MySQL：Linux / python

linux ---MySQL

• mysql 登录账号

>mysql -u name -p
Enter password:
Welcome to the MySQL monitor.  Commands end with ; or \g.

• 显示数据库

mysql> show databases;

• 选中数据库

mysql> use database_name;

• 查看选中数据库里的文件

mysql> show tables;

• 查看某个文件的结构

mysql> desc file_name;
mysql> DESCRIBE file_name;

• 退出mysql

mysql> exit

• 关键字和函数名不区分大小写 为了直观 关键字大写 其他小写

mysql> CREATE DATABASE test_db;  创建一个名为 test_db 的数据库
mysql> CREATE DATABASE IF NOT EXISTS test_db;

mysql> CREATE DATABASE IF NOT EXISTS test_db_char  创建一个测试数据库，命名为 test_db_char
    -> DEFAULT CHARACTER SET utf8  指定其默认字符集为 utf8
    -> DEFAULT COLLATE utf8_chinese_ci;  默认校对规则为 utf8_chinese_ci（简体中文，不区分大小写）

mysql> SHOW CREATE DATABASE test_db;  查看 test_db 数据库的定义声明
mysql> ALTER DATABASE test_db  
    -> DEFAULT CHARACTER SET gb2312  指定字符集修改为 gb2312
    -> DEFAULT COLLATE gb2312_chinese_ci;  默认校对规则修改为 gb2312_unicode_ci

mysql> DROP DATABASE test_db;  删除数据库 test_db
mysql> DROP DATABASE IF EXISTS test_db;

mysql> SHOW DATABASES LIKE 'test_db';  查看与 test_db 完全匹配的数据库
mysql> SHOW DATABASES LIKE '%test%';  查看名字中包含 test 的数据库
mysql> SHOW DATABASES LIKE 'db%';  查看名字以 db 开头的数据库
mysql> SHOW DATABASES LIKE '%db';  查看名字以 db 结尾的数据库

mysql> ALTER TABLE old_name RENAME TO new_name;  修改表名

mysql> SELECT name,height FROM tb_students_info ORDER BY height DESC,name ASC;
        选择                                     排序            降序      升序

mysql> SELECT name,age,height FROM tb_students_info 
    -> WHERE age>21 AND height>=175;

mysql> SELECT name FROM tb_students_info
    -> WHERE name LIKE '____y';  查找所有以字母“y”结尾，且“y”前面只有 4 个字母的学生姓名

mysql> SELECT name,age FROM tb_students_info 
    -> WHERE age BETWEEN 20 AND 23;  查询年龄在 20 到 23 之间的学生姓名和年龄

mysql> SELECT `sex`, GROUP_CONCAT(name) FROM tb_students_info   GROUP_CONCAT() 函数会把每个分组的字段值都显示出来
    -> GROUP BY sex;  分组

mysql> SELECT sex,COUNT(sex) FROM tb_students_info   COUNT() 函数计算每一组的记录数
    -> GROUP BY sex;

mysql> SELECT sex,GROUP_CONCAT(name) FROM tb_students_info 
    ->GROUP BY sex WITH ROLLUP;   WITH ROLLUP 显示记录的总和

select 
    name as 姓名,   # as  重命名列名
    case sex when 1 then '男' else '女' end as 性别,  #  case ... when .. then .. else .. end   1是男否则是女结束
    birth as 生日
from student 
where birth between '1980-1-1' and '1989-12-31';

截取字符串 https://www.jb51.net/article/173358.htm

select substring_index(input,split,index)
该字段 = ‘1,2,3’  获取中间的值
mysql> select substring_index(substring_index(该字段, ',', 2) , ',', -1) from 表名;

mysql> select left(string, 3);   # 左边数截前3个
mysql> select right(string, 3);    # 右边数截前3个
mysql> select substring(string, pos, length);   # 从pos位置截length长度

年龄计算
   floor(datediff(curdate(), stu_birth) / 365) 
floor ：向下取整
datediff：计算时间差
curdate()：获取当前时间

连接查询
select cou_name, cou_credit, tea_name from tb_course t1 
inner join tb_teacher t2 on t1.tea_id=t2.tea_id;

统计查到的结果数目
select count(*) from example_table where 条件
显示查到的结果前5行
select * from example_table where 条件 limit 5

---

python pymysql 调用数据库数据

import pymysql

db = pymysql.connect(host='localhost', user='', passwd='', database='')
cursor = db.cursor()
sql_string = 'select * from database where pos between 100 and 200'
cursor.execute(sql_string)

一行一行读取返回值  
while True:
    row = cursor.fetchone()  元组
    if not row:
        break
    
一次性读取返回值
results = cursor.fetchall()   # [(), ()]

本地就写 host='localhost'，非本地就 host='地址'