用聚合函数查询MySQL数据

1、COUNT()函数

统计数据表中包含的记录行的总数，或者根据查询结果返回列中包含的数据行数

MariaDB [vincen]> select COUNT(*) AS f_num  #计算所有行数，并改名为f_num名输出
    -> from fruits                          #从fruits表中
    -> ;                                    #结束
+-------+
| f_num |
+-------+
|    16 |
+-------+
1 row in set (0.01 sec)

MariaDB [vincen]> select s_id,   #查询s_id列
    -> COUNT(f_id)               #查询f_id列总的行数，以COUNT(f_id)名输出
    -> from fruits               #从fruits表中
    -> GROUP BY s_id             #按s_id列排序
    -> ;                         #结束
+------+-------------+
| s_id | COUNT(f_id) |
+------+-------------+
|  101 |           3 |
|  102 |           3 |
|  103 |           2 |
|  104 |           2 |
|  105 |           3 |
|  106 |           1 |
|  107 |           2 |
+------+-------------+
7 rows in set (0.00 sec)

2、SUM()函数

求总和的函数，返回指定列值的总和

MariaDB [vincen]> select s_id,      #查询s_id列
    -> SUM(f_price) AS sum_price    #查询f_price列数值的总和并改名为sum_price输出
    -> from fruits                  #从fruits表中
    -> GROUP BY s_id                #按s_id列排序
    -> ;                            #结束
+------+-----------+
| s_id | sum_price |
+------+-----------+
|  101 |     18.60 |
|  102 |     26.80 |
|  103 |     11.40 |
|  104 |     14.00 |
|  105 |     22.40 |
|  106 |     15.60 |
|  107 |      7.20 |
+------+-----------+
7 rows in set (0.01 sec)

3、AVG()函数

计算返回的行数和每一行数据的和，指定列数据的平均值

MariaDB [vincen]> select s_id,     #查询s_id列
    -> AVG(f_price) AS avg_price   #查询f_price列数值的平均值并改名为avg_price输出
    -> from fruits                 #从fruits表中
    -> GROUP BY s_id               #按s_id列排序
    -> ;                           #结束
+------+-----------+
| s_id | avg_price |
+------+-----------+
|  101 |  6.200000 |
|  102 |  8.933333 |
|  103 |  5.700000 |
|  104 |  7.000000 |
|  105 |  7.466667 |
|  106 | 15.600000 |
|  107 |  3.600000 |
+------+-----------+
7 rows in set (0.01 sec)

4、MAX()函数

返回指定列的最大值

MariaDB [vincen]> select s_id,       #查询s_id列
    -> MAX(f_price) AS max_price     #查询f_price列中最大的数值并改名为max_price输出
    -> from fruits                   #从fruits表中 
    -> GROUP BY s_id                 #按s_id列排序
    -> ;                             #结束
+------+-----------+
| s_id | max_price |
+------+-----------+
|  101 |     10.20 |
|  102 |     11.20 |
|  103 |      9.20 |
|  104 |      7.60 |
|  105 |     11.60 |
|  106 |     15.60 |
|  107 |      3.60 |
+------+-----------+
7 rows in set (0.00 sec)

5、MIN()函数

返回指定列的最小值

MariaDB [vincen]> select s_id,     #查询s_id列
    -> MIN(f_price) AS min_price   #查询f_price列中最小的数值并改名为min_price输出
    -> from fruits                 #从fruits表中
    -> GROUP BY s_id;              #以s_id列排序
+------+-----------+
| s_id | min_price |
+------+-----------+
|  101 |      3.20 |
|  102 |      5.30 |
|  103 |      2.20 |
|  104 |      6.40 |
|  105 |      2.60 |
|  106 |     15.60 |
|  107 |      3.60 |
+------+-----------+
7 rows in set (0.01 sec)

用聚合函数查询MySQL数据

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