Join Lookup from 1 table to multiple columns - sql

Question

How do I link 1 table with multiple columns in another table without using mutiple JOIN query?
Below is my scenario:
I have table User with ID and Name
User
+---------+------------+
| Id | Name |
+---------+------------+
| 1 | John |
| 2 | Mike |
| 3 | Charles |
+---------+------------+
And table Product with multiple columns, but just focus on 2 columns CreateBy And ModifiedBy
+------------+-----------+-------------+
| product_id | CreateBy | ModifiedBy |
+------------+-----------+-------------+
| 1 | 1 | 3 |
| 2 | 1 | 3 |
| 3 | 2 | 3 |
| 4 | 2 | 1 |
| 5 | 2 | 3 |
+------------+-----------+-------------+
With normal JOIN, i will need to do 2 JOIN:
SELECT p.Product_id,
u1.Name AS CreateByName,
u2.Name AS ModifiedByName
FROM Product p
JOIN USER user u1 ON p.CreateBy = u1.Id,
JOIN USER user u2 ON p.ModifiedBy = u2.Id
to come out result
+------------+---------------+-----------------+
| product_id | CreateByName | ModifiedByName |
+------------+---------------+-----------------+
| 1 | John | Charles |
| 2 | John | Charles |
| 3 | Mike | Charles |
| 4 | Mike | John |
| 5 | Mike | Charles |
+------------+---------------+-----------------+
How do i avoid that 2 times JOIN?
I'm using MS-SQL , but open to all SQL query for my own learning curious

Answer 1

Your current design/approach is acceptable, I think, and the need for two joins is a function of there being two user ID columns. Each of the two columns requires a separate join.
For fun, here is a table design which you may consider if you really want to have to perform only one join:
+------------+-----------+-------------+
| product_id | user_id | type |
+------------+-----------+-------------+
| 1 | 1 | created |
| 2 | 1 | created |
| 3 | 2 | created |
| 4 | 2 | created |
| 5 | 2 | created |
| 1 | 3 | modified |
| 2 | 3 | modified |
| 3 | 3 | modified |
| 4 | 1 | modified |
| 5 | 3 | modified |
+------------+-----------+-------------+
Now, you can get away with a just a single join followed by an aggregation:
SELECT
p.product_id,
MAX(CASE WHEN t.type = 'created' THEN u.Name END) AS CreateByName,
MAX(CASE WHEN t.type = 'modified' THEN u.Name END) AS ModifiedByName
FROM Product p
INNER JOIN user u
ON p.user_id = u.Id
GROUP BY
p.product_id;
Note that I don't recommend this approach at all. It is much cleaner to use your current approach and use two joins. Joins can fairly easily be optimized using one or more indices. The above aggregation approach would probably not perform as well as what you already have.

Answer 2

If you use natural keys instead of surrogates, you won't need to join at all.
I don't know how you tell your products apart in the real world, but for the example I will assume you have a UPC
CREATE TABLE User
(Name VARCHAR(20) PRIMARY KEY);
CREATE TABLE Product
(UPC CHAR(12) PRIMARY KEY,
CreatedBy VARCHAR(20) REFERENCES User(Name),
ModifiedBy VARCHAR(20) REFERENCES User(Name)
);
Now your query is a simple select, and you also enforce uniqueness of your user names as a bonus, and don't need additional indexes.
Try it...
HTH

Answer 3

Join is the best Approach, but if looking for alternate approach you can use Inline Query.
SELECT P.PRODUCT_ID,
(SELECT [NAME] FROM #USER WHERE ID = CREATED_BY) AS CREATED_BY,
(SELECT [NAME] FROM #USER WHERE ID = MODIFIED_BY) AS MODIFIED_BY
FROM #PRODUCT P
DEMO