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概述
相信我們經(jīng)常會遇到這樣的場景：想要了解雙十一天貓購買化妝品的人員中平均消費額度是多少（這可能有利于對商品價格區(qū)間的定位）；或者不同年齡段的化妝品消費占比是多少（這可能有助于對商品備貨量的預(yù)估）。
這個時候就要用到分組查詢，分組查詢的目的是為了把數(shù)據(jù)分成多個邏輯組（購買化妝品的人員是一個組，不同年齡段購買化妝品的人員也是組），并對每個組進行聚合計算的過程:。
分組查詢的語法格式如下：
select cname, group_fun,... from tname [where condition]
group by group_expression [having group_condition];
說明一下：
1、group_fun 代表聚合函數(shù)，是指對分組的數(shù)據(jù)進行聚合計算的函數(shù)。
2、group_expression 代表分組表達式，允許多個，多個之間使用逗號隔開。
3、group_condition 分組之后，再對分組后的數(shù)據(jù)進行條件過濾的過程。
4、分組語法中，select后面出現(xiàn)的字段 要么是group by后面的字段，要么是聚合函數(shù)的列，其他類型會報異常，我們下面的內(nèi)容中會詳細(xì)說明。
說分組之前，先來看看聚合函數(shù)，聚合函數(shù)是分組查詢語法格式中重要的一部分。我們經(jīng)常需要匯總數(shù)據(jù)而不用把它們實際檢索出來，所以MySQL提供了專門的函數(shù)。使用這些函數(shù)，可用于計算我們需要的數(shù)據(jù)，以便分析和生成報表。
聚合函數(shù)
聚合函數(shù)有以下幾種。
AVG()函數(shù)
AVG()通過對表中行數(shù)計數(shù)并計算特定列值之和，求得該列的平均值。 AVG()可用來返回所有列的平均值，也可以用來返回特定列或行的平均值。
下面示例返回用戶表中用戶的平均年齡：
mysql> select * from user2;
+----+--------+------+----------+-----+
| id | name | age | address | sex |
+----+--------+------+----------+-----+
| 1 | brand | 21 | fuzhou | 1 |
| 2 | helen | 20 | quanzhou | 0 |
| 3 | sol | 21 | xiamen | 0 |
| 4 | weng | 33 | guizhou | 1 |
| 5 | selina | 25 | NULL | 0 |
| 6 | anny | 23 | shanghai | 0 |
| 7 | annd | 24 | shanghai | 1 |
| 8 | sunny | NULL | guizhou | 0 |
+----+--------+------+----------+-----+
8 rows in set
mysql> select avg(age) from user2;
+----------+
| avg(age) |
+----------+
| 23.8571 |
+----------+
1 row in set
注意點：
1、AVG()只能用來確定特定數(shù)值列的平均值 。
2、AVG()函數(shù)忽略列值為NULL的行，所以上圖中age值累加之后是除以7，而不是除以8。
COUNT()函數(shù)
COUNT()函數(shù)進行計數(shù)。 可以用COUNT()確定表中符合條件的行的數(shù)目。
count 有 count(*)、count(具體字段)、count(常量) 三種方式來體現(xiàn) 下面 演示了count(*) 和 count(cname)的用法。
mysql> select * from user2;
+----+--------+------+----------+-----+
| id | name | age | address | sex |
+----+--------+------+----------+-----+
| 1 | brand | 21 | fuzhou | 1 |
| 2 | helen | 20 | quanzhou | 0 |
| 3 | sol | 21 | xiamen | 0 |
| 4 | weng | 33 | guizhou | 1 |
| 5 | selina | 25 | NULL | 0 |
| 6 | anny | 23 | shanghai | 0 |
| 7 | annd | 24 | shanghai | 1 |
| 8 | sunny | NULL | guizhou | 0 |
+----+--------+------+----------+-----+
8 rows in set
mysql> select count(*) from user2 where sex=0;
+----------+
| count(*) |
+----------+
| 5 |
+----------+
1 row in set
mysql> select count(age) from user2 where sex=0;
+------------+
| count(age) |
+------------+
| 4 |
+------------+
1 row in set
可以看到，都是取出女生的用戶數(shù)量，count(*) 比 count(age) 多一個，那是因為age中包含null值。
所以：如果指定列名，則指定列的值為空的行被COUNT()函數(shù)忽略，但如果COUNT()函數(shù)中用的是星號（ *），則不忽略。
MAX()和MIN()函數(shù)
MAX()返回指定列中的最大值，MIN()返回指定列中的最小值。
mysql> select * from user2;
+----+--------+------+----------+-----+
| id | name | age | address | sex |
+----+--------+------+----------+-----+
| 1 | brand | 21 | fuzhou | 1 |
| 2 | helen | 20 | quanzhou | 0 |
| 3 | sol | 21 | xiamen | 0 |
| 4 | weng | 33 | guizhou | 1 |
| 5 | selina | 25 | NULL | 0 |
| 6 | anny | 23 | shanghai | 0 |
| 7 | annd | 24 | shanghai | 1 |
| 8 | sunny | NULL | guizhou | 0 |
+----+--------+------+----------+-----+
8 rows in set
mysql> select max(age),min(age) from user2;
+----------+----------+
| max(age) | min(age) |
+----------+----------+
| 33 | 20 |
+----------+----------+
1 row in set
注意：同樣的，MAX()、MIN()函數(shù)忽略列值為NULL的行。
SUM函數(shù)
SUM()用來返回指定列值的和（總計） ，下面返回了所有年齡的總和，同樣的，忽略了null的值
mysql> select * from user2;
+----+--------+------+----------+-----+
| id | name | age | address | sex |
+----+--------+------+----------+-----+
| 1 | brand | 21 | fuzhou | 1 |
| 2 | helen | 20 | quanzhou | 0 |
| 3 | sol | 21 | xiamen | 0 |
| 4 | weng | 33 | guizhou | 1 |
| 5 | selina | 25 | NULL | 0 |
| 6 | anny | 23 | shanghai | 0 |
| 7 | annd | 24 | shanghai | 1 |
| 8 | sunny | NULL | guizhou | 0 |
+----+--------+------+----------+-----+
8 rows in set
mysql> select sum(age) from user2;
+----------+
| sum(age) |
+----------+
| 167 |
+----------+
1 row in set
分組查詢
數(shù)據(jù)準(zhǔn)備，假設(shè)我們有一個訂貨單表如下（記載用戶的訂單金額和下單時間）：
mysql> select * from t_order;
+---------+-----+-------+--------+---------------------+------+
| orderid | uid | uname | amount | time | year |
+---------+-----+-------+--------+---------------------+------+
| 20 | 1 | brand | 91.23 | 2018-08-20 17:22:21 | 2018 |
| 21 | 1 | brand | 87.54 | 2019-07-16 09:21:30 | 2019 |
| 22 | 1 | brand | 166.88 | 2019-04-04 12:23:55 | 2019 |
| 23 | 2 | helyn | 93.73 | 2019-09-15 10:11:11 | 2019 |
| 24 | 2 | helyn | 102.32 | 2019-01-08 17:33:25 | 2019 |
| 25 | 2 | helyn | 106.06 | 2019-12-24 12:25:25 | 2019 |
| 26 | 2 | helyn | 73.42 | 2020-04-03 17:16:23 | 2020 |
| 27 | 3 | sol | 55.55 | 2019-08-05 19:16:23 | 2019 |
| 28 | 3 | sol | 69.96 | 2020-09-16 19:23:16 | 2020 |
| 29 | 4 | weng | 199.99 | 2020-06-08 19:55:06 | 2020 |
+---------+-----+-------+--------+---------------------+------+
10 rows in set
單字段分組
即對于某個字段進行分組，比如針對用戶進行分組，輸出他們的用戶Id，訂單數(shù)量和總額：
mysql> select uid,count(uid),sum(amount) from t_order group by uid;
+-----+------------+-------------+
| uid | count(uid) | sum(amount) |
+-----+------------+-------------+
| 1 | 3 | 345.65 |
| 2 | 4 | 375.53 |
| 3 | 2 | 125.51 |
| 4 | 1 | 199.99 |
+-----+------------+-------------+
4 rows in set
多字段分組
即對于多個字段進行分組，比如針對用戶進行分組，再對他們不同年份的訂單數(shù)據(jù)進行分組，輸出訂單數(shù)量和消費總額：
mysql> select uid,count(uid) as nums,sum(amount) as totalamount,year from t_order group by uid,year;
+-----+------+-------------+------+
| uid | nums | totalamount | year |
+-----+------+-------------+------+
| 1 | 1 | 91.23 | 2018 |
| 1 | 2 | 254.42 | 2019 |
| 2 | 3 | 302.11 | 2019 |
| 2 | 1 | 73.42 | 2020 |
| 3 | 1 | 55.55 | 2019 |
| 3 | 1 | 69.96 | 2020 |
| 4 | 1 | 199.99 | 2020 |
+-----+------+-------------+------+
7 rows in set
分組前的條件過濾：where
這個很簡單，就是再分組（group by）之前通過where關(guān)鍵字進行條件過濾，取出我們需要的數(shù)據(jù)，假設(shè)我們只要列出2019年8月之后的數(shù)據(jù)，源數(shù)據(jù)只有6條合格的，有兩條年份一樣被分組的：
mysql> select uid,count(uid) as nums,sum(amount) as totalamount,year from t_order where time > '2019-08-01' group by uid,year;
+-----+------+-------------+------+
| uid | nums | totalamount | year |
+-----+------+-------------+------+
| 2 | 2 | 199.79 | 2019 |
| 2 | 1 | 73.42 | 2020 |
| 3 | 1 | 55.55 | 2019 |
| 3 | 1 | 69.96 | 2020 |
| 4 | 1 | 199.99 | 2020 |
+-----+------+-------------+------+
5 rows in set
分組后的條件過濾：having
有時候我們需要再分組之后再對數(shù)據(jù)進行過濾，這時候就需要使用having關(guān)鍵字進行數(shù)據(jù)過濾，再上述條件下，我們需要取出消費次數(shù)超過一次的數(shù)據(jù)：
mysql> select uid,count(uid) as nums,sum(amount) as totalamount,year from t_order where time > '2019-08-01' group by uid,year having nums>1;
+-----+------+-------------+------+
| uid | nums | totalamount | year |
+-----+------+-------------+------+
| 2 | 2 | 199.79 | 2019 |
+-----+------+-------------+------+
1 row in set
這邊需要注意區(qū)分where和having：
where是在分組（聚合）前對記錄進行篩選，而having是在分組結(jié)束后的結(jié)果里篩選，最后返回過濾后的結(jié)果。
可以把having理解為兩級查詢，即含having的查詢操作先獲得不含having子句時的sql查詢結(jié)果表，然后在這個結(jié)果表上使用having條件篩選出符合的記錄，最后返回這些記錄，因此，having后是可以跟聚合函數(shù)的，并且這個聚集函數(shù)不必與select后面的聚集函數(shù)相同。
分組后的排序處理
order條件接在group by后面，也就是統(tǒng)計出每個用戶的消費總額和消費次數(shù)后，對用戶的消費總額進行降序排序的過程。
mysql> select uid,count(uid) as nums,sum(amount) as totalamount from t_order group by uid;
+-----+------+-------------+
| uid | nums | totalamount |
+-----+------+-------------+
| 1 | 3 | 345.65 |
| 2 | 4 | 375.53 |
| 3 | 2 | 125.51 |
| 4 | 1 | 199.99 |
+-----+------+-------------+
4 rows in set
mysql> select uid,count(uid) as nums,sum(amount) as totalamount from t_order group by uid order by totalamount desc;
+-----+------+-------------+
| uid | nums | totalamount |
+-----+------+-------------+
| 2 | 4 | 375.53 |
| 1 | 3 | 345.65 |
| 4 | 1 | 199.99 |
| 3 | 2 | 125.51 |
+-----+------+-------------+
4 rows in set
分組后的limit 限制
limit限制關(guān)鍵字一般放在語句的最末尾，比如基于我們上面的搜索，我們再limit 1，只取出消費額最高的那條，其他跳過。
mysql> select uid,count(uid) as nums,sum(amount) as totalamount from t_order group by uid order by totalamount desc limit 1;
+-----+------+-------------+
| uid | nums | totalamount |
+-----+------+-------------+
| 2 | 4 | 375.53 |
+-----+------+-------------+
1 row in set
關(guān)鍵字的執(zhí)行順序
我們看到上面那我們用了 where、group by、having、order by、limit這些關(guān)鍵字，如果一起使用，他們是有先后順序，順序錯了會導(dǎo)致異常，語法格式如下：
select cname from tname
where [原表查詢條件]
group by [分組表達式]
having [分組過濾條件]
order by [排序條件]
limit [offset,] count;
mysql> select uid,count(uid) as nums,sum(amount) as totalamount from t_order where time > '2019-08-01' group by uid having totalamount>100 order by totalamount desc limit 1;
+-----+------+-------------+
| uid | nums | totalamount |
+-----+------+-------------+
| 2 | 3 | 273.21 |
+-----+------+-------------+
1 row in set
總結(jié)
1、分組語法中，select后面出現(xiàn)的字段 要么是group by后面的字段，要么是聚合函數(shù)的列，其他類型會報異常：可以自己試試。
2、分組關(guān)鍵字的執(zhí)行順序：where、group by、having、order by、limit，順序不能調(diào)換，否則會報異常：可以自己試試。
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