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