Python学习笔记（六）——Python数据库开发

1 一个简单的例子

注意：例中所用的“mbox-short.txt”文件可在这里查看

import sqlite3 # 导入SQLite库

conn = sqlite3.connect('emaildb.sqlite') # 将数据库存储在文件emaildb.sqlite中
cur = conn.cursor()

cur.execute('''
DROP TABLE IF EXISTS Counts''')

cur.execute('''
CREATE TABLE Counts (email TEXT, count INTEGER)''')

fname = raw_input('Enter file name: ')
if ( len(fname) < 1 ) : fname = 'mbox-short.txt'
fh = open(fname)
for line in fh:
    if not line.startswith('From: ') : continue
    pieces = line.split()
    email = pieces[1]
    print email
    cur.execute('SELECT count FROM Counts WHERE email = ? ', (email, )) # 元组中的第一项将会替代问号
    row = cur.fetchone()
    if row is None:
        cur.execute('''INSERT INTO Counts (email, count) 
                VALUES ( ?, 1 )''', ( email, ) )
    else : 
        cur.execute('UPDATE Counts SET count=count+1 WHERE email = ?', 
            (email, ))
    # This statement commits outstanding changes to disk each 
    # time through the loop - the program can be made faster 
    # by moving the commit so it runs only after the loop completes
    conn.commit() # 保存到数据库文件

# https://www.sqlite.org/lang_select.html
sqlstr = 'SELECT email, count FROM Counts ORDER BY count DESC LIMIT 10' # DESC：降序排列，LIMIT 10：只取最前面的10个结果

print
print "Counts:"
for row in cur.execute(sqlstr) : # row是一个数组
    print str(row[0]), row[1] # 为了保证在字符是Unicode的情况下可以正确打印，将其转换为string

cur.close() # 关闭cursor

• **cursor.fetchone()**：把结果集的下一行保存为 list 或者None
• **commit()**：把自上次保存命令以来的所有事务保存到数据库
• 打印数据库数据的时候经常用str()

2 一对多关系和 XML

例：
注意：例中所用的“Library.xml”文件可在这里查看

import xml.etree.ElementTree as ET
import sqlite3

conn = sqlite3.connect('trackdb.sqlite')
cur = conn.cursor()

# Make some fresh tables using executescript()
cur.executescript('''
DROP TABLE IF EXISTS Artist;
DROP TABLE IF EXISTS Album;
DROP TABLE IF EXISTS Track;

CREATE TABLE Artist (
    id  INTEGER NOT NULL PRIMARY KEY AUTOINCREMENT UNIQUE,
    name    TEXT UNIQUE
);

CREATE TABLE Album (
    id  INTEGER NOT NULL PRIMARY KEY AUTOINCREMENT UNIQUE,
    artist_id  INTEGER,
    title   TEXT UNIQUE
);

CREATE TABLE Track (
    id  INTEGER NOT NULL PRIMARY KEY 
        AUTOINCREMENT UNIQUE,
    title TEXT  UNIQUE,
    album_id  INTEGER,
    len INTEGER, rating INTEGER, count INTEGER
);
''')


fname = raw_input('Enter file name: ')
if ( len(fname) < 1 ) : fname = 'Library.xml'

# <key>Track ID</key><integer>369</integer>
# <key>Name</key><string>Another One Bites The Dust</string>
# <key>Artist</key><string>Queen</string>
def lookup(d, key):
    found = False
    for child in d:
        if found : return child.text
        if child.tag == 'key' and child.text == key :
            found = True
    return None

stuff = ET.parse(fname) # 利用ElementTree.parse将文件直接解析成xml对象
all = stuff.findall('dict/dict/dict')
print 'Dict count:', len(all)
for entry in all:
    if ( lookup(entry, 'Track ID') is None ) : continue

    name = lookup(entry, 'Name')
    artist = lookup(entry, 'Artist')
    album = lookup(entry, 'Album')
    count = lookup(entry, 'Play Count')
    rating = lookup(entry, 'Rating')
    length = lookup(entry, 'Total Time')

    if name is None or artist is None or album is None : 
        continue

    print name, artist, album, count, rating, length

    cur.execute('''INSERT OR IGNORE INTO Artist (name) 
        VALUES ( ? )''', ( artist, ) )
    cur.execute('SELECT id FROM Artist WHERE name = ? ', (artist, ))
    artist_id = cur.fetchone()[0]

    cur.execute('''INSERT OR IGNORE INTO Album (title, artist_id) 
        VALUES ( ?, ? )''', ( album, artist_id ) )
    cur.execute('SELECT id FROM Album WHERE title = ? ', (album, ))
    album_id = cur.fetchone()[0]

    cur.execute('''INSERT OR REPLACE INTO Track
        (title, album_id, len, rating, count) 
        VALUES ( ?, ?, ?, ?, ? )''', 
        ( name, album_id, length, rating, count ) )

    conn.commit()

注：

• 如果需要多次实验，可用**DROP TABLE IF EXISTS TableName**删除指定表格
• 可利用**ElementTree.parse**将文件直接解析成xml对象

3 多对多关系和 JSON^1

多对多关系需要添加一张连接表，将多对多关系分解为多对一关系。例如，“课程”和“用户”是多对多关系，所以添加一张表，如下图所示：

例：
注意：例中所用的“roster_data_sample.json”文件可在这里查看

import json
import sqlite3

conn = sqlite3.connect('rosterdb.sqlite')
cur = conn.cursor()

# Do some setup
cur.executescript('''
DROP TABLE IF EXISTS User;
DROP TABLE IF EXISTS Member;
DROP TABLE IF EXISTS Course;

CREATE TABLE User (
    id     INTEGER NOT NULL PRIMARY KEY AUTOINCREMENT UNIQUE,
    name   TEXT UNIQUE
);

CREATE TABLE Course (
    id     INTEGER NOT NULL PRIMARY KEY AUTOINCREMENT UNIQUE,
    title  TEXT UNIQUE
);

CREATE TABLE Member (
    user_id     INTEGER,
    course_id   INTEGER,
    role        INTEGER,
    PRIMARY KEY (user_id, course_id)
)
''')

fname = raw_input('Enter file name: ')
if ( len(fname) < 1 ) : fname = 'roster_data_sample.json'

# [
#   [ "Charley", "si110", 1 ],
#   [ "Mea", "si110", 0 ],

str_data = open(fname).read()
json_data = json.loads(str_data)

for entry in json_data:

    name = entry[0];
    title = entry[1];
    role = entry[2];

    print name, title

    cur.execute('''INSERT OR IGNORE INTO User (name) 
        VALUES ( ? )''', ( name, ) )
    cur.execute('SELECT id FROM User WHERE name = ? ', (name, ))
    user_id = cur.fetchone()[0]

    cur.execute('''INSERT OR IGNORE INTO Course (title) 
        VALUES ( ? )''', ( title, ) )
    cur.execute('SELECT id FROM Course WHERE title = ? ', (title, ))
    course_id = cur.fetchone()[0]

    cur.execute('''INSERT OR REPLACE INTO Member
        (user_id, course_id, role) VALUES ( ?, ?, ?)''', 
        ( user_id, course_id, role ) )

    conn.commit()