How do you solve Combine Two Tables on LeetCode?

Combine Two Tables (LeetCode #175) can be solved using Brute Force or Optimal Solution. The optimal approach is Optimal Solution with O(n) time complexity and O(n) space complexity. Key insight: Understanding how to use JOIN operations can significantly improve performance when combining data from multiple tables.

What is the brute force solution for Combine Two Tables?

The brute force approach for Combine Two Tables is Brute Force, with O(n²) time complexity and O(1) space complexity. In the brute force approach, we will iterate through each person in the Person table and for each person, we will search for their corresponding address in the Address table. This is straightforward but inefficient as it involves nested loops. The optimal Optimal Solution approach improves this to O(n).

What algorithm or data structure is used to solve Combine Two Tables?

Combine Two Tables uses the following concepts: Database. The recommended patterns to study are: Hash Map, Array.

What are the interview tips for Combine Two Tables?

Always clarify the requirements regarding null values and how to handle them. Practice writing SQL queries with different types of JOINs to become familiar with their behavior. Be prepared to explain your thought process and the reasoning behind your chosen approach.

What are common mistakes when solving Combine Two Tables?

Not using LEFT JOIN when it's required to include all records from the primary table. Overlooking null values in the result when there is no matching address.

#175

Combine Two Tables

Easy

DatabaseHash MapArray

LeetCode ↗

Approaches

Brute ForceOptimal

Complexity Comparison

	Brute Force	Optimal Solution★
Time	O(n²)	O(n)
Space	O(1)	O(n)

💡

Intuition

Time O(n)Space O(n)

The optimal approach uses a LEFT JOIN operation to efficiently combine the two tables based on the personId. This method leverages the database's ability to handle joins, which is much faster than manually iterating through both tables.

⚙️

Algorithm

3 steps

1Step 1: Use a LEFT JOIN to combine the Person table with the Address table on personId.
2Step 2: Select the firstName, lastName, city, and state from the combined result.
3Step 3: Return the result set.

solution.py4 lines

1# Full working Python code
2SELECT p.firstName, p.lastName, a.city, a.state
3FROM Person p
4LEFT JOIN Address a ON p.personId = a.personId;

ℹ

Complexity note: The time complexity is O(n) because the LEFT JOIN operation processes each row of the Person table once, making it linear in relation to the number of persons.

1Understanding how to use JOIN operations can significantly improve performance when combining data from multiple tables.
2Using LEFT JOIN ensures that we retain all records from the Person table, even if there are no matching records in the Address table.

Solutions and explanations are original Tejav content. Problem titles © LeetCode — use the LeetCode button above for the full problem statement.