日韩欧美人妻无码精品白浆,www.大香蕉久久网,狠狠的日狠狠的操,日本好好热在线观看

[p]　結(jié)構(gòu)化查詢（sql）的另一個(gè)強(qiáng)大的功能是分類匯總，也就是group子句；mysql當(dāng)然也提供這個(gè)功能?，F(xiàn)在還以我在《select查詢的應(yīng)用（二）join子句的用法》中的數(shù)據(jù)庫(kù)為例說說group子句的用法。 [br][br]　　一、查詢每個(gè)客戶的交易次數(shù)。count()是一個(gè)與group子句一起使用的函數(shù)，它的作用是計(jì)數(shù)： [br][br]select customer,count(*) as sale_count from sales group by customer [br][br]返回的查詢可能結(jié)果為： [br][br]+----------+------------+ [br]| customer | sale_count | [br]+----------+------------+ [br]| 1 | 1 | [br]+----------+------------+ [br]| 2 | 1 | [br]+----------+------------+ [br][br]表示了代碼為1和2的兩個(gè)客戶分別有一次交易。 [br][br]　　二、查詢每個(gè)客戶的交易總額。sum()是一個(gè)與group子句一起使用的函數(shù)，它的作用是求和： [br][br]select customer,sum(price*amount) as total_price from sales group by customer [br][br]返回的查詢結(jié)果可能為： [br][br]+----------+-------------+ [br]| customer | total_price | [br]+----------+-------------+ [br]| 1 | 12000.00 | [br]+----------+-------------+ [br]| 2 | 12000.00 | [br]+----------+-------------+ [br][br]表示了代碼為1和2的兩個(gè)客戶各有12000元的交易額。其中sum(price*amount)表示先將price和amount即單價(jià)和數(shù)量相乘后再求和，亦即總價(jià)。 [br][br]　　三、查詢每個(gè)客戶的平均每次交易額。avg()是求平均值的函數(shù)： [br][br]select customer,avg(price*amount) as avg_price from sales group by customer [br][br]返回的查詢結(jié)果可能為： [br][br]+----------+-----------+ [br]| customer | avg_price | [br]+----------+-----------+ [br]| 1 | 12000.00 | [br]+----------+-----------+ [br]| 2 | 12000.00 | [br]+----------+-----------+ [br][br]表示了代碼為1和2的兩個(gè)客戶的平均每次交易的交易額都是12000元。由于我預(yù)設(shè)的數(shù)據(jù)量比較小，查詢的結(jié)果不甚明顯，請(qǐng)讀者諒解?？梢噪S意向sales表中添加一些數(shù)據(jù)，數(shù)據(jù)量越大，結(jié)果越明顯。 [br][br]　　四、查詢每個(gè)客戶最大的和最小的一筆成交額。max()和min()函數(shù)分別是取最大值和最小值的函數(shù)： [br][br]select customer,max(price*amount) as max_price,min(price*amount) as min_price from sales group by customer [br][br]返回的查詢結(jié)果可能為： [br][br]+----------+-----------+-----------+ [br]| customer | max_price | min_price | [br]+----------+-----------+-----------+ [br]| 1 | 12000.00 | 12000.00 | [br]+----------+-----------+-----------+ [br]| 2 | 12000.00 | 12000.00 | [br]+----------+-----------+-----------+ [br]　　 [br]　　五、查詢每一種貨物售出的平均價(jià)格。 [br][br]select good_code,avg(price) as avg_price from sales group by good_code [br][br]對(duì)么？不對(duì)！這樣查詢到的是每種貨物各條銷售記錄中價(jià)格的平均值，并不是實(shí)際售出的所有該貨物的平均價(jià)格；對(duì)我們來說，有用的是按照銷售數(shù)量加權(quán)平均的價(jià)格： [br][br]select good_code,sum(price*amount)/sum(amount) as avg_price from sales group by good_code [br][br]返回的查詢結(jié)果可能為： [br][br]+-----------+-----------+ [br]| good_code | avg_price | [br]+-----------+-----------+ [br]| a0001 | 1200.00 | [br]+-----------+-----------+ [br][br]　　六、查詢售給不同客戶的每一種貨物售出的平均價(jià)格。只要在group子句中多加一個(gè)關(guān)鍵字： [br][br]select good_code,customer,sum(price*amount)/sum(amount) as avg_price from sales group by good_code,customer [br][br]返回的查詢結(jié)果可能為： [br][br]+-----------+----------+-----------+ [br]| good_code | customer | avg_price | [br]+-----------+----------+-----------+ [br]| a0001 | 1 | 1200.00 | [br]+-----------+----------+-----------+ [br]| a0001 | 2 | 1200.00 | [br]+-----------+----------+-----------+ [br][br]所有客戶和所售貨物兩項(xiàng)相同的記錄匯總到一起來求平均，就形成了售給不同客戶的每一種貨物售出的平均價(jià)格。[/p]
[p]按月統(tǒng)計(jì)：[color=#0000ff]select[/color][color=#000000] [/color][color=#ff00ff]convert[/color][color=#000000]([/color][color=#0000ff]char[/color][color=#000000]([/color][color=#800000][b]7[/b][/color][color=#000000]),createtime,[/color][color=#800000][b]20[/b][/color][color=#000000]) [/color][color=#0000ff]as[/color][color=#000000] time,[/color][color=#ff00ff]sum[/color][color=#000000](price*amount) [/color][color=#0000ff]as[/color][color=#000000] hits [/color][color=#0000ff]from[/color][color=#000000] sales [br][/color][color=#0000ff]group[/color][color=#000000] [/color][color=#0000ff]by[/color][color=#000000] [/color][color=#ff00ff]convert[/color][color=#000000]([/color][color=#0000ff]char[/color][color=#000000]([/color][color=#800000][b]7[/b][/color][color=#000000]),createtime,[/color][color=#800000][b]20[/b][/color][color=#000000])[/color][/p]