Case or If, What to choose in SQL - sql

Question

How can i manage this query. I want to have only one person in the query. If there is an id in empId column, then the name of him. Otherwise the name of the boss.
Product empId teamId
----------------------
A 1 3
B 2 4
C 3
D 2 3
E 4
User Table
Id Name
-----------
1 Jim
2 Carrey
3 Bill
4 Clinton
Team Table
Team_Id BossId
-----------
3 3
4 4
The result should look like:
Product user.name
-----------------
A Jim
B Carrey
C Bill
D Carrey
E Clinton

Answer 1

Use CASE or the COALESCE() function:
SELECT
x.Product
, COALESCE(emp.Name, boss.Name) AS Name
FROM
TableX AS x
LEFT JOIN
User AS emp
ON emp.Id = x.empId
LEFT JOIN
User AS boss
ON boss.Id = x.bossId
Updated:
SELECT
x.Product
, COALESCE(emp.Name, boss.Name) AS Name
FROM
TableX AS x
LEFT JOIN
User AS emp
ON emp.Id = x.empId
LEFT JOIN
Team As t
JOIN
User AS boss
ON boss.Id = t.bossId
ON t.team_Id = x.teamId

Answer 2

Something like this:
SELECT
Table1.Product,
CASE
WHEN Table1.empId IS NULL
THEN Boss.name
ELSE Emp.name
END
FROM
Table1
LEFT JOIN [User] AS Emp
ON Emp.Id =Table1.empId
LEFT JOIN Team
ON Team.Team_Id =Table1.teamId
LEFT JOIN [User] AS Boss
ON Boss.Id=Team.BossId

Answer 3

SELECT Y.Product, U.Name
FROM YourTable AS Y
JOIN Users AS U ON Y.empId = U.Id
UNION
SELECT Y.Product, U.Name
FROM YourTable AS Y
JOIN Team AS T ON Y.teamId = T.Team_Id
JOIN Users AS U ON T.BossId = U.Id
WHERE Y.empId IS NULL;

Answer 4

-- SET search_path='tmp';
DROP TABLE tmp.products;
CREATE TABLE products
( product CHAR(1)
, emp_id INTEGER
, team_id INTEGER
);
INSERT INTO products(product,emp_id,team_id)
VALUES ('A',1,3), ('B',2,4), ('C',NULL,3), ('D',2,3), ('E',NULL,4);
DROP TABLE tmp.names;
CREATE TABLE names
(id INTEGER
, zname varchar
);
INSERT INTO names(id,zname)
VALUES ( 1, 'Jim') ,( 2, 'Carrey') ,( 3, 'Bill') ,( 4, 'Clinton') ;
DROP TABLE tmp.teams;
CREATE TABLE teams
( team_id INTEGER NOT NULL
, boss_id INTEGER NOT NULL
);
INSERT INTO teams(team_id,boss_id) VALUES ( 3,4) , (4,4);
WITH lutser(prod,id,team) AS
(
SELECT k1.product AS prod
, k1.emp_id AS id
, k1.team_id AS team
FROM tmp.products k1
UNION
SELECT k2.product AS prod
, t.boss_id AS id
, k2.team_id AS team
FROM tmp.products k2
JOIN tmp.teams t ON t.team_id = k2.team_id
WHERE k2.emp_id IS NULL
)
SELECT l.prod
, l.id
, l.team
, n.zname
FROM lutser l
JOIN names n ON n.id = l.id
;
extra bonus point for a recursive version of this CTE ...