一、MySql中函數(shù)定義：

• 函數(shù)就是MySQL服務(wù)內(nèi)置的命令
• 函數(shù)的格式：函數(shù)名()
• 通常使用的方法=>函數(shù)名(表頭名)

二、常用函數(shù)：

三、案例：

• 字符函數(shù)例子

mysql> select name,length(name) from tarena.employees where employee_id = 8 ;
+--------+-------------------+
| name   | char_length(name) |
+--------+-------------------+
| 汪云   |                 6   |
+--------+-------------------+
1 row in set (0.00 sec)

• 數(shù)學(xué)函數(shù)例子

# 輸出偶數(shù)行 行號(hào)和用戶(hù)名
mysql> select id , name  from tarena.user where  mod(id,2) = 0 ;
+----+-----------------+
| id | name            |
+----+-----------------+
|  2 | bin             |
|  4 | adm             |
|  6 | sync            |
|  8 | halt            |
+----+-----------------+
4 rows in set (0.00 sec)

• 日期時(shí)間函數(shù)例子

# 輸出當(dāng)前時(shí)間
mysql> select now();   
+---------------------+
| now()               |
+---------------------+
| 2021-10-12 09:27:39 |
+---------------------+
1 row in set (0.00 sec)

# 獲取當(dāng)前年份
mysql> select year(now());
+-------------+
| year(now()) |
+-------------+
|        2021 |
+-------------+
1 row in set (0.00 sec)

• 聚集函數(shù)例子

# 輸出uid列的最小值和最大值
mysql> select min(uid),max(uid) from  tarena.user;
+----------+----------+
| min(uid) | max(uid) |
+----------+----------+
|        0  |    65534 |
+----------+----------+
1 row in set (0.00 sec)

# 統(tǒng)計(jì)使用解釋器/sbin/nologin 用戶(hù)的個(gè)數(shù)
mysql> select count(name) from  tarena.user where shell="/sbin/nologin";
+-------------+
| count(name) |
+-------------+
|          19 |
+-------------+
1 row in set (0.01 sec)