Conditional JOIN different tables - sql

Question

I want to know if a user has an entry in any of 2 related tables.
Tables
USER (user_id)
EMPLOYEE (id, user_id)
STUDENT (id, user_id)
A User may have an employee and/or student entry. How can I get that info in one query?
I tried:
select * from [user] u
inner join employee e
on e.user_id = case when e.user_id is not NULL
then u.user_id
else null
end
inner join student s
on s.user_id = case when s.user_id is not NULL
then u.user_id
else null
end
But it will return only users with entries in both tables.
SQL Fiddle example

Answer 1

You could use an outer join:
select *
from USER u
left outer join EMPLOYEE e ON u.user_id = e.user_id
left outer join STUDENT s ON u.user_id = s.user_id
where s.user_id is not null or e.user_id is not null
alternatively (if you're not interested in the data from the EMPLOYEE or STUDENT table)
select *
from USER u
where exists (select 1 from EMPLOYEE e where e.user_id = u.user_id)
or exists (select 1 from STUDENT s where s.user_id = u.user_id)

Answer 2

If you want to get all user data together You might have:
SELECT
user_id
,'Employee' AS Source
FROM
employee
UNION
SELECT
user_id
,'Student' AS Source
FROM
student
http://sqlfiddle.com/#!3/90216/22
Which can also be done with a full join and a CASE statement:
SELECT
ISNULL(e.user_id,s.user_id) AS user_id
,CASE WHEN e.user_id IS NULL THEN 'Student'
ELSE 'Employee'
END AS SOURCE
FROM
employee AS e
FULL JOIN student AS s
ON s.user_id = e.user_id
http://sqlfiddle.com/#!3/90216/29
the latter will combine people who are both students adn employees into one row and call them and employee. compare:
http://sqlfiddle.com/#!3/2aa3e/1
and
http://sqlfiddle.com/#!3/2aa3e/2
where I have made user 1 a student and a employee

Answer 3

Such solution also can help you.
SELECT S.*, P.*
,CASE
WHEN S.ShipmentType = 'import' THEN SP.SupplierName
WHEN S.ShipmentType = 'export' THEN C.CustomerName
END AS ShipmentDesination
FROM tblShippments S
INNER JOIN tblProducts P ON S.productId = P.productID
LEFT OUTER JOIN tblCustomers C ON S.companyId = C.customerId AND S.ShipmentType = 'export'
LEFT OUTER JOIN tblSuppliers SP ON S.companyId = SP.supplierId AND S.ShipmentType = 'import'

Answer 4

If you look at employee and student tables as one, you can use left join:
select *
from user u
left join
(
select 'Employee' as UserType,
id,
user_id
from employee e
union all
select 'Student',
id,
user_id
from student s
) r
ON u.user_id = r.user_id

Answer 5

How about UNION which you write as 2 separate SELECT statements
For example:
SELECT * FROM User U
JOIN Employee E ON E.User_Id = U.User_Id
UNION
SELECT * FROM User U
JOIN student S ON S.User_Id = U.User_Id
I couldn't see why you needed the CASE statement it looked superfluous. If you wanted all Users and to show nulls then use LEFT OUTER JOIN.

Answer 6

work for mysql v5.6+
schema:
CREATE TABLE USER (user_id INT);
CREATE TABLE employee (id INT, user_id INT);
CREATE TABLE student (id INT, user_id INT);
INSERT INTO USER SELECT 1;
INSERT INTO USER SELECT 2;
INSERT INTO USER SELECT 3;
INSERT INTO USER SELECT 4;
INSERT INTO employee SELECT 1, 1;
INSERT INTO employee SELECT 2, 4;
INSERT INTO student SELECT 1, 2;
first solution
,full outer join does not work for mysql v5.6+:
SELECT
IF(e.user_id IS NULL,s.user_id,e.user_id) AS user_id
,CASE WHEN e.user_id IS NULL THEN 'Student'
ELSE 'Employee'
END AS SOURCE
FROM
employee AS e
LEFT JOIN student AS s
ON (s.user_id = e.user_id)
UNION
SELECT
IF(e.user_id IS NULL,s.user_id,e.user_id) AS user_id
,CASE WHEN e.user_id IS NULL THEN 'Student'
ELSE 'Employee'
END AS SOURCE
FROM
employee AS e
RIGHT JOIN student AS s
ON (s.user_id = e.user_id)
alternative solution:
SELECT IF(e.user_id IS NULL,s.user_id,e.user_id) AS user_id,
CASE WHEN e.user_id IS NULL THEN 'Student'
ELSE 'Employee'
END AS SOURCE
FROM USER AS u
LEFT OUTER JOIN employee e ON (u.user_id = e.user_id)
LEFT OUTER JOIN student s ON (u.user_id = s.user_id)
WHERE s.user_id IS NOT NULL OR e.user_id IS NOT NULL
other, alternative solution:
SELECT u.user_id,SOURCE
FROM USER u
INNER JOIN
(
SELECT 'Employee' AS SOURCE,id,user_id FROM employee e
UNION ALL
SELECT 'Student',id,user_id FROM student s
) r ON u.user_id = r.user_id

Answer 7

SELECT OrderID, Quantity, O.ProductID , ProductName,
CASE
WHEN Quantity > 3 THEN 'The quantity is More than 3'
WHEN Quantity = 3 THEN 'The quantity is Equal to 3'
ELSE 'The quantity is Less than 3'
END AS QuantityText
FROM tb_OrderDetail O
INNER JOIN tb_Product P ON O.ProductID = P.ProductID