LeetCode 175: Combine Two Tables
LeetCode 175 Explained: Merge Two Tables with Ease
I'm Varchasv, a Data Engineer working on enterprise data integration.
Currently on a 90-problem challenge to level up my technical skills and switch to a more development-focused role.
What I'm doing:
- Solving 2 Leetcode problems daily (SQL + DSA).
- Blogging about each problem.
- Building in public.
My Goal - Land a better data engineering role by mid-2026.
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Date: January 13, 2026
Category: SQL
Time Taken: 2 minutes
Difficulty: Easy
Problem Statement
Table: Person and Address
Write a solution to report the first name, last name, city, and state of each person in the Person table. If the address of a personId is not present in the Address table, report null instead.
Return the result table in any order.
Link: 175: Combine Two Tables
My Approach
Initial thought:
It looked like a pretty basic inner join at first glance, as we needed only 2 columns from each table, but it resulted in the wrong solution because we also need null values in case personId is not present in the Address table.
Final solution:
I used left join instead of inner join to combine the tables.
Why this works:
This works because the left join will also include the values that are null along with the common values.
Solution Code
SELECT firstName, lastName, city, state
FROM Person AS P
LEFT JOIN Address AS A
USING (personId)
Key Takeaway: Importance of Left and Right Joins.
Pattern: Table Join
Mistakes I Made: Chose Inner Join instead of Left Join.
Series: 90 Days of Data Engineering Progress: 1/90 problems completed
Tags: #LearnInPublic #DEQuest #LeetCode #SQL #DataEngineering