leetcode 1378. Replace Employee ID With The Unique Identifier (SQL Solution)

(1378. Replace Employee ID With The Unique) This is a LeetCode easy question. However, if you are not familiar with joins, it can become tricky. This question primarily requires knowledge of joins, and if you don’t know when to use which join, you may find it challenging even though it’s considered an easy question. I have explained at the end how to solve this type of question by simply looking at the output.

The Question :

Table: Employees

id is the primary key (column with unique values) for this table.
Each row of this table contains the id and the name of an employee in a company.

Table: EmployeeUNI

(id, unique_id) is the primary key (combination of columns with unique values) for this table.
Each row of this table contains the id and the corresponding unique id of an employee in the company.

Write a solution to show the unique ID of each user, If a user does not have a unique ID replace just show null.

Return the result table in any order.

The result format is in the following example.

Example 1:

Input:
Employees table:

EmployeeUNI table:

Output:

Explanation:

Alice and Bob do not have a unique ID, We will show null instead.
The unique ID of Meir is 2.
The unique ID of Winston is 3.
The unique ID of Jonathan is 1.

---

The Solution :

select Unique_id, name from employees e1 left join employeeUNI e2 
on e1.id=e2.id 

Explanation:

We can observe that in the expected output table, there is one column from the left table (name) and one column from the right table (unique_id). In the left table’s column (name), all the values are present, while in the right table’s column (unique_id), some null values are present. In this situation, we use a left join. A left join provides us with matching values from the right table and retains all values from the left table.

Other LeetCode Questions :

• LeetCode 1683. Invalid Tweets (SQL Query Solution)
• LeetCode 1148. Article Views I – SQL Query Solution