В чем отличие между конструкциями WHERE и HAVING в SQL?

WHERE filters rows before grouping (GROUP BY), affecting which rows participate in aggregate functions. HAVING filters groups after grouping, based on the results of aggregate functions.

Feature	WHERE	HAVING
Application	To individual rows	To groups of rows
Processing time	Before GROUP BY	After GROUP BY
Usage	With table fields but without aggregate functions	With results of aggregate functions or fields from GROUP BY

Example of using WHERE:

SELECT product_name, price
FROM products
WHERE category_id = 1; -- Filters rows before grouping (if it were to occur)

Example of using HAVING:

SELECT category_id, COUNT(*) AS total_products
FROM products
GROUP BY category_id
HAVING COUNT(*) > 5; -- Filters groups after counting total products

Example of combined usage:

SELECT category_id, AVG(price) AS average_price
FROM products
WHERE stock > 0 -- Filters only in-stock products BEFORE grouping
GROUP BY category_id
HAVING AVG(price) < 100; -- Filters groups with average price less than 100 AFTER grouping and averaging