Python - 数据库和SQL

Python编程语言具有强大的数据库编程功能。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。Python数据库接口的标准是Python DB-API。大多数Python数据库接口都遵循此标准。

以下是可用的Python数据库接口列表：Python数据库接口和API。您必须为需要访问的每个数据库下载单独的DB API模块。

本章我们将学习如何在Python编程语言中使用SQLite数据库。这是通过使用Python内置的sqlite3模块来实现的。您应该首先创建一个表示数据库的连接对象，然后创建一些游标对象来执行SQL语句。

连接数据库

下面的Python代码演示了如何连接到现有数据库。如果数据库不存在，则将创建它，最后返回一个数据库对象。

#!/usr/bin/python

import sqlite3

conn = sqlite3.connect('test.db')

print "Opened database successfully";

在这里，您还可以提供数据库名称作为特殊名称:memory: 来在RAM中创建数据库。现在，让我们运行上面的程序来在当前目录中创建我们的数据库test.db。您可以根据需要更改您的路径。将上面的代码保存在sqlite.py文件中，并按如下所示执行它。如果数据库成功创建，则会显示以下消息。

$chmod +x sqlite.py
$./sqlite.py
Open database successfully

创建表

下面的Python程序将用于在前面创建的数据库中创建表。

#!/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，它将在您的test.db中创建COMPANY表，并显示以下消息：

Opened database successfully
Table created successfully

插入操作

下面的Python程序演示了如何在上面示例中创建的COMPANY表中创建记录。

#!/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', 20000.00 )");

conn.execute("INSERT INTO COMPANY (ID,NAME,AGE,ADDRESS,SALARY) \
      VALUES (2, 'Allen', 25, 'Texas', 15000.00 )");

conn.execute("INSERT INTO COMPANY (ID,NAME,AGE,ADDRESS,SALARY) \
      VALUES (3, 'Teddy', 23, 'Norway', 20000.00 )");

conn.execute("INSERT INTO COMPANY (ID,NAME,AGE,ADDRESS,SALARY) \
      VALUES (4, 'Mark', 25, 'Rich-Mond ', 65000.00 )");

conn.commit()
print "Records created successfully";
conn.close()

执行上述程序后，它将在COMPANY表中创建给定的记录，并显示以下两行：

Opened database successfully
Records created successfully

选择操作

下面的Python程序演示了如何从上面示例中创建的COMPANY表中提取和显示记录。

#!/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()

执行上述程序后，将产生以下结果。

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

下面的Python代码演示了如何使用UPDATE语句更新任何记录，然后从COMPANY表中提取和显示更新后的记录。

#!/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()

执行上述程序后，将产生以下结果。

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

下面的Python代码演示了如何使用DELETE语句删除任何记录，然后从COMPANY表中提取和显示剩余的记录。

#!/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()

执行上述程序后，将产生以下结果。

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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