Find each employee and their manager name using a self join on the `employees` table.

Problem Statement

<p>Find each employee and their manager name using a self join on the <code>employees</code> table.</p>

Examples

Input: employees table: +----+---------+------------+ | id | name | manager_id | +----+---------+------------+ | 1 | Boss | NULL | | 2 | Alice | 1 | | 3 | Bob | 1 | | 4 | Charlie | 2 | +----+---------+------------+

Explanation: The query joins the matching records on the related keys and projects the requested fields.

Complexity

Time Complexity: -

Space Complexity: -

Hints

💡 Hint 1: Determine which tables contain the required columns and relate them using LEFT JOIN on the corresponding foreign/primary keys. 💡 Hint 2: Write out the ON condition matching key fields (e.g. ON a.key = b.key) and apply any filtering in the WHERE clause. 💡 Hint 3: Select only the requested columns in the SELECT clause with clear aliases if needed: SELECT ... FROM ... LEFT JOIN ... ON ...;

Editorial & Approach

Problem Overview & Intuition

To solve "Self Join", we query the relational database engine using declarative SQL. The goal is to find each employee and their manager name using a self join on the `employees` table. By formulating an optimal execution plan with appropriate projection and filtering, the database engine executes the query with minimal overhead.

Step-by-Step Approach

Analyze Schema: Identify the target tables, necessary foreign keys, and expected output columns.
Construct Filtering & Logic: Apply table joins to isolate the requested data.
Format & Order: Ensure columns match the expected project schema in order.

Optimal Implementation (SQL)

SELECT e.name AS employee, m.name AS manager FROM employees e LEFT JOIN employees m ON e.manager_id = m.id;

Complexity Analysis

Time Complexity O(N * M) worst-case, O(N + M) with hash/merge join on indexed keys.

Space Complexity O(N) for join buffer and result set.

Key Considerations & Edge Cases

Empty Tables: The query executes safely returning zero rows without syntax error.
NULL Values: Columns containing NULL values are properly handled by standard ANSI SQL semantics.
Case Sensitivity: String comparisons and keywords adhere to PostgreSQL/standard SQL rules.

Self Join

Medium

Find each employee and their manager name using a self join on the employees table.

Example Scenarios

1Example 1

Input:

employees table

id	name	manager_id
1	Boss	NULL
2	Alice	1
3	Bob	1
4	Charlie	2

Output:

employee	manager
Boss	null
Alice	Boss
Bob	Boss
Charlie	Alice

Explanation:

The query joins the matching records on the related keys and projects the requested fields.

SQL Editor

Loading Editor...

Query Results

Run a query to see results here.