Aggregate Functions

COUNT, SUM, AVG, MIN, MAX — compute summary values across rows

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Explanation

Aggregate functions compute a single value from many rows.

| Function | What it does | |----------|-------------| | COUNT(*) | Count rows | | COUNT(col) | Count non-NULL values | | SUM(col) | Total of values | | AVG(col) | Average of values | | MIN(col) | Smallest value | | MAX(col) | Largest value |

sql

SELECT
    COUNT(*)             AS total_orders,
    COUNT(shipped_at)    AS shipped_orders,   -- NULL shipped_at excluded
    SUM(amount)          AS total_revenue,
    AVG(amount)          AS avg_order,
    MIN(amount)          AS smallest_order,
    MAX(amount)          AS largest_order
FROM orders;

NULL handling: COUNT(*) counts all rows. COUNT(col) skips NULLs. AVG also ignores NULLs (only averages non-NULL values).

DISTINCT with aggregates:

sql COUNT(DISTINCT customer_id) -- count unique customers, not total orders

Examples

Sales summary

ROUND() cleans up floating-point decimals in AVG results

SELECT
    COUNT(*)                   AS num_sales,
    COUNT(DISTINCT customer_id) AS unique_customers,
    SUM(amount)                AS total_revenue,
    ROUND(AVG(amount), 2)      AS avg_sale,
    MAX(amount)                AS biggest_sale
FROM sales
WHERE sale_date >= '2024-01-01';

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