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聚集函数

Sum:求和 Avg:求平均数 Max:求最大值 Min:求最小值 Count:求记录

SELECT SUM(item_price*quanlity) AS total_price
FROM orderitems
WHERE order_num = 2005;

聚集不同值：

SELECT AVG(DISTINCT prod_price) AS avg_price #只考虑不同价格
FROM products
WHERE vend_id = 1003;