What does the DISTINCT keyword do when used with aggregate functions like COUNT()?

Q: What does the DISTINCT keyword do when used with aggregate functions like COUNT()?

The DISTINCT keyword removes duplicate values **before** the aggregate function is applied. When used with functions like COUNT(), SUM(), AVG(), etc., it ensures the function operates only on unique values. Counts **unique** (non-duplicate) values. NULL values are **ignored**. SELECT COUNT(color) FROM products; -- counts all non-NULL colors (including duplicates) SELECT COUNT(DISTINCT color) FROM products; -- counts only unique colors `SUM(DISTINCT col)` – Adds only unique values. `AVG(DISTINCT col)` – Averages only unique values. SELECT SUM(price) FROM sales; -- sum of all prices SELECT SUM(DISTINCT price) FROM sales; -- sum of unique price values DISTINCT filters duplicates **before** applying the aggregate. Useful when you want aggregation over unique values only. COUNT(DISTINCT col1, col2) can count unique combinations of columns (MySQL-specific). In summary: DISTINCT ensures that aggregate functions operate only on unique values, making results more precise when duplicates exist.

Difficulty: 5/10

COUNT DISTINCT, aggregate functions, query optimization

Using DISTINCT with Aggregate Functions like COUNT()

The DISTINCT keyword removes duplicate values before the aggregate function is applied. When used with functions like COUNT(), SUM(), AVG(), etc., it ensures the function operates only on unique values.

1. COUNT(DISTINCT column)

1
Counts unique (non-duplicate) values.
2
NULL values are ignored.

Example

What does the DISTINCT keyword do when used with aggregate functions like COUNT()?

Difficulty: 5/10

COUNT DISTINCT, aggregate functions, query optimization

Using DISTINCT with Aggregate Functions like COUNT()

1. COUNT(DISTINCT column)

1
Counts unique (non-duplicate) values.
2
NULL values are ignored.

Example

What does the DISTINCT keyword do when used with aggregate functions like COUNT()?

Using DISTINCT with Aggregate Functions like COUNT()

What does the DISTINCT keyword do when used with aggregate functions like COUNT()?

Using DISTINCT with Aggregate Functions like COUNT()

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