mysql 怎样统计分组数

CREATE TABLE sales (
    id INT AUTO_INCREMENT PRIMARY KEY,
    product_category VARCHAR(50),
    sale_amount DECIMAL(10, 2)
);

SELECT product_category, COUNT(*) AS count
FROM sales
GROUP BY product_category;

SELECT product_category, sale_month, COUNT(*) AS count
FROM sales
GROUP BY product_category, sale_month;

SELECT product_category, COUNT(DISTINCT sale_amount) AS count
FROM sales
WHERE sale_amount IS NOT NULL
GROUP BY product_category;

CREATE INDEX idx_product_category ON sales(product_category);

SELECT product_category, COUNT(*) AS count
FROM sales
GROUP BY product_category
LIMIT 10 OFFSET 0;