Adding a Query to a Subquery then produces no results - sql

Question

The lastReports subquery by itself, returns 10 rows. The "wrapper" records do exist, but when I join them, I get 0 rows returned in 12ms from: This is on SQLite.
My joins is good, I'm not making a selection mistake mistake(SQL LEFT JOIN Subquery Alias)... what am I missing, or not understanding?
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

You can try to make row_number by TEACHER_ID column and order by REPORTINGPERIOD, which mean closer the date in subquery, get rn = 1 the data in each TEACHER_ID the max date and JOIN on TEACHERS.
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');
Query 1:
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
Results:
| t.NAME | t1.REPORTINGTYPE | t1.REPORTINGPERIOD |
|----------------|------------------|--------------------|
| Mr John Smith | Final | Mar 2017 |
| Ms Janet Smith | Draft | Jun 2018 |

Answer 2

The REPORTINGPERIOD values do not compare correctly, because Jul comes before Jun in the alphabet. You should use a format like yyyy-mm where the most significant field comes first.
In SQLite, you can simply 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