mysql学习笔记（三）

字符函数

函数名称	描述
CONCAT()	字符连接
CONCAT_WS()	使用指定的分隔符进行字符连接
FORMAT()	数字格式化
LOWER()	转换成小写字母
UPPER()	转换成大写字母
LEFT()	获取左侧字符
RIGHT()	获取右侧字符
LENGTH()	获取字符串的长度
LTRIM	删除前导空格
RTRIM	删除后续空格
TRIM	删除前导和后续空格
SUBSTRING()	字符串获取
[NOT] LIKE	模式匹配
REPLACE()	字符串替换

# 将2个字符连接在一起
mysql> SELECT CONCAT('I love ', 'you!');
+---------------------------+
| CONCAT('I love ', 'you!') |
+---------------------------+
| I love you!               |
+---------------------------+
1 row in set (0.00 sec)

# 可指定第二个参数连接符号
mysql> SELECT CONCAT('I love ','--', 'you!');
+--------------------------------+
| CONCAT('I love ','--', 'you!') |
+--------------------------------+
| I love --you!                  |
+--------------------------------+
1 row in set (0.00 sec)

# 使用指定的分隔符进行字符连接
mysql> SELECT CONCAT_WS('#', 'A', 'B', 'C');
+-------------------------------+
| CONCAT_WS('#', 'A', 'B', 'C') |
+-------------------------------+
| A#B#C                         |
+-------------------------------+
1 row in set (0.00 sec)

# 数字格式化，第二个参数为保留的小数位数
mysql> SELECT FORMAT(1112333.567, 1);
+------------------------+
| FORMAT(1112333.567, 1) |
+------------------------+
| 1,112,333.6            |
+------------------------+
1 row in set (0.00 sec)

# 将所有字母改为小写
mysql> SELECT LOWER('MySql');
+----------------+
| LOWER('MySql') |
+----------------+
| mysql          |
+----------------+
1 row in set (0.00 sec)

# 将所有字符改为大写
mysql> SELECT UPPER('mysql');
+----------------+
| UPPER('mysql') |
+----------------+
| MYSQL          |
+----------------+
1 row in set (0.00 sec)

# 取左侧前2个字符
mysql> SELECT LEFT('mysql', 2);
+------------------+
| LEFT('mysql', 2) |
+------------------+
| my               |
+------------------+
1 row in set (0.00 sec)

# 取右侧后3个字符
mysql> SELECT RIGHT('mysql', 3);
+-------------------+
| RIGHT('mysql', 3) |
+-------------------+
| sql               |
+-------------------+
1 row in set (0.00 sec)

#获取字符串的长度
mysql> SELECT LENGTH('mysql');
+-----------------+
| LENGTH('mysql') |
+-----------------+
|               5 |
+-----------------+
1 row in set (0.00 sec)

# 删除前导字符'$'
mysql> SELECT TRIM(LEADING '$' FROM '$$$mysql$$$$$$$$');
+-------------------------------------------+
| TRIM(LEADING '$' FROM '$$$mysql$$$$$$$$') |
+-------------------------------------------+
| mysql$$$$$$$$                             |
+-------------------------------------------+
1 row in set (0.00 sec)

# 删除后续字符'$'
mysql> SELECT TRIM(TRAILING '$' FROM '$$$mysql$$$$$$$$');
+--------------------------------------------+
| TRIM(TRAILING '$' FROM '$$$mysql$$$$$$$$') |
+--------------------------------------------+
| $$$mysql                                   |
+--------------------------------------------+
1 row in set (0.00 sec)

# 删除前导和后续字符
mysql> SELECT TRIM(BOTH '$' FROM '$$$mysql$$$$$$$$');
+----------------------------------------+
| TRIM(BOTH '$' FROM '$$$mysql$$$$$$$$') |
+----------------------------------------+
| mysql                                  |
+----------------------------------------+
1 row in set (0.00 sec)

# 将字符'$'替换为空字符
mysql> SELECT REPLACE('$$$my$$$$sql$$$$$$$$', '$', '');
+------------------------------------------+
| REPLACE('$$$my$$$$sql$$$$$$$$', '$', '') |
+------------------------------------------+
| mysql                                    |
+------------------------------------------+
1 row in set (0.00 sec)

# 截取字符串前2个字符，注意：下标从1开始。第一个参数为字符串，第二个参数为截取字符的起始位置，第三个参数为截取字符的终止位置，都是闭区间！其操作和python切片类似。
mysql> SELECT SUBSTRING('mysql', 1, 2);
+--------------------------+
| SUBSTRING('mysql', 1, 2) |
+--------------------------+
| my                       |
+--------------------------+
1 row in set (0.00 sec)

# 用LIKE关键字来匹配字符串
mysql> SELECT * FROM cyc;
+----+------+-----+
| id | name | age |
+----+------+-----+
|  1 | 张三 |  20 |
|  2 | 张刘 |  11 |
|  3 | 李四 |  16 |
|  4 | tom% |  16 |
+----+------+-----+
4 rows in set (0.00 sec)

mysql> SELECT * FROM cyc WHERE name LIKE '张%';
+----+------+-----+
| id | name | age |
+----+------+-----+
|  1 | 张三 |  20 |
|  2 | 张刘 |  11 |
+----+------+-----+
2 rows in set (0.00 sec)

# 匹配含'%'的记录，中间的'%'前面添加字符表示转义，旁边2个为通配符。
mysql> SELECT * FROM cyc WHERE name LIKE '%1%%' ESCAPE '1';
+----+------+-----+
| id | name | age |
+----+------+-----+
|  4 | tom% |  16 |
+----+------+-----+
1 row in set (0.00 sec)

%（百分号）：匹配任意多个字符；
-（下划线）：匹配任意一个字符。
数值运算符与函数

名称	描述
CEIL()	进一取整
DIV	整数除法（结果没有小数）
FLOOR()	舍一取整
MOD	取余数（取模）
POWER()	幂运算
ROUND()	四舍五入
TRUNCATE()	数字截取，以小数点为参照点

mysql> SELECT TRUNCATE(3.55555,1);
+---------------------+
| TRUNCATE(3.55555,1) |
+---------------------+
|                 3.5 |
+---------------------+
1 row in set (0.00 sec)

mysql> SELECT TRUNCATE(3.55555,0);
+---------------------+
| TRUNCATE(3.55555,0) |
+---------------------+
|                   3 |
+---------------------+
1 row in set (0.00 sec)

mysql> SELECT TRUNCATE(13.55555,-1);
+-----------------------+
| TRUNCATE(13.55555,-1) |
+-----------------------+
|                    10 |
+-----------------------+
1 row in set (0.00 sec)

比较运算符与函数

名称	描述
[NOT] BETWEEN...AND...	[不]在范围之内（闭合区间）
[NOT] IN()	[不]在列出值范围内
IS [NOT] NULL	[不]为空

mysql> SELECT 1 BETWEEN 1 AND 23;
+--------------------+
| 1 BETWEEN 1 AND 23 |
+--------------------+
|                  1 |
+--------------------+
1 row in set (0.00 sec)

mysql> SELECT 15 IN (2, 3, 4, 54, 100);
+--------------------------+
| 15 IN (2, 3, 4, 54, 100) |
+--------------------------+
|                        0 |
+--------------------------+
1 row in set (0.00 sec)

mysql> SELECT NULL IS NULL;
+--------------+
| NULL IS NULL |
+--------------+
|            1 |
+--------------+
1 row in set (0.00 sec)

mysql> SELECT * FROM cyc WHERE name IS NOT NULL;
+----+------+-----+
| id | name | age |
+----+------+-----+
|  1 | 张三 |  20 |
|  2 | 张刘 |  11 |
|  3 | 李四 |  16 |
|  4 | tom% |  16 |
+----+------+-----+
4 rows in set (0.00 sec)

日期时间函数

名称	描述
NOW()	当前日期和时间
CURDATE()	当前日期
CURTIME()	当前时间
DATE_ADD()	日期变化
DATEDIFF()	日期差值
DATE_FORMAT()	日期格式化

# 返回当前日期和时间
mysql> SELECT NOW();
+---------------------+
| NOW()               |
+---------------------+
| 2019-08-24 10:27:34 |
+---------------------+
1 row in set (0.00 sec)

# 返回当前日期
mysql> SELECT CURDATE();
+------------+
| CURDATE()  |
+------------+
| 2019-08-24 |
+------------+
1 row in set (0.00 sec)

# 返回当前时间
mysql> SELECT CURTIME();
+-----------+
| CURTIME() |
+-----------+
| 10:27:59  |
+-----------+
1 row in set (0.00 sec)

# 使用关键字 INTERVAL 来计算日期，注意：WEEK不为复数形式，不加S
mysql> SELECT DATE_ADD('2019-08-24', INTERVAL 3 WEEK);
+-----------------------------------------+
| DATE_ADD('2019-08-24', INTERVAL 3 WEEK) |
+-----------------------------------------+
| 2019-09-14                              |
+-----------------------------------------+
1 row in set (0.00 sec)

# 日期差值计算
mysql> SELECT DATEDIFF('2019-08-24', '2019-10-23');
+--------------------------------------+
| DATEDIFF('2019-08-24', '2019-10-23') |
+--------------------------------------+
|                                  -60 |
+--------------------------------------+
1 row in set (0.00 sec)

# 日期格式化
mysql> SELECT DATE_FORMAT('2018-3-8','%m/%d/%Y');
+------------------------------------+
| DATE_FORMAT('2018-3-8','%m/%d/%Y') |
+------------------------------------+
| 03/08/2018                         |
+------------------------------------+
1 row in set (0.00 sec)

信息函数

名称	描述
CONNECTION_ID()	连接ID或者thread线程ID
DATABASE()	当前数据库
LAST_INSERT_ID()	最后插入记录的ID号
USER()	当前用户
VERSION()	版本信息

# 查询连接ID
mysql> SELECT CONNECTION_ID();
+-----------------+
| CONNECTION_ID() |
+-----------------+
|               3 |
+-----------------+
1 row in set (0.00 sec)

# 当前连接的数据库
mysql> SELECT DATABASE();
+------------+
| DATABASE() |
+------------+
| test       |
+------------+
1 row in set (0.00 sec)

mysql> INSERT cyc(name, age) VALUES('nick', 23);
Query OK, 1 row affected (0.00 sec)

# 返回最后插入一条记录的ID，注意：有自增id才有结果，若同时插入多条记录，则返回第一条记录的id值
mysql> SELECT LAST_INSERT_ID();
+------------------+
| LAST_INSERT_ID() |
+------------------+
|                5 |
+------------------+
1 row in set (0.00 sec)

mysql> SELECT * FROM cyc;
+----+------+-----+
| id | name | age |
+----+------+-----+
|  1 | 张三 |  20 |
|  2 | 张刘 |  11 |
|  3 | 李四 |  16 |
|  4 | tom% |  16 |
|  5 | nick |  23 |
+----+------+-----+
5 rows in set (0.00 sec)

# 查看当前用户
mysql> SELECT USER();
+----------------+
| USER()         |
+----------------+
| root@localhost |
+----------------+
1 row in set (0.00 sec)

# 查看当前版本号
mysql> SELECT VERSION();
+-----------+
| VERSION() |
+-----------+
| 5.7.26    |
+-----------+
1 row in set (0.00 sec)

聚合函数，只有一个返回值

名称	描述
AVG()	平均值
COUNT()	计数
MAX()	最大值
MIN()	最小值
SUM()	求和

加密函数

名称描述
MD5()	信息摘要（加密）算法（主要用在web项目）
PASSWORD()	密码计算法（主要用在mysql客户端）

mysql> SELECT MD5('root');
+----------------------------------+
| MD5('root')                      |
+----------------------------------+
| 63a9f0ea7bb98050796b649e85481845 |
+----------------------------------+
1 row in set (0.00 sec)

mysql> SELECT PASSWORD('root');
+-------------------------------------------+
| PASSWORD('root')                          |
+-------------------------------------------+
| *81F5E21E35407D884A6CD4A731AEBFB6AF209E1B |
+-------------------------------------------+
1 row in set, 1 warning (0.00 sec)

# 提示PASSWORD()这个函数将会被丢弃
mysql> SHOW WARNINGS;
+---------+------+-------------------------------------------------------------------+
| Level   | Code | Message                                                           |
+---------+------+-------------------------------------------------------------------+
| Warning | 1681 | 'PASSWORD' is deprecated and will be removed in a future release. |
+---------+------+-------------------------------------------------------------------+
1 row in set (0.00 sec)

mysql> SET PASSWORD=PASSWORD('root');
Query OK, 0 rows affected, 1 warning (0.00 sec)

# 查看警告
mysql> SHOW WARNINGS;
+---------+------+-------------------------------------------------------------------------------------------------------------------------------------------------------------------+
| Level   | Code | Message                                                                                                                                                           |
+---------+------+-------------------------------------------------------------------------------------------------------------------------------------------------------------------+
| Warning | 1287 | 'SET PASSWORD = PASSWORD('')' is deprecated and will be removed in a future release. Please use SET PASSWORD = '' instead |
+---------+------+-------------------------------------------------------------------------------------------------------------------------------------------------------------------+
1 row in set (0.00 sec)

# 更改客户端登陆密码
mysql> SET PASSWORD='admin';
Query OK, 0 rows affected (0.00 sec)
# 退出客户端
mysql> exit;
Bye
# 重新登陆mysql客户端，回车即可登陆成功
C:\Users\17283>mysql -uroot -padmin

创建自定义函数（简称UDF）

CREATE FUNCTION function_name 
RETURNS {STRING | INTEGER | REAL | DECIMAL | VARCHAR(cap)} routine_body;

函数体：
1、由合法的sql语句构成；
2、可以是简单的SELECT或INSERT语句；
3、若为复合结构（多条sql语句）则使用BEGIN...END语句；复合结构可以包含声明，循环，控制结构。

mysql> SELECT DATE_FORMAT(NOW(), '%Y年%m月%d日 %H点：%i分：%s秒');
+-----------------------------------------------------+
| DATE_FORMAT(NOW(), '%Y年%m月%d日 %H点：%i分：%s秒') |
+-----------------------------------------------------+
| 2019年08月24日 11点：16分：41秒                     |
+-----------------------------------------------------+
1 row in set (0.00 sec)

# 创建自定义函数，2个必要条件：（零个或多个）参数和（只能有一个）返回值
mysql> CREATE FUNCTION f1() RETURNS VARCHAR(30) RETURN DATE_FORMAT(NOW(), '%Y年%m月%d日 %H点：%i分：%s秒');
Query OK, 0 rows affected (0.00 sec)

# 调用自定义函数
mysql> SELECT f1();
+---------------------------------+
| f1()                            |
+---------------------------------+
| 2019年08月24日 11点：27分：32秒 |
+---------------------------------+
1 row in set (0.00 sec)

# 创建一个计算平均值的函数，返回结果保留2位小数
mysql> CREATE FUNCTION f2(num1 SMALLINT UNSIGNED,num2 SMALLINT UNSIGNED) RETURNS FLOAT(10, 2) UNSIGNED RETURN (num1 + num2) / 2;
Query OK, 0 rows affected (0.00 sec)

mysql> SELECT f2(10.1, 2);
+-------------+
| f2(10.1, 2) |
+-------------+
|        6.00 |
+-------------+
1 row in set (0.00 sec)

# 创建带参数的复合函数，由于mysql默认结束符号是分号';'。
# 因此创建复合函数时会出错，暂时修改默认结束符为'//'，创建完之后再修改为分号！
mysql> CREATE FUNCTION adduser(username VARCHAR(20), user_age SMALLINT UNSIGNED) RETURNS INT UNSIGNED
    -> BEGIN
    -> INSERT cyc(name, age) VALUES(username, user_age);
ERROR 1064 (42000): You have an error in your SQL syntax; check the manual that corresponds to your MySQL server version for the right syntax to use near '' at line 3
mysql> DELIMITER //
mysql> CREATE FUNCTION adduser(username VARCHAR(20), user_age SMALLINT UNSIGNED) RETURNS INT UNSIGNED
    -> BEGIN
    -> INSERT cyc(name, age) VALUES(username, user_age);
    -> RETURN LAST_INSERT_ID();
    -> END
    -> //
Query OK, 0 rows affected (0.00 sec)

mysql> SELECT adduser('王五',56);
    -> //
+--------------------+
| adduser('王五',56) |
+--------------------+
|                  6 |
+--------------------+
1 row in set (0.00 sec)

# 修改结束符号为分号
mysql> DELIMITER ;
mysql> SELECT adduser('赵六',26);
+--------------------+
| adduser('赵六',26) |
+--------------------+
|                  7 |
+--------------------+
1 row in set (0.00 sec)

mysql> SELECT * FROM cyc;
+----+------+-----+
| id | name | age |
+----+------+-----+
|  1 | 张三 |  20 |
|  2 | 张刘 |  11 |
|  3 | 李四 |  16 |
|  4 | tom% |  16 |
|  5 | nick |  23 |
|  6 | 王五 |  56 |
|  7 | 赵六 |  26 |
+----+------+-----+
7 rows in set (0.00 sec)

mysql学习笔记（三）

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