MySQL基礎之 統計函數總結
阿新 • • 發佈:2018-11-10
sum() sql基礎 max ted where match span sele affect
五種統計函數:count()、max()、avg()、min()、max()函數
count()函數
count()函數在進行計算的時候,是分情況進行計算的,主要是一下兩種
1、采用count(*)對表中的記錄進行統計,不管表字段中包含的是NULL值還是非NULL值,都會參與計算
2、而采用count(field)方式對表中的記錄進行統計,如果所在的field中包含有NULL值,將忽略NULL值。
舉例所示:
mysql> select * from test1_1; #可以看出來表中一共有6條記錄,其中有條記錄的name列包含NULL值 +----+--------+| id | name | +----+--------+ | 1 | 超峰 | | 2 | b | | 3 | c | | 4 | a | | 5 | chen | | 6 | NULL | +----+--------+ 6 rows in set (0.00 sec) mysql> select count(*) from test1_1; +----------+ | count(*) | +----------+ | 6 | +----------+ 1 row in set (0.00 sec) mysql>select count(name) from test1_1; #NULL值將不參與計算 +-------------+ | count(name) | +-------------+ | 5 | +-------------+ 1 row in set (0.00 sec)
sum()函數
sum()函數實現統計指定字段值之和,在具體統計時將忽略NULL值,但是不會忽略0值
舉例子:
mysql> select * from test1_1; +----+--------+-------+ | id | name | grade | +----+--------+-------+| 1 | 超峰 | 99 | | 2 | b | 90 | | 3 | c | 95 | | 4 | a | 85 | | 5 | chen | 90 | | 6 | NULL | 90 | +----+--------+-------+ 6 rows in set (0.00 sec) mysql> select sum(grade) from test1_1; +------------+ | sum(grade) | +------------+ | 549 | +------------+ 1 row in set (0.00 sec) mysql> update test1_1 set grade=NULL where id=6; Query OK, 1 row affected (0.00 sec) Rows matched: 1 Changed: 1 Warnings: 0 mysql> select * from test1_1; +----+--------+-------+ | id | name | grade | +----+--------+-------+ | 1 | 超峰 | 99 | | 2 | b | 90 | | 3 | c | 95 | | 4 | a | 85 | | 5 | chen | 90 | | 6 | NULL | NULL | +----+--------+-------+ 6 rows in set (0.00 sec) mysql> select sum(grade) from test1_1; #會發現NULL值沒有參與計算 +------------+ | sum(grade) | +------------+ | 459 | +------------+ 1 row in set (0.00 sec)
MAX()和MIN()函數
MAX函數和MIN函數會忽略值為NULL的數據,但是不會忽略值為0的數據
在此不做舉例
針對沒有任何數據的表,這五種函數的返回值是什麽呢?
舉例子:
mysql> select * from test1_5; Empty set (0.00 sec) mysql> select count(*),avg(id),sum(id),max(id),min(id) from test1_5; +----------+---------+---------+---------+---------+ | count(*) | avg(id) | sum(id) | max(id) | min(id) | +----------+---------+---------+---------+---------+ | 0 | NULL | NULL | NULL | NULL | +----------+---------+---------+---------+---------+ 1 row in set (0.00 sec)
看的出來只有count函數的返回值是0,其他的函數返回值都是NULL。
MySQL基礎之 統計函數總結