AVG() 平均值
COUNT() 计数
MAX() 最大值
MIN() 最小值
SUM() 求和
先预备一张表，结构如下
mysql> DESC goodstest;
+----------+----------------------+------+-----+---------+----------------+
| Field | Type | Null | Key | Default | Extra |
+----------+----------------------+------+-----+---------+----------------+
| id | smallint(6) | NO | PRI | NULL | auto_increment |
| goodname | varchar(20) | NO | | NULL | |
| price | smallint(5) unsigned | YES | | NULL | |
+----------+----------------------+------+-----+---------+----------------+
3 rows in set (0.01 sec)
记录如下
mysql> SELECT * FROM goodstest;
+----+----------+-------+
| id | goodname | price |
+----+----------+-------+
| 1 | A | 1 |
| 2 | B | 2 |
| 3 | C | 3 |
| 4 | D | 4 |
| 5 | E | 5 |
| 6 | F | 6 |
| 7 | G | 7 |
+----+----------+-------+
7 rows in set (0.00 sec)
例1:求平均值
mysql> SELECT AVG(id) FROM goodstest;
+---------+
| AVG(id) |
+---------+
| 4.0000 |
+---------+
1 row in set (0.00 sec)
例2:取出id排名前三的记录
mysql> SELECT * FROM goodstest LIMIT 3;
+----+----------+-------+
| id | goodname | price |
+----+----------+-------+
| 1 | A | 1 |
| 2 | B | 2 |
| 3 | C | 3 |
+----+----------+-------+
3 rows in set (0.00 sec)
算出平均值并按照新的名字输出
mysql> SELECT AVG(price) AS ave_price FROM goodstest ;
+-----------+
| ave_price |
+-----------+
| 4.0000 |
+-----------+
1 row in set (0.00 sec)
例3:四舍五入
mysql> SELECT ROUND(2.4561,2);
+-----------------+
| ROUND(2.4561,2) |
+-----------------+
| 2.46 |
+-----------------+
1 row in set (0.00 sec)
mysql> SELECT ROUND(2.4561,3);
+-----------------+
| ROUND(2.4561,3) |
+-----------------+
| 2.456 |
+-----------------+
1 row in set (0.00 sec)
例4:COUNT 计数
mysql> SELECT COUNT(id) AS counts FROM goodstest;
+--------+
| counts |
+--------+
| 7 |
+--------+
1 row in set (0.00 sec)
例5:求最大值
mysql> SELECT MAX(price) AS counts FROM goodstest;
+--------+
| counts |
+--------+
| 7 |
+--------+
1 row in set (0.00 sec)
例6:求最小值
mysql> SELECT MIN(price) AS counts FROM goodstest;
+--------+
| counts |
+--------+
| 1 |
+--------+
1 row in set (0.00 sec)
例7:求总数
mysql> SELECT SUM(price) AS counts FROM goodstest;
+--------+
| counts |
+--------+
| 28 |
+--------+
1 row in set (0.00 sec)