Eliminating Duplicate Records with DISTINCT

Data often contains redundant entries, which can skew analysis and lead to inaccurate results. This challenge focuses on using the DISTINCT keyword in SQL to efficiently identify and remove duplicate rows from a table, ensuring you work with clean and representative data. You'll be crafting SQL queries to extract unique combinations of values from a given dataset.

Problem Description

You are given a table named Employees with the following columns: EmployeeID, FirstName, LastName, Department, and Salary. Your task is to write SQL queries that return only the unique combinations of FirstName, LastName, and Department from the Employees table. The query should ignore duplicate entries where these three columns have the same values.

Key Requirements:

• The query must use the DISTINCT keyword.
• The output should include only the FirstName, LastName, and Department columns.
• The order of the output rows is not important.
• The query should handle cases where there are no duplicate entries (i.e., all rows are unique).
• The query should handle cases where all rows are duplicates.

Expected Behavior:

The query should return a result set containing only the unique combinations of FirstName, LastName, and Department. Each row in the result set represents a distinct employee profile based on these three attributes.

Edge Cases to Consider:

• Empty Table: If the Employees table is empty, the query should return an empty result set.
• Null Values: The query should correctly handle NULL values in the FirstName, LastName, and Department columns. Two rows with NULL in the same column are considered duplicates for the purpose of DISTINCT.
• Case Sensitivity: The behavior of DISTINCT regarding case sensitivity might depend on the specific SQL database system. Assume case-insensitive comparison for this challenge unless otherwise specified by the database system.

Examples

Example 1:

Input:
Employees Table:
EmployeeID | FirstName | LastName | Department | Salary
----------|-----------|----------|------------|--------
1         | John      | Doe      | Sales      | 50000
2         | Jane      | Smith    | Marketing  | 60000
3         | John      | Doe      | Sales      | 55000
4         | Peter     | Jones    | IT         | 70000
5         | Jane      | Smith    | Marketing  | 62000

Output:
FirstName | LastName | Department
----------|----------|------------
John      | Doe      | Sales
Jane      | Smith    | Marketing
Peter     | Jones    | IT

Explanation:
The query removes the duplicate row (John Doe, Sales) because the combination of FirstName, LastName, and Department is already present.

Example 2:

Input:
Employees Table:
EmployeeID | FirstName | LastName | Department | Salary
----------|-----------|----------|------------|--------
1         | John      | Doe      | Sales      | 50000
2         | John      | Doe      | Sales      | 50000
3         | John      | Doe      | Sales      | 50000

Output:
FirstName | LastName | Department
----------|----------|------------
John      | Doe      | Sales

Explanation:
All rows are duplicates of each other. The query returns only one row with the unique combination.

Example 3: (Handling NULLs)

Input:
Employees Table:
EmployeeID | FirstName | LastName | Department | Salary
----------|-----------|----------|------------|--------
1         | John      | Doe      | Sales      | 50000
2         | NULL      | Smith    | Marketing  | 60000
3         | John      | Doe      | Sales      | 55000
4         | NULL      | Smith    | Marketing  | 62000

Output:
FirstName | LastName | Department
----------|----------|------------
John      | Doe      | Sales
NULL      | Smith    | Marketing

Explanation:
The query correctly identifies the unique combinations, including those with NULL values.  The two rows with NULL FirstName and Smith LastName in Marketing are treated as duplicates.

Constraints

• The Employees table will always exist.
• The table will contain at least one row.
• The data types of the columns are as follows: EmployeeID (INT), FirstName (VARCHAR), LastName (VARCHAR), Department (VARCHAR), Salary (INT).
• The maximum length of FirstName, LastName, and Department is 255 characters.
• The query must execute within a reasonable time (e.g., less than 1 second) for tables with up to 10,000 rows.

Notes

• Focus on using the DISTINCT keyword effectively.

• Consider how NULL values are handled by DISTINCT in your chosen SQL dialect.

• The problem emphasizes selecting specific columns, not the entire row.

• Think about the order of columns in the DISTINCT clause – it doesn't affect the result in this case, but it's good practice to be explicit.

• Pseudocode:

  SELECT DISTINCT FirstName, LastName, Department
  FROM Employees;