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MySQL 分組查詢和聚合函數(shù)

概述

相信我們經(jīng)常會(huì)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(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，分組查詢的目的是為了把數(shù)據(jù)分成多個(gè)邏輯組（購買化妝品的人員是一個(gè)組，不同年齡段購買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(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。

3、group_condition 分組之后，再對(duì)分組后的數(shù)據(jù)進(jìn)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(huì)報(bào)異常，我們下面的內(nèi)容中會(huì)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(shí)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(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)	說明
AVG()	返回指定字段的平均值
COUNT()	返回查詢結(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(diǎn)：

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(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(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

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)用戶進(jìn)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(duì)于多個(gè)字段進(jìn)行分組，比如針對(duì)用戶進(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

分組前的條件過濾：where

這個(gè)很簡單，就是再分組（group by）之前通過where關(guān)鍵字進(jìn)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(shí)候我們需要再分組之后再對(duì)數(shù)據(jù)進(jìn)行過濾，這時(shí)候就需要使用having關(guān)鍵字進(jìn)行數(shù)據(jù)過濾，再上述條件下，我們需要取出消費(fèi)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(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，最后返回過濾后的結(jié)果。

可以把having理解為兩級(jí)查詢，即含having的查詢操作先獲得不含having子句時(shí)的sql查詢結(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)用戶的消費(fèi)總額和消費(fèi)次數(shù)后，對(duì)用戶的消費(fèi)總額進(jìn)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(fèi)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(cuò)了會(huì)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(dá)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(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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