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MySQL 分組查詢(xún)和聚合函數(shù)

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概述

相信我們經(jīng)常會(huì)遇到這樣的場(chǎng)景:想要了解雙十一天貓購(gòu)買(mǎi)化妝品的人員中平均消費(fèi)額度是多少(這可能有利于對(duì)商品價(jià)格區(qū)間的定位);或者不同年齡段的化妝品消費(fèi)占比是多少(這可能有助于對(duì)商品備貨量的預(yù)估)。

這個(gè)時(shí)候就要用到分組查詢(xún),分組查詢(xún)的目的是為了把數(shù)據(jù)分成多個(gè)邏輯組(購(gòu)買(mǎi)化妝品的人員是一個(gè)組,不同年齡段購(gòu)買(mǎi)化妝品的人員也是組),并對(duì)每個(gè)組進(jìn)行聚合計(jì)算的過(guò)程:。

分組查詢(xún)的語(yǔ)法格式如下:

 select cname, group_fun,... from tname [where condition]
 group by group_expression [having group_condition]; 

說(shuō)明一下:

1、group_fun 代表聚合函數(shù),是指對(duì)分組的數(shù)據(jù)進(jìn)行聚合計(jì)算的函數(shù)。

2、group_expression 代表分組表達(dá)式,允許多個(gè),多個(gè)之間使用逗號(hào)隔開(kāi)。

3、group_condition 分組之后,再對(duì)分組后的數(shù)據(jù)進(jìn)行條件過(guò)濾的過(guò)程。

4、分組語(yǔ)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(lèi)型會(huì)報(bào)異常,我們下面的內(nèi)容中會(huì)詳細(xì)說(shuō)明。 

說(shuō)分組之前,先來(lái)看看聚合函數(shù),聚合函數(shù)是分組查詢(xún)語(yǔ)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(shí)際檢索出來(lái),所以MySQL提供了專(zhuān)門(mén)的函數(shù)。使用這些函數(shù),可用于計(jì)算我們需要的數(shù)據(jù),以便分析和生成報(bào)表。

聚合函數(shù)

聚合函數(shù)有以下幾種。 

函數(shù) 說(shuō)明
AVG() 返回指定字段的平均值
COUNT() 返回查詢(xún)結(jié)果行數(shù)
MAX() 返回指定字段的最大值 
MIN() 返回指定字段的最小值
SUM() 返回指定字段的求和值

AVG()函數(shù)

AVG()通過(guò)對(duì)表中行數(shù)計(jì)數(shù)并計(jì)算特定列值之和,求得該列的平均值。 AVG()可用來(lái)返回所有列的平均值,也可以用來(lái)返回特定列或行的平均值。

下面示例返回用戶(hù)表中用戶(hù)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(diǎn):

1、AVG()只能用來(lái)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(jìn)行計(jì)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(lái)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(hù)數(shù)量,count(*) 比 count(age) 多一個(gè),那是因?yàn)閍ge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(hào)( *),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(lái)返回指定列值的和(總計(jì)) ,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(xún)

數(shù)據(jù)準(zhǔn)備,假設(shè)我們有一個(gè)訂貨單表如下(記載用戶(hù)的訂單金額和下單時(shí)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(duì)于某個(gè)字段進(jìn)行分組,比如針對(duì)用戶(hù)進(jìn)行分組,輸出他們的用戶(hù)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(duì)于多個(gè)字段進(jìn)行分組,比如針對(duì)用戶(hù)進(jìn)行分組,再對(duì)他們不同年份的訂單數(shù)據(jù)進(jìn)行分組,輸出訂單數(shù)量和消費(fèi)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(guò)濾:where

這個(gè)很簡(jiǎn)單,就是再分組(group by)之前通過(guò)where關(guān)鍵字進(jìn)行條件過(guò)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(guò)濾:having

有時(shí)候我們需要再分組之后再對(duì)數(shù)據(jù)進(jìn)行過(guò)濾,這時(shí)候就需要使用having關(guān)鍵字進(jìn)行數(shù)據(jù)過(guò)濾,再上述條件下,我們需要取出消費(fèi)次數(shù)超過(guò)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(duì)記錄進(jìn)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(guò)濾后的結(jié)果。

可以把having理解為兩級(jí)查詢(xún),即含having的查詢(xún)操作先獲得不含having子句時(shí)的sql查詢(xún)結(jié)果表,然后在這個(gè)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(gè)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(jì)出每個(gè)用戶(hù)的消費(fèi)總額和消費(fèi)次數(shù)后,對(duì)用戶(hù)的消費(fèi)總額進(jìn)行降序排序的過(guò)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(yǔ)句的最末尾,比如基于我們上面的搜索,我們?cè)賚imit 1,只取出消費(fèi)額最高的那條,其他跳過(guò)。

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(cuò)了會(huì)導(dǎo)致異常,語(yǔ)法格式如下:

 select cname from tname
 where [原表查詢(xún)條件]
 group by [分組表達(dá)式]
 having [分組過(guò)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(yǔ)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(lèi)型會(huì)報(bào)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(huì)報(bào)異常:可以自己試試。

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標(biāo)簽:沈陽(yáng) 呼和浩特 阿里 天津 合肥 牡丹江 惠州 公主嶺

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