| 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，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（ *），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(jìn)行分組，比如針對用戶進(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
即對于多個字段進(jìn)行分組，比如針對用戶進(jìn)行分組，再對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
有時候我們需要再分組之后再對數(shù)據(jù)進(jìn)行過濾，這時候就需要使用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(jì)出每個用戶的消費(fèi)總額和消費(fèi)次數(shù)后，對用戶的消費(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(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，否則會報(bào)異常：可以自己試試。
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標(biāo)簽：沈陽 呼和浩特 阿里 天津 合肥 牡丹江 惠州 公主嶺
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