MySQL数据库编程01
文章目录
- MySQL编程基础
-
- 常量与变量
-
- 常量
-
- 字符串常量
- 数值常量
- 日期和时间常量
- 布尔值常量
- NULL值
- 变量
-
- 局部变量
- 全局变量
- 常用系统函数
-
- 字符串函数
-
- 计算字符串字符数和长度
- 合并字符串
- 字符串大小写转换
- 删除空格
- 取子串
- 数学函数
- 日期和时间函数
-
- 获取当前系统的日期及取日期的年月日
- 获取当前系统的日期时间
- 系统信息函数
- 条件控制函数
-
- IF
- CASE
- 数据类型转换函数
MySQL编程基础
常量与变量
常量
字符串常量
字符串常量指用单引号或双引号括起来的字符序列。在MySQL中推荐使用单引号。
mysql> SELECT * FROM products WHERE prod_id='BR01';
+----+---------+-------------------+------------+
| id | prod_id | prod_name | prod_price |
+----+---------+-------------------+------------+
| 4 | BR01 | 8 inch teddy bear | 5.99 |
+----+---------+-------------------+------------+
1 row in set (0.00 sec)
考虑下面的SQL命令为什么没有输出
mysql> SELECT * FROM products WHERE 'prod_id'='BR01';
Empty set (0.04 sec)
数值常量
数值常量可以分为整数常量和小数常量
将产品编号为BR03的产品价格改为1250（用科学计数法表示）
mysql> UPDATE products SET prod_price=1.25E+3 WHERE prod_id='BR03';
Query OK, 1 row affected (0.14 sec)
Rows matched: 1 Changed: 1 Warnings: 0
mysql> SELECT * FROM products WHERE prod_id='BR03';
+----+---------+--------------------+------------+
| id | prod_id | prod_name | prod_price |
+----+---------+--------------------+------------+
| 6 | BR03 | 18 inch teddy bear | 1250.00 |
+----+---------+--------------------+------------+
1 row in set (0.00 sec)
日期和时间常量
新增一列为产品生产日期，默认值为’2022-04-02 12:00:00’
对数值稍作修改
查询生产日期大于’2022-04-02 13:30:00’的产品
mysql> SELECT * FROM products WHERE prod_date>'2022-04-02 13:30:00';
+----+---------+-------------+------------+---------------------+
| id | prod_id | prod_name | prod_price | prod_date |
+----+---------+-------------+------------+---------------------+
| 8 | RYL01 | King doll | 9.99 | 2022-04-03 12:00:00 |
| 9 | RYL02 | Queen dool | 9.99 | 2022-04-03 12:00:00 |
| 19 | MR01 | toy dog | 5.00 | 2022-04-03 12:00:00 |
| 23 | MR02 | toy dog | 2.00 | 2022-04-03 12:00:00 |
+----+---------+-------------+------------+---------------------+
4 rows in set (0.00 sec)
布尔值常量
查询产品价格是否大于7的返回结果
mysql> SELECT prod_name,prod_price>7 FROM products;
+---------------------+--------------+
| prod_name | prod_price>7 |
+---------------------+--------------+
| Fish bean bag toy | 0 |
| Bird bean bag toy | 0 |
| Rabbit bean bag toy | 0 |
| 8 inch teddy bear | 0 |
| 12 inch teddy bear | 1 |
| 18 inch teddy bear | 1 |
| Raggedy Ann | 0 |
| King doll | 1 |
| Queen dool | 1 |
| toy dog | 0 |
| toy dog | 0 |
+---------------------+--------------+
11 rows in set (0.00 sec)
NULL值
新增一行产品价格为空的信息
mysql> SELECT * FROM products WHERE prod_id='MR03';
+----+---------+-----------+------------+---------------------+
| id | prod_id | prod_name | prod_price | prod_date |
+----+---------+-----------+------------+---------------------+
| 12 | MR03 | NULL | NULL | 2022-04-02 12:00:00 |
+----+---------+-----------+------------+---------------------+
1 row in set (0.00 sec)
给为NULL的产品价格加10会返回什么？
给为1250的产品价格加NULL会返回什么？
mysql> UPDATE products SET prod_price=prod_price+10 WHERE prod_id='MR03';
Query OK, 0 rows affected (0.04 sec)
Rows matched: 1 Changed: 0 Warnings: 0
mysql> SELECT * FROM products WHERE prod_id='MR03';
+----+---------+-----------+------------+---------------------+
| id | prod_id | prod_name | prod_price | prod_date |
+----+---------+-----------+------------+---------------------+
| 12 | MR03 | NULL | NULL | 2022-04-02 12:00:00 |
+----+---------+-----------+------------+---------------------+
1 row in set (0.00 sec)
mysql> UPDATE products SET prod_price=prod_price+NULL WHERE prod_price=1250;
Query OK, 1 row affected (0.07 sec)
Rows matched: 1 Changed: 1 Warnings: 0
mysql> SELECT * FROM products WHERE prod_id='BR03';
+----+---------+--------------------+------------+---------------------+
| id | prod_id | prod_name | prod_price | prod_date |
+----+---------+--------------------+------------+---------------------+
| 6 | BR03 | 18 inch teddy bear | NULL | 2022-04-02 12:00:00 |
+----+---------+--------------------+------------+---------------------+
1 row in set (0.00 sec)
可以看到NULL+10=NULL;1250+NULL=NULL;
变量
局部变量
查询产品编号为BR01的产品价格并赋值给变量price
mysql> SET @price=(SELECT prod_price FROM products WHERE prod_id='BR01');
Query OK, 0 rows affected (0.04 sec)
mysql> SELECT @price;
+--------+
| @price |
+--------+
| 5.99 |
+--------+
1 row in set (0.03 sec)
查询产品编号为BR02的产品名称和产品价格并分别赋值给name_v和price_v
mysql> SELECT prod_name,prod_price INTO @name_v,@price_v FROM products WHERE prod_id='BR02';
Query OK, 1 row affected (0.00 sec)
mysql> SELECT @name_v,@price_v;
+--------------------+----------+
| @name_v | @price_v |
+--------------------+----------+
| 12 inch teddy bear | 8.99 |
+--------------------+----------+
1 row in set (0.00 sec)
根据变量所给的值查询指定产品的信息
mysql> SET @p_id='BR03';
Query OK, 0 rows affected (0.00 sec)
mysql> SELECT * FROM products WHERE prod_id=@p_id;
+----+---------+--------------------+------------+---------------------+
| id | prod_id | prod_name | prod_price | prod_date |
+----+---------+--------------------+------------+---------------------+
| 6 | BR03 | 18 inch teddy bear | NULL | 2022-04-02 12:00:00 |
+----+---------+--------------------+------------+---------------------+
1 row in set (0.00 sec)
全局变量
mysql> SELECT @@back_log; /*返回MySQL主要连接请求的数量*/
+------------+
| @@back_log |
+------------+
| 151 |
+------------+
1 row in set (0.00 sec)
mysql> SELECT @@basedir; /*返回MySQL安装基准目录*/
+-----------+
| @@basedir |
+-----------+
| D:\MySQL\ |
+-----------+
1 row in set (0.00 sec)
mysql> SELECT @@license; /*返回服务器的许可类型*/
+-----------+
| @@license |
+-----------+
| GPL |
+-----------+
1 row in set (0.00 sec)
mysql> SELECT @@port; /*返回服务器侦听TCP/IP连接所用的端口*/
+--------+
| @@port |
+--------+
| 3306 |
+--------+
1 row in set (0.00 sec)
mysql> SELECT @@version; /*返回服务器版本号*/
+-----------+
| @@version |
+-----------+
| 8.0.19 |
+-----------+
1 row in set (0.00 sec)
查看所有的全局变量
mysql> SHOW GLOBAL VARIABLES;
会返回很多信息，建议模糊查找，比如
mysql> SHOW GLOBAL VARIABLES LIKE 'sto%';
+---------------------------------+-------+
| Variable_name | Value |
+---------------------------------+-------+
| stored_program_cache | 256 |
| stored_program_definition_cache | 256 |
+---------------------------------+-------+
2 rows in set, 1 warning (0.00 sec)
常用系统函数
字符串函数
计算字符串字符数和长度
CHAR_LENGTH:返回字符串所包含的字符个数
LENGTH:返回字符串的字节长度，一个汉字3个字节，一个数字或字母是一个字节
mysql> SELECT CHAR_LENGTH('mysql');
+----------------------+
| CHAR_LENGTH('mysql') |
+----------------------+
| 5 |
+----------------------+
1 row in set (0.04 sec)
mysql> SELECT LENGTH('mysql');
+-----------------+
| LENGTH('mysql') |
+-----------------+
| 5 |
+-----------------+
1 row in set (0.00 sec)
mysql> SELECT CHAR_LENGTH('汉字');
+-----------------------+
| CHAR_LENGTH('汉字') |
+-----------------------+
| 2 |
+-----------------------+
1 row in set (0.00 sec)
mysql> SELECT LENGTH('汉字');
+------------------+
| LENGTH('汉字') |
+------------------+
| 6 |
+------------------+
1 row in set (0.00 sec)
合并字符串
mysql> SELECT CONCAT('MySQL版本:',@@version) 版本信息;
+--------------------+
| 版本信息 |
+--------------------+
| MySQL版本:8.0.19 |
+--------------------+
1 row in set (0.00 sec)
字符串大小写转换
mysql> SELECT LOWER(@str1) 转换为小写,UPPER(@str1) 转换为大写;
+-----------------+-----------------+
| 转换为小写 | 转换为大写 |
+-----------------+-----------------+
| mysql | MYSQL |
+-----------------+-----------------+
删除空格
只能删除前后空格，不能删除字符串中间的空格
mysql> SET @str1=' MySQL ';
mysql> SELECT LTRIM(@str1) 删除前导空格,RTRIM(@str1) 删除尾部空格,TRIM(@str1) 删除两侧空格;
+--------------------+--------------------+--------------------+
| 删除前导空格 | 删除尾部空格 | 删除两侧空格 |
+--------------------+--------------------+--------------------+
| MySQL | MySQL | MySQL |
+--------------------+--------------------+--------------------+
取子串
SUBSTRING(str,start,length)
首字母为M的产品编号
mysql> SELECT * FROM products WHERE SUBSTRING(prod_id,1,1)='M';
+----+---------+-----------+------------+---------------------+
| id | prod_id | prod_name | prod_price | prod_date |
+----+---------+-----------+------------+---------------------+
| 10 | MR01 | toy dog | 5.00 | 2022-04-03 12:00:00 |
| 11 | MR02 | toy dog | 2.00 | 2022-04-03 12:00:00 |
| 12 | MR03 | NULL | NULL | 2022-04-02 12:00:00 |
+----+---------+-----------+------------+---------------------+
第四个字符为3的产品编号
mysql> SELECT * FROM products WHERE SUBSTRING(prod_id,4,1)='3';
+----+---------+--------------------+------------+---------------------+
| id | prod_id | prod_name | prod_price | prod_date |
+----+---------+--------------------+------------+---------------------+
| 6 | BR03 | 18 inch teddy bear | NULL | 2022-04-02 12:00:00 |
| 12 | MR03 | NULL | NULL | 2022-04-02 12:00:00 |
+----+---------+--------------------+------------+---------------------+
数学函数
mysql> SELECT ABS(-1) 绝对值,PI() AS π,SQRT(9) AS 平方根,MOD(9,4) AS 余数,ROUND(2.568,2) AS 四舍五入保留2位小数;
+-----------+----------+-----------+--------+------------------------------+
| 绝对值 | π | 平方根 | 余数 | 四舍五入保留2位小数 |
+-----------+----------+-----------+--------+------------------------------+
| 1 | 3.141593 | 3 | 1 | 2.57 |
+-----------+----------+-----------+--------+------------------------------+
日期和时间函数
获取当前系统的日期及取日期的年月日
CURDATE():返回当前系统日期
YEAR(d)、MONTH(d)、DAY(d):分别返回日期或日期时间d的年月日的值
mysql> SET @d=CURDATE();
Query OK, 0 rows affected (0.00 sec)
mysql> SELECT @d,YEAR(@d),MONTH(@d),DAY(@d);
+------------+----------+-----------+---------+
| @d | YEAR(@d) | MONTH(@d) | DAY(@d) |
+------------+----------+-----------+---------+
| 2022-04-03 | 2022 | 4 | 3 |
+------------+----------+-----------+---------+
查询产品生产日期距今几天（也可以是几年）
mysql> SELECT prod_name 产品名称,DAY(CURDATE())-DAY(prod_date) 距今几日 FROM products;
+---------------------+--------------+
| 产品名称 | 距今几日 |
+---------------------+--------------+
| Fish bean bag toy | 2 |
| Bird bean bag toy | 2 |
| Rabbit bean bag toy | 2 |
| 8 inch teddy bear | 1 |
| 12 inch teddy bear | 1 |
| 18 inch teddy bear | 1 |
| Raggedy Ann | 1 |
| King doll | 0 |
| Queen dool | 0 |
| toy dog | 0 |
| toy dog | 0 |
| NULL | 1 |
+---------------------+--------------+
获取当前系统的日期时间
mysql> SELECT CURRENT_TIMESTAMP(),LOCALTIME(),NOW(),SYSDATE();
+---------------------+---------------------+---------------------+---------------------+
| CURRENT_TIMESTAMP() | LOCALTIME() | NOW() | SYSDATE() |
+---------------------+---------------------+---------------------+---------------------+
| 2022-04-03 21:42:10 | 2022-04-03 21:42:10 | 2022-04-03 21:42:10 | 2022-04-03 21:42:10 |
+---------------------+---------------------+---------------------+---------------------+
系统信息函数
mysql> SELECT USER() AS 用户,DATABASE() AS 当前数据库;
+----------------+-----------------+
| 用户 | 当前数据库 |
+----------------+-----------------+
| root@localhost | learn |
+----------------+-----------------+
条件控制函数
IF
返回前6条记录，如果产品价格为NULL时显示0，否则显示当前字段值。
mysql> SELECT prod_id 产品编号,IF(prod_price IS NULL,0,prod_price) 产品价格 FROM products LIMIT 6;
+--------------+--------------+
| 产品编号 | 产品价格 |
+--------------+--------------+
| BNBG01 | 3.49 |
| BNBG02 | 3.49 |
| BNBG03 | 3.49 |
| BR01 | 5.99 |
| BR02 | 8.99 |
| BR03 | 0 |
+--------------+--------------+
CASE
如果产品价格小于5元，为便宜，否则为贵；
mysql> SELECT prod_id 产品编号,prod_price 产品价格,
-> CASE
-> WHEN prod_price < 5 THEN '便宜'
-> ELSE '贵'
-> END 价格类型
-> FROM products;
+--------------+--------------+--------------+
| 产品编号 | 产品价格 | 价格类型 |
+--------------+--------------+--------------+
| BNBG01 | 3.49 | 便宜 |
| BNBG02 | 3.49 | 便宜 |
| BNBG03 | 3.49 | 便宜 |
| BR01 | 5.99 | 贵 |
| BR02 | 8.99 | 贵 |
| BR03 | 9.00 | 贵 |
| RGAN01 | 4.99 | 便宜 |
| RYL01 | 9.99 | 贵 |
| RYL02 | 9.99 | 贵 |
| MR01 | 5.00 | 贵 |
| MR02 | 2.00 | 便宜 |
| MR03 | 5.00 | 贵 |
+--------------+--------------+--------------+
12 rows in set (0.00 sec)
数据类型转换函数
CAST(x AS 新类型)
CONVERT(x 新类型)
mysql> SELECT prod_name,prod_price INTO @name,@price FROM products WHERE prod_id='BR02';
Query OK, 1 row affected (0.00 sec)
mysql> SELECT CONCAT(@name,'的价格是',CAST(@price AS CHAR(7))) 信息;
+------------------------------------+
| 信息 |
+------------------------------------+
| 12 inch teddy bear的价格是8.99 |
+------------------------------------+