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Python 数据库和SQL

Python编程语言具有用于数据库编程的强大功能。Python支持各种数据库，例如SQLite，MySQL，Oracle，Sybase，PostgreSQL等。Python还支持数据定义语言(DDL)，数据操作语言(DML)和数据查询语句。用于数据库接口的Python标准是Python DB-API。大多数Python数据库接口都遵守该标准。

以下是可用的Python数据库接口的列表：Python数据库接口和API。需要为访问的每个数据库下载单独的DB API模块。

在本章中，我们演示python编程语言连接SQLite数据库。这是通过使用python内置的sqlite3模块完成的。应该首先创建一个代表数据库的连接对象，然后创建一些游标对象以执行SQL语句。

连接到数据库

以下Python代码显示了如何连接到现有数据库。如果数据库不存在，则将创建该数据库，最后将返回一个数据库对象。

 
# Filename : example.py
# Copyright : 2020 By Codebaoku
# Author by : www.codebaoku.com
# Date : 2020-08-25
#!/usr/bin/python
 import sqlite3
 conn = sqlite3.connect('test.db')
 print "Opened database successfully";

在这里，还可以提供数据库名称作为特殊名称:memory:以在RAM中创建数据库。现在，我们运行上面的程序在当前目录中创建数据库test.db。可以根据需要更改路径，将上面的代码保存在sqlite.py文件中，并如下所示执行它。如果成功创建了数据库，则它将显示以下消息。

 
# Filename : example.py
# Copyright : 2020 By Codebaoku
# Author by : www.codebaoku.com
# Date : 2020-08-25
$chmod +x sqlite.py
 $./sqlite.py
 Open database successfully

创建表

以下Python程序将用于在先前创建的数据库中创建表。

 
# Filename : example.py
# Copyright : 2020 By Codebaoku
# Author by : www.codebaoku.com
# Date : 2020-08-25
#!/usr/bin/python
 import sqlite3
 conn = sqlite3.connect('test.db')
 print "Opened database successfully";
 conn.execute('''CREATE TABLE COMPANY
          (ID INT PRIMARY KEY not NULL,
          NAME TEXT not NULL,
          AGE INT not NULL,
          ADDRESS CHAR(50),
          SALARY REAL);''')
 print "Table created successfully";
 conn.close()

执行上述程序后，它将在test.db中创建COMPANY表，并显示以下消息：

 
# Filename : example.py
# Copyright : 2020 By Codebaoku
# Author by : www.codebaoku.com
# Date : 2020-08-25
Opened database successfully
 Table created successfully

插入操作

以下Python程序显示了如何在上面的示例中创建的COMPANY表中创建记录。

 
# Filename : example.py
# Copyright : 2020 By Codebaoku
# Author by : www.codebaoku.com
# Date : 2020-08-25
#!/usr/bin/python
 import sqlite3
 conn = sqlite3.connect('test.db')
 print "Opened database successfully";
 conn.execute("INSERT INTO COMPANY (ID,NAME,AGE,ADDRESS,SALARY) \
       VALUES (1, 'Paul', 32, 'California', 19000.00 )");
 conn.execute("INSERT INTO COMPANY (ID,NAME,AGE,ADDRESS,SALARY) \
       VALUES (2, 'Allen', 25, 'Texas', 19800.00 )");
 conn.execute("INSERT INTO COMPANY (ID,NAME,AGE,ADDRESS,SALARY) \
       VALUES (3, 'Teddy', 23, 'Norway', 22000.00 )");
 conn.execute("INSERT INTO COMPANY (ID,NAME,AGE,ADDRESS,SALARY) \
       VALUES (4, 'Mark', 25, 'Rich-Mond ', 55000.00 )");
 conn.commit()
 print "Records created successfully";
 conn.close()

执行上述程序后，它将在COMPANY表中创建给定记录，并将显示以下两行：

 
# Filename : example.py
# Copyright : 2020 By Codebaoku
# Author by : www.codebaoku.com
# Date : 2020-08-25
Opened database successfully
 Records created successfully

选择操作

以下Python程序演示了如何从上例中创建的COMPANY表中查询获取并显示记录。

 
# Filename : example.py
# Copyright : 2020 By Codebaoku
# Author by : www.codebaoku.com
# Date : 2020-08-25
#!/usr/bin/python
 import sqlite3
 conn = sqlite3.connect('test.db')
 print "Opened database successfully";
 cursor = conn.execute("SELECT id, name, address, salary from COMPANY")
 for row in cursor:
    print "ID = ", row[0]
    print "NAME = ", row[1]
    print "ADDRESS = ", row[2]
    print "SALARY = ", row[3], "\n"
 print "Operation done successfully";
 conn.close()

执行上述程序时，将产生以下结果：

 
# Filename : example.py
# Copyright : 2020 By Codebaoku
# Author by : www.codebaoku.com
# Date : 2020-08-25
Opened database successfully
 ID = 1
 NAME = Paul
 ADDRESS = California
 SALARY = 20000.0
 ID = 2
 NAME = Allen
 ADDRESS = Texas
 SALARY = 15000.0
 ID = 3
 NAME = Teddy
 ADDRESS = Norway
 SALARY = 20000.0
 ID = 4
 NAME = Mark
 ADDRESS = Rich-Mond
 SALARY = 65000.0
 Operation done successfully

更新操作

以下Python代码显示了如何使用UPDATE语句更新任何记录，然后从COMPANY表中查询获取并显示更新的记录。

 
# Filename : example.py
# Copyright : 2020 By Codebaoku
# Author by : www.codebaoku.com
# Date : 2020-08-25
#!/usr/bin/python
 import sqlite3
 conn = sqlite3.connect('test.db')
 print "Opened database successfully";
 conn.execute("UPDATE COMPANY set SALARY = 25000.00 where ID = 1")
 conn.commit
 print "Total number of rows updated :", conn.total_changes
 cursor = conn.execute("SELECT id, name, address, salary from COMPANY")
 for row in cursor:
    print "ID = ", row[0]
    print "NAME = ", row[1]
    print "ADDRESS = ", row[2]
    print "SALARY = ", row[3], "\n"
 print "Operation done successfully";
 conn.close()

执行上述程序时，将产生以下结果：

 
# Filename : example.py
# Copyright : 2020 By Codebaoku
# Author by : www.codebaoku.com
# Date : 2020-08-25
Opened database successfully
 Total number of rows updated : 1
 ID = 1
 NAME = Paul
 ADDRESS = California
 SALARY = 25000.0
 ID = 2
 NAME = Allen
 ADDRESS = Texas
 SALARY = 15000.0
 ID = 3
 NAME = Teddy
 ADDRESS = Norway
 SALARY = 20000.0
 ID = 4
 NAME = Mark
 ADDRESS = Rich-Mond
 SALARY = 65000.0
 Operation done successfully

删除操作

以下Python代码显示了如何使用DELETE语句删除任何记录，然后从COMPANY表中查询获取并显示其余记录。

 
# Filename : example.py
# Copyright : 2020 By Codebaoku
# Author by : www.codebaoku.com
# Date : 2020-08-25
#!/usr/bin/python
 import sqlite3
 conn = sqlite3.connect('test.db')
 print "Opened database successfully";
 conn.execute("DELETE from COMPANY where ID = 2;")
 conn.commit()
 print "Total number of rows deleted :", conn.total_changes
 cursor = conn.execute("SELECT id, name, address, salary from COMPANY")
 for row in cursor:
    print "ID = ", row[0]
    print "NAME = ", row[1]
    print "ADDRESS = ", row[2]
    print "SALARY = ", row[3], "\n"
 print "Operation done successfully";
 conn.close()

执行上述程序时，将产生以下结果：

 
# Filename : example.py
# Copyright : 2020 By Codebaoku
# Author by : www.codebaoku.com
# Date : 2020-08-25
Opened database successfully
 Total number of rows deleted : 1
 ID = 1
 NAME = Paul
 ADDRESS = California
 SALARY = 20000.0
 ID = 3
 NAME = Teddy
 ADDRESS = Norway
 SALARY = 20000.0
 ID = 4
 NAME = Mark
 ADDRESS = Rich-Mond
 SALARY = 65000.0
 Operation done successfully

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