資料準備：
mysql> CREATE TABLE users(
    -> id SMALLINT UNSIGNED PRIMARY KEY AUTO_INCREMENT,
    -> username VARCHAR(20) NOT NULL,
    -> age SMALLINT UNSIGNED DEFAULT 10,
    -> sex BOOLEAN
    -> );
Query OK, 0 rows affected (0.12 sec)

mysql> INSERT users VALUES(DEFAULT,'tom',23,1);
Query OK, 1 row affected (0.06 sec)

mysql> INSERT users VALUES(DEFAULT,'tomi',23,1);
Query OK, 1 row affected (0.06 sec)

mysql> INSERT users VALUES(DEFAULT,'jerry',26,0);
Query OK, 1 row affected (0.05 sec)

mysql> INSERT users VALUES(DEFAULT,'jerry',26,NULL);
Query OK, 1 row affected (0.05 sec)

mysql> INSERT users VALUES(DEFAULT,'marry',DEFAULT,NULL);
Query OK, 1 row affected (0.06 sec)

mysql> INSERT users VALUES(DEFAULT,'marry',DEFAULT,1);
Query OK, 1 row affected (0.11 sec)

mysql> INSERT users VALUES(DEFAULT,'tom',34,0);
Query OK, 1 row affected (0.05 sec)

mysql> SELECT * FROM users;
+----+----------+------+------+
| id | username | age  | sex  |
+----+----------+------+------+
|  1 | tom      |   23 |    1 |
|  2 | tomi     |   23 |    1 |
|  3 | jerry    |   26 |    0 |
|  4 | jerry    |   26 | NULL |
|  5 | marry    |   10 | NULL |
|  6 | marry    |   10 |    1 |
|  7 | tom      |   34 |    0 |
+----+----------+------+------+
7 rows in set (0.00 sec)

1、聚集函式
聚集函式是執行在行組上，計算和返回單個值的函式。
SQL聚集函式
函式說明
AVG()返回某列的平均值
COUNT()返回某列的行數
MAX()返回某列的最大值
MIN()返回某列的最小值
SUM()返回某個列之和
（1）、AVG()函式
可以返回所有列的平均值，也可以返回特定列的平均值。

mysql> SELECT AVG(age) AS avg_age FROM users; #計算所有記錄中age欄位的均值。
+---------+
| avg_age |
+---------+
| 21.7143 |
+---------+
1 row in set (0.00 sec)

mysql> SELECT AVG(age) AS avg_age FROM users WHERE sex=1;#計算所有的sex=1的記錄中age欄位的均值。
+---------+
| avg_age |
+---------+
| 18.6667 |
+---------+
1 row in set (0.00 sec)

（2）、COUNT()函式
COUNT(*)對錶中行的數目進行計數，不管表列中包含的是空（NULL）還是非空值；
COUNT(column)對特定列中有值的行進行計算，忽略NULL值。
mysql> SELECT COUNT(*) FROM users;#對錶中行的數目進行計數
+----------+
| COUNT(*) |
+----------+
|        7 |
+----------+
1 row in set (0.00 sec)

mysql> SELECT COUNT(sex) FROM users;#對sex列中，有值的行進行計算，忽略NULL值。
+------------+
| COUNT(sex) |
+------------+
|          5 |
+------------+
1 row in set (0.00 sec)
（3）、MAX()函式
MAX()返回指定列的最大值，要求指定列名，忽略NULL值。
在MySQL中，MAX()函式可以對非資料列使用，在用於文字資料時，如果資料按相應的列排序，MAX()返回最後一行。
mysql> SELECT MAX(age) FROM users;
+----------+
| MAX(age) |
+----------+
|       34 |
+----------+
1 row in set (0.00 sec)

mysql> SELECT MAX(sex) FROM users;
+----------+
| MAX(sex) |
+----------+
|        1 |
+----------+
1 row in set (0.00 sec)

mysql> SELECT MAX(username) FROM users;
+---------------+
| MAX(username) |
+---------------+
| tomi          |
+---------------+
1 row in set (0.00 sec)

mysql> SELECT username FROM users ORDER BY username ASC; #可以看到，tomi排在最後。
+----------+
| username |
+----------+
| jerry    |
| jerry    |
| marry    |
| marry    |
| tom      |
| tom      |
| tomi     |
+----------+
7 rows in set (0.00 sec)

（4）、MIN()函式
MIN()返回指定列的最小值，要求指定列名，忽略NULL值。
在MySQL中，MIN()函式可以對非資料列使用，在用於文字資料時，如果資料按相應的列排序，MIN()返回最前面的一行。
mysql> SELECT MIN(sex) FROM users;
+----------+
| MIN(sex) |
+----------+
|        0 |
+----------+
1 row in set (0.00 sec)

mysql> SELECT MIN(age) FROM users;
+----------+
| MIN(age) |
+----------+
|       10 |
+----------+
1 row in set (0.00 sec)

（5）、SUM()函式
用來返回指定列的和（總計），忽略NULL值的行。
mysql> SELECT SUM(age) FROM users;
+----------+
| SUM(age) |
+----------+
|      152 |
+----------+
1 row in set (0.00 sec)

mysql> SELECT SUM(sex) FROM users;
+----------+
| SUM(sex) |
+----------+
|        3 |
+----------+
1 row in set (0.00 sec)

mysql> SELECT SUM(age) AS sum_age FROM users WHERE sex=1;#對sex=1的記錄，求年齡之和。
+---------+
| sum_age |
+---------+
|      56 |
+---------+
1 row in set (0.00 sec)

SUM()也可以合計計算值。
mysql> SELECT * FROM users WHERE sex=0;
+----+----------+------+------+
| id | username | age  | sex  |
+----+----------+------+------+
|  3 | jerry    |   26 |    0 |
|  7 | tom      |   34 |    0 |
+----+----------+------+------+
2 rows in set (0.00 sec)

mysql> SELECT SUM(age*id) AS sum_age_id FROM users WHERE sex=0;
+------------+
| sum_age_id |
+------------+
|        316 |
+------------+
1 row in set (0.00 sec)

2、聚集不同值
DISTINCT關鍵字：如果使用DISTINCT關鍵字，則需要去掉重複值。（即：重複的數值只能使用1次）
mysql> SELECT * FROM users WHERE sex=1;
+----+----------+------+------+
| id | username | age  | sex  |
+----+----------+------+------+
|  1 | tom      |   23 |    1 |
|  2 | tomi     |   23 |    1 |
|  6 | marry    |   10 |    1 |
+----+----------+------+------+
3 rows in set (0.00 sec)

mysql> SELECT AVG(DISTINCT age) FROM users WHERE sex=1;#對23,10求均值。
+-------------------+
| AVG(DISTINCT age) |
+-------------------+
|           16.5000 |
+-------------------+
1 row in set (0.03 sec)

mysql> SELECT AVG(age) FROM users WHERE sex=1;#對23,23,10求均值
+----------+
| AVG(age) |
+----------+
|  18.6667 |
+----------+
1 row in set (0.00 sec)

3、組合聚集函式
聚集函式可以組合使用
mysql> SELECT COUNT(*) AS num,MIN(age) AS age_min,MAX(age) AS age_max,AVG(age) AS age_avg FROM users;
+-----+---------+---------+---------+
| num | age_min | age_max | age_avg |
+-----+---------+---------+---------+
|   7 |      10 |      34 | 21.7143 |
+-----+---------+---------+---------+

1 row in set (0.00 sec)

mysql> SELECT COUNT(*) AS num,MIN(age) AS age_min,MAX(age) AS age_max,AVG(age) AS age_avg FROM users WHERE sex=1;
+-----+---------+---------+---------+
| num | age_min | age_max | age_avg |
+-----+---------+---------+---------+
|   3 |      10 |      23 | 18.6667 |
+-----+---------+---------+---------+
1 row in set (0.00 sec)

mysql> SELECT * FROM users WHERE sex=1;
+----+----------+------+------+
| id | username | age  | sex  |
+----+----------+------+------+
|  1 | tom      |   23 |    1 |
|  2 | tomi     |   23 |    1 |
|  6 | marry    |   10 |    1 |
+----+----------+------+------+
3 rows in set (0.00 sec)

mysql> SELECT COUNT(*) AS num,MIN(age) AS age_min,MAX(age) AS age_max,AVG(DISTINCT age) AS age_avg FROM users WHERE sex=1;
+-----+---------+---------+---------+
| num | age_min | age_max | age_avg |
+-----+---------+---------+---------+
|   3 |      10 |      23 | 16.5000 |
+-----+---------+---------+---------+
1 row in set (0.00 sec)