记述python中关于数据库的基本操作。

一、数据库编程接口

1.连接对象

数据库连接对象（Connection Object）提供获取数据库游标对象、回滚事物的方法，以及连接、关闭数据库连接。

1.1 获取连接对象

使用connect()方法获得连接对象，不同的数据库有不同的参数。

connect()函数常用参数及说明（具体参数以数据库模块为准）

参数	说明
dsn	数据源名称，给出该参数表示数据库依赖
user	用户名
password	用户密码
host	主机名
database	数据库名称

1.2 连接对象方法

连接对象方法

方法	说明
close()	关闭数据库
commit()	提交事务 主要用于处理数据量大，复杂度搞的数据，如一些列的操作，使用事务可以保证操作全部执行，要么全部不执行。
rollback()	回滚事务
cursor()	获取游标对象，操作数据库，如执行DML操作，调用存储过程等

2. 游标对象

游标对象（Cursor Object）代表数据库中的游标，用于进行数据库操作，执行sql语句，调用存储过程，增删改查等。

• 通过连接对象的cursor()方法获取游标对象，需要先连接
• description：数据库列类型和值的描述信息
• rowcount：返回结果的行数统计信息，如SELECT、UPDATE、CALLPROC等

游标对象方法

方法	说明
callproc(procname[,parameter])	调用存储过程，需要数据库支持
close()	关闭当前游标
execute(operation[,parameter])	执行数据库操作，sql语句或者数据库命令
executemany(operation,seq_of_params)	用于批量操作，如批量更新
fetchone()	获取查询结果集中的吓一条记录
fetchmany(size)	获取指定数量的记录
fetchall()	获取结果集中的所有记录
nextset()	跳到下一个可用的结果集
arraysize	指定使用fetchmany获取的行数，默认为1
setinputsize(sizes)	设置在调用execute()方法时分配的内存区域大小
setoutputsize(sizes)	设置列缓冲区大小，对大数据列（如long和blobs）尤其有用

二、使用SQLite数据库

SQLite是一个数据库文件可以直接操作，体积小，可以直接嵌入到软件中。Python中内置了SQLite3，可以使用import3直接导入SQLite3模块。

 1.创建SQLite数据库文件

使用import直接导入sqlite3模块然后进行数据库操作。

connect()方法参数直接写sqlite文件名即可，若文件不存在时会自动创建数据库文件。

示例：

# _*_ coding:utf-8 _*_


import sqlite3      #导入SQLie3数据库模块


# 创建连接对象
sqlite3conn = sqlite3.connect("demo.db")

# 获取光标对象
cursor = sqlite3conn.cursor()

# 执行sql语句
sql = r"create table if not exists user (id int primary key,name varchar,age int)"

cursor.execute(sql)

# 关闭光标
cursor.close()

# 关闭连接对象
sqlite3conn.close()

结果：

 在统一目录下创建了"demo.db"数据库文件，然后添加了表user

2.操作SQLite

2.1 新增数据

insert into 表名(字段1,字段2,...,字段n) values (字段值1,字段值2,...,字段值n)

使用insert插入数据。

示例：

# _*_ coding:utf-8 _*_


import sqlite3      #导入SQLie3数据库模块


# 创建连接对象
sqlite3conn = sqlite3.connect("demo.db")

# 获取光标对象
cursor = sqlite3conn.cursor()

# 执行sql语句
sql1 = r"insert into user (id, name, age) values (1,'小明',18)"
sql2 = r"insert into user (id, name, age) values (2,'小李',20)"

cursor.execute(sql1)
cursor.execute(sql2)

# 关闭光标
cursor.close()

# 提交事务,将数据变更保存到数据库中。
sqlite3conn.commit()

# 关闭连接对象
sqlite3conn.close()

结果：

成功在标准插入了2条数据。

2.2 查看数据

select * from table where ...

• fetchone()：获取查询结果集的下一条记录
• fetchmany(size)：获取指定数量的记录
• fetchall()：获取所有就
• 结果集取出的数据会自动从结果集中删除。
• where可以使用“?”当占位符，然后后面添加元组做判定条件。

示例：

# _*_ coding:utf-8 _*_


import sqlite3      #导入SQLie3数据库模块


# 创建连接对象
sqlite3conn = sqlite3.connect("demo.db")

# 获取光标对象
cursor = sqlite3conn.cursor()

# 执行sql语句
sql = r"select * from user"

cursor.execute(sql)

print("fetchone():{}".format(cursor.fetchone()))
print("fetchmany():{}".format(cursor.fetchmany(2)))
print("fetchall():{}".format(cursor.fetchall()))

# 使用?当占位符，execute语句中用元组替换符号
sql = r"select * from user where id>?"

cursor.execute(sql,(2,))
print("fetchall():{}".format(cursor.fetchall()))

# 关闭光标
cursor.close()

# 提交事务,将数据变更保存到数据库中。
sqlite3conn.commit()

# 关闭连接对象
sqlite3conn.close()

结果：

========================== RESTART: D:\Desktop\Demo.py =========================
fetchone():(1, '小明', 18)
fetchmany():[(2, '小李', 20), (3, '老王', 30)]
fetchall():[(4, '老赵', 40), (5, '小天', 10), (6, '老钱', 70)]
fetchall():[(3, '老王', 30), (4, '老赵', 40), (5, '小天', 10), (6, '老钱', 70)]
>>> 

2.3 修改数据

update table set 字段名=字段值 where 查询条件

示例：

# _*_ coding:utf-8 _*_


import sqlite3      #导入SQLie3数据库模块


# 创建连接对象
sqlite3conn = sqlite3.connect("demo.db")

# 获取光标对象
cursor = sqlite3conn.cursor()

# 执行sql语句
sql = r"update user set name = ? where id = ?"
cursor.execute(sql,("new name",1))

# 关闭光标
cursor.close()

# 提交事务,将数据变更保存到数据库中。
sqlite3conn.commit()

# 关闭连接对象
sqlite3conn.close()

结果：

 2.4 删除数据

delete from table where 查询条件

示例：

# _*_ coding:utf-8 _*_


import sqlite3      #导入SQLie3数据库模块


# 创建连接对象
sqlite3conn = sqlite3.connect("demo.db")

# 获取光标对象
cursor = sqlite3conn.cursor()

# 执行sql语句
sql = r"delete from user where id = ?"
cursor.execute(sql,(1,))
cursor.execute(sql,(4,))

# 关闭光标
cursor.close()

# 提交事务,将数据变更保存到数据库中。
sqlite3conn.commit()

# 关闭连接对象
sqlite3conn.close()

结果：

三、使用MySQL

1.下载安装MySql模块

参考：MySQL 8.0 安装、卸载教程（图文+说明）_鱼听禅的博客-CSDN博客

2.安装PyMySQL

PyMySQL在python的基本库中，可直接在cmd中使用pip指令安装。

pip install PyMySQL

C:\Users\LJM>pip install PyMySQL
Collecting PyMySQL
  Downloading PyMySQL-1.0.2-py3-none-any.whl (43 kB)
     |████████████████████████████████| 43 kB 1.1 MB/s
Installing collected packages: PyMySQL
Successfully installed PyMySQL-1.0.2
WARNING: You are using pip version 20.1.1; however, version 22.3.1 is available.
You should consider upgrading via the 'c:\users\ljm\appdata\local\programs\python\python37\python.exe -m pip install --upgrade pip' command.

C:\Users\LJM>

注意：

pip指令如果无效时需要在环境变量中添加pip.exe的目录路径。

3.连接数据库

PyMySQL的connect()信息：

    Connect = class Connection(builtins.object)
     |  Connect(*, user=None, password='', host=None, database=None, unix_socket=None, port=0, charset='', sql_mode=None, read_default_file=None, conv=None, use_unicode=True, client_flag=0, cursorclass=<class 'pymysql.cursors.Cursor'>, init_command=None, connect_timeout=10, read_default_group=None, autocommit=False, local_infile=False, max_allowed_packet=16777216, defer_connect=False, auth_plugin_map=None, read_timeout=None, write_timeout=None, bind_address=None, binary_prefix=False, program_name=None, server_public_key=None, ssl=None, ssl_ca=None, ssl_cert=None, ssl_disabled=None, ssl_key=None, ssl_verify_cert=None, ssl_verify_identity=None, compress=None, named_pipe=None, passwd=None, db=None)
     |  
     |  Representation of a socket with a mysql server.
     |  
     |  The proper way to get an instance of this class is to call
     |  connect().
     |  
     |  Establish a connection to the MySQL database. Accepts several
     |  arguments:
     |  
     |  :param host: Host where the database server is located
     |  :param user: Username to log in as
     |  :param password: Password to use.
     |  :param database: Database to use, None to not use a particular one.
     |  :param port: MySQL port to use, default is usually OK. (default: 3306)
     |  :param bind_address: When the client has multiple network interfaces, specify
     |      the interface from which to connect to the host. Argument can be
     |      a hostname or an IP address.
     |  :param unix_socket: Optionally, you can use a unix socket rather than TCP/IP.
     |  :param read_timeout: The timeout for reading from the connection in seconds (default: None - no timeout)
     |  :param write_timeout: The timeout for writing to the connection in seconds (default: None - no timeout)
     |  :param charset: Charset you want to use.
     |  :param sql_mode: Default SQL_MODE to use.
     |  :param read_default_file:
     |      Specifies  my.cnf file to read these parameters from under the [client] section.
     |  :param conv:
     |      Conversion dictionary to use instead of the default one.
     |      This is used to provide custom marshalling and unmarshalling of types.
     |      See converters.
     |  :param use_unicode:
     |      Whether or not to default to unicode strings.
     |      This option defaults to true.
     |  :param client_flag: Custom flags to send to MySQL. Find potential values in constants.CLIENT.
     |  :param cursorclass: Custom cursor class to use.
     |  :param init_command: Initial SQL statement to run when connection is established.
     |  :param connect_timeout: Timeout before throwing an exception when connecting.
     |      (default: 10, min: 1, max: 31536000)
     |  :param ssl:
     |      A dict of arguments similar to mysql_ssl_set()'s parameters.
     |  :param ssl_ca: Path to the file that contains a PEM-formatted CA certificate
     |  :param ssl_cert: Path to the file that contains a PEM-formatted client certificate
     |  :param ssl_disabled: A boolean value that disables usage of TLS
     |  :param ssl_key: Path to the file that contains a PEM-formatted private key for the client certificate
     |  :param ssl_verify_cert: Set to true to check the validity of server certificates
     |  :param ssl_verify_identity: Set to true to check the server's identity
     |  :param read_default_group: Group to read from in the configuration file.
     |  :param autocommit: Autocommit mode. None means use server default. (default: False)
     |  :param local_infile: Boolean to enable the use of LOAD DATA LOCAL command. (default: False)
     |  :param max_allowed_packet: Max size of packet sent to server in bytes. (default: 16MB)
     |      Only used to limit size of "LOAD LOCAL INFILE" data packet smaller than default (16KB).
     |  :param defer_connect: Don't explicitly connect on construction - wait for connect call.
     |      (default: False)
     |  :param auth_plugin_map: A dict of plugin names to a class that processes that plugin.
     |      The class will take the Connection object as the argument to the constructor.
     |      The class needs an authenticate method taking an authentication packet as
     |      an argument.  For the dialog plugin, a prompt(echo, prompt) method can be used
     |      (if no authenticate method) for returning a string from the user. (experimental)
     |  :param server_public_key: SHA256 authentication plugin public key value. (default: None)
     |  :param binary_prefix: Add _binary prefix on bytes and bytearray. (default: False)
     |  :param compress: Not supported
     |  :param named_pipe: Not supported
     |  :param db: **DEPRECATED** Alias for database.
     |  :param passwd: **DEPRECATED** Alias for password.
     |  
     |  See `Connection <https://www.python.org/dev/peps/pep-0249/#connection-objects>`_ in the
     |  specification.

示例：

# _*_ coding:utf-8 _*_


import pymysql      #导入数据库模块


# 创建连接对象
mysqlconn = pymysql.connect(host = "127.0.0.1",port = 3306 ,user = "root",passwd = "root", database = "demo")

# 获取光标对象
cursor = mysqlconn.cursor()

# 执行sql语句
sql = r"select version()"
cursor.execute(sql)

data = cursor.fetchone()
print("Database Version:{}".format(data))

# 关闭光标
cursor.close()

# 关闭连接对象
mysqlconn.close()
print("Closed database.")

结果：

Database Version:('8.0.18',)
Closed database.