SQL to count staff working at time with join - mysql

Question

I am new to SQL.
I want to count staff working at a particular time.
The data schema has a Person table and a Shifts table. They are joined by a StaffShifts table which has both a user_id field and a shift_id field.
Each staff member can have many shifts, and each shift can have many staff.
create table Person
(
user_id INT,
rank_id INT,
groupschedule_id INT,
personnum VARCHAR(6),
PRIMARY KEY (user_id)
);
INSERT INTO Person (user_id, rank_id, groupschedule_id, personnum)
VALUES
(1, 1, 1, 'ABC123'),
(2, 1, 2, 'DEF456'),
(3, 2, 3, 'GHI789'),
(4, 1, 1, 'JKL123'),
(5, 3, 2, 'NOP123'),
(6, 1, 3, 'RST789'),
(7, 2, 1, 'WXY789'),
(8, 1, 2, 'ABC432'),
(9, 1, 3, 'DEF789')
;
CREATE TABLE Groupschedule
(
groupschedule_id INT,
shortnm char(20)
);
INSERT INTO Groupschedule
VALUES
(1,'TEAM 1'),
(2,'TEAM 2'),
(3,'TEAM 3')
;
CREATE TABLE Shifts
(
shift_id INT PRIMARY KEY,
shift_start datetime,
shift_end datetime
);
INSERT INTO Shifts
VALUES
(1, '2021-03-08 06:45:00', '2021-03-08 15:00:00'),
(2, '2021-03-08 14:00:00', '2021-03-08 23:00:00'),
(3, '2021-03-08 23:00:00', '2021-04-09 07:00:00')
;
CREATE TABLE Osl
(
shift_id INT,
osl INT,
area char(10),
FOREIGN KEY (shift_id) REFERENCES Shifts(shift_id)
);
INSERT INTO Osl
VALUES
(1,3, 'EAST'),
(2,2, 'EAST'),
(3,2, 'EAST')
;
CREATE TABLE StaffShifts
( shift_id INT,
user_id INT,
FOREIGN KEY (shift_id) REFERENCES Shifts(shift_id),
FOREIGN KEY (user_id) REFERENCES Person(user_id)
);
WHATS BEEN TRIED
I tried to start with retrieving all the staff working at the time with:
SELECT shift_id FROM Shifts WHERE shift_start < '2021-03-08 11:00:00' AND shift_end > '2021-03-08 11:00:00'
INNER JOIN StaffShifts ON Person.user_id=StaffShifts.user_id
On a fiddle this results in an error that references the INNER JOIN but does not elaborate.
I have created a fiddle here https://dbfiddle.uk/?rdbms=mysql_8.0&fiddle=1572ed28005766d30a18521557aadb90
UPDATE
I have tried SQL statement:
SELECT shift_id FROM Shifts INNER JOIN StaffShifts ON Person.user_id=StaffShifts.user_id
WHERE shift_start < '2021-03-08 11:00:00' AND shift_end > '2021-03-08 11:00:00'
However this produces and error shift_id is ambiguous.
UPDATE
Since i really want to COUNT the users working a given shift I am trying to return a list of users - using JOINs for the two one to many relationships:
SELECT Person.user_id
FROM Person
INNER JOIN StaffShifts ON Person.user_id=StaffShifts.user_id
INNER JOIN StaffShifts ON Shifts.shift_id=StaffShifts.shift_id
WHERE Shifts.shift_start < '2021-03-08 11:00:00'
AND Shifts.shift_end > '2021-03-08 11:00:00'
But this results in 'Not unique table/alias: 'StaffShifts''
Note I have not tried to use COUNT until I return a list of Persons.

Answer 1

You need to include Person - which you want to count, StaffShifts - where a person is assigned to a shift so you could also join Shifts based on that, and Shifts - where you check the hour range you wish.
SELECT
COUNT(*)
FROM `Person`
INNER JOIN `StaffShifts` ON `Person`.`user_id` = `StaffShifts`.`user_id`
INNER JOIN `Shifts` ON `StaffShifts`.`shift_id` = `Shifts`.`shift_id`
WHERE
Shifts.shift_start < '2021-03-08 11:00:00'
AND Shifts.shift_end > '2021-03-08 14:00:00'
;