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10.1 数据分组

分组可以将数据分为多个逻辑组，对每个组进行聚集计算。

10.2 创建分组

# GROUP BY 子句指示DBMS按照vend_id 排序并分组数据select vend_id, count(*) as num_prodsfrom productsgroup by vend_id;

10.3 过滤分组

通过HAVING子句

# 过滤出两个以上订单的分组select cust_id, count(*) as orders from orders group by cust_idhaving count(*) >= 2;

# 列出具有两个以上产品且其价格大于等于4的供应商select vend_id, count(*) as onum_prods from productswhere prod_price >= 4group by vend_idhaving count(*) >= 2

10.4 分组和排序

group by 和 order by 的区别

一般在使用GROUP BY子句时，应该也给出ORDER BY子句。这是保证数据正确排序的唯一方法。

# 按订购物品的数目排序输出select order_num, count(*) as items from orderItemsgroup by order_numhaving count(*) >= 3order by items, order_num;

10.5 select 子句顺序

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