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📜  SQLite- Python

📅  最后修改于: 2021-01-04 05:14:12             🧑  作者: Mango

在本章中，您将学习如何在Python程序中使用SQLite。

安装

可以使用由Gerhard Haring编写的sqlite3模块将SQLite3与Python集成。它提供了一个符合PEP 249描述的DB-API 2.0规范的SQL接口。您不需要单独安装此模块，因为它是默认随Python 2.5.x版一起提供的。

要使用sqlite3模块，必须首先创建一个代表数据库的连接对象，然后可以选择创建一个游标对象，该对象将帮助您执行所有SQL语句。

Python sqlite3模块API

以下是重要的sqlite3模块例程，这些例程可以满足您从Python程序使用SQLite数据库的要求。如果您正在寻找更复杂的应用程序，则可以查看Python sqlite3模块的官方文档。

Sr.No. API & Description
1

sqlite3.connect(database [,timeout ,other optional arguments])

This API opens a connection to the SQLite database file. You can use “:memory:” to open a database connection to a database that resides in RAM instead of on disk. If database is opened successfully, it returns a connection object.

When a database is accessed by multiple connections, and one of the processes modifies the database, the SQLite database is locked until that transaction is committed. The timeout parameter specifies how long the connection should wait for the lock to go away until raising an exception. The default for the timeout parameter is 5.0 (five seconds).

If the given database name does not exist then this call will create the database. You can specify filename with the required path as well if you want to create a database anywhere else except in the current directory.
2

connection.cursor([cursorClass])

This routine creates a cursor which will be used throughout of your database programming with Python. This method accepts a single optional parameter cursorClass. If supplied, this must be a custom cursor class that extends sqlite3.Cursor.
3

cursor.execute(sql [, optional parameters])

This routine executes an SQL statement. The SQL statement may be parameterized (i. e. placeholders instead of SQL literals). The sqlite3 module supports two kinds of placeholders: question marks and named placeholders (named style).

For example − cursor.execute(“insert into people values (?, ?)”, (who, age))
4

connection.execute(sql [, optional parameters])

This routine is a shortcut of the above execute method provided by the cursor object and it creates an intermediate cursor object by calling the cursor method, then calls the cursor’s execute method with the parameters given.
5

cursor.executemany(sql, seq_of_parameters)

This routine executes an SQL command against all parameter sequences or mappings found in the sequence sql.
6

connection.executemany(sql[, parameters])

This routine is a shortcut that creates an intermediate cursor object by calling the cursor method, then calls the cursor.s executemany method with the parameters given.
7

cursor.executescript(sql_script)

This routine executes multiple SQL statements at once provided in the form of script. It issues a COMMIT statement first, then executes the SQL script it gets as a parameter. All the SQL statements should be separated by a semi colon (;).
8

connection.executescript(sql_script)

This routine is a shortcut that creates an intermediate cursor object by calling the cursor method, then calls the cursor’s executescript method with the parameters given.
9

connection.total_changes()

This routine returns the total number of database rows that have been modified, inserted, or deleted since the database connection was opened.
10

connection.commit()

This method commits the current transaction. If you don’t call this method, anything you did since the last call to commit() is not visible from other database connections.
11

connection.rollback()

This method rolls back any changes to the database since the last call to commit().
12

connection.close()

This method closes the database connection. Note that this does not automatically call commit(). If you just close your database connection without calling commit() first, your changes will be lost!
13

cursor.fetchone()

This method fetches the next row of a query result set, returning a single sequence, or None when no more data is available.
14

cursor.fetchmany([size = cursor.arraysize])

This routine fetches the next set of rows of a query result, returning a list. An empty list is returned when no more rows are available. The method tries to fetch as many rows as indicated by the size parameter.
15

cursor.fetchall()

This routine fetches all (remaining) rows of a query result, returning a list. An empty list is returned when no rows are available.

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。

#!/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：以在RAM中创建数据库。现在，让我们运行上面的程序在当前目录中创建数据库test.db。您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。

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

建立表格

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()

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

Opened database successfully
Table created successfully

插入操作

以下Python程序显示了如何在上例中创建的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表中创建给定记录，并将显示以下两行-

Opened database successfully
Records created successfully

选择操作

以下Python程序演示了如何从上面的示例中创建的COMPANY表中获取并显示记录。

#!/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，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

UPDATE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，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

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表中获取并显示其余记录。

#!/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，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