Python - 数据库和 SQL

简述

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，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。数据库接口的 Python 标准是 Python DB-API。大多数 Python 数据库接口都遵循这个标准。

以下是可用 Python 数据库接口的列表：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。

在本章中，我们将看到 SQLite 数据库在 Python 编程语言中的使用。它是通过使用 python 的内置 sqlite3 模块完成的。您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来执行 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:在 RAM 中创建数据库。现在，让我们运行上面的程序来创建我们的数据库test.db在当前目录中。您可以根据需要更改路径。将上述代码保存在 sqlite.py 文件中并执行如下所示。如果数据库创建成功，那么它将显示以下消息。


$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。


#!/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()

执行上述程序时，它将在您的test.db它将显示以下消息 -


Opened database successfully
Table created successfully

插入操作

下面的 Python 程序显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()

执行上述程序时，它将在 COMPANY 表中创建给定的记录，并将显示以下两行 -


Opened database successfully
Records created successfully

选择操作

下面的 Python 程序显示了如何从上面示例中创建的 COMPANY 表中获取和显示记录。


#!/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()

执行上述程序时，会产生如下结果。


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

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 表中获取并显示更新的记录。


#!/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()

执行上述程序时，会产生如下结果。


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 代码展示了如何使用 DELETE 语句删除任何记录，然后从 COMPANY 表中获取并显示剩余的记录。


#!/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()

执行上述程序时，会产生如下结果。


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