What does the DISTINCT clause do in a SELECT query?

Q: What does the DISTINCT clause do in a SELECT query?

In MySQL, the **DISTINCT clause** is used in a `SELECT` query to remove duplicate rows from the result set. It ensures that each row returned is unique based on the columns specified in the SELECT clause. Eliminates duplicate rows from the query result. Can be applied to a single column or multiple columns, e.g., `SELECT DISTINCT department_id FROM employees;` Helps in analyzing unique values, such as distinct categories, names, or IDs. Often used with aggregate functions to get counts of unique items, e.g., `SELECT COUNT(DISTINCT department_id) FROM employees;` The DISTINCT clause is applied **after filtering with WHERE** but **before ordering with ORDER BY**, ensuring that duplicates are removed from the intermediate result set before final sorting. SELECT DISTINCT department_id FROM employees WHERE salary > 50000 ORDER BY department_id;

Difficulty: 2/10

duplicate elimination, query performance, index interaction

Understanding the DISTINCT Clause in MySQL

In MySQL, the DISTINCT clause is used in a SELECT query to remove duplicate rows from the result set. It ensures that each row returned is unique based on the columns specified in the SELECT clause.

Key Features of the DISTINCT Clause

1
Eliminates duplicate rows from the query result.
2
Can be applied to a single column or multiple columns, e.g., SELECT DISTINCT department_id FROM employees;
3
Helps in analyzing unique values, such as distinct categories, names, or IDs.
4
Often used with aggregate functions to get counts of unique items, e.g., SELECT COUNT(DISTINCT department_id) FROM employees;

The DISTINCT clause is applied after filtering with WHERE but before ordering with ORDER BY, ensuring that duplicates are removed from the intermediate result set before final sorting.

Example of DISTINCT Clause Usage

Scenario Questions

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Follow-up Questions