 2018', 'Draft');
INSERT INTO REPORTS VALUES (4,2,'Jul 2018', 'Draft');
INSERT INTO REPORTS VALUES (5,1,'Mar 2017', 'Final');

Results ：

SELECT t.NAME,
       t1.REPORTINGTYPE,
       t1.REPORTINGPERIOD 
FROM teachers as t INNER JOIN
(
    SELECT *,(SELECT COUNT(*) FROM REPORTS tt WHERE tt.TEACHER_ID = t1.TEACHER_ID and tt.REPORTINGPERIOD>=t1.REPORTINGPERIOD) rn
    FROM REPORTS t1
) as t1 on t1.TEACHER_ID = t.id and rn = 1
ORDER BY t.NAME

Answer 2

REPORTINGPERIOD的值不能正确比较，因为Jul在字母表中的Jun之前。您应该使用yyyy-mm之类的格式，其中最重要的字段在前。

在SQLite中，您可以简单地use MAX() to select entire rows：

SELECT t.Name,
       r.ReportingType,
       max(r.ReportingPeriod)
FROM Teachers t
JOIN Reports r ON t.ID = r.Teacher_ID
GROUP BY r.Teacher_ID;

NAME            REPORTINGTYPE  max(r.ReportingPeriod)
--------------  -------------  ----------------------
Mr John Smith   Final          2017-03               
Ms Janet Smith  Draft          2018-07

然后将查询添加到子查询不会产生任何结果

2 个答案: