Coursera课程《Using Databases with Python》 密歇根大学
Week3 Data Models and Relational SQL
15.4 Designing a Data Model
主要介绍了数据模型的重要性，以及数据模型构建的一些思考过程。
15.5 Representing a Data Model in Tables
概念模型
主键(Primary key)，指的是一个列或多列的组合，其值能唯一地标识表中的每一行，通过它可强制表的实体完整性。主键主要是用于其他表的外键关联，以及本记录的修改与删除。
外键(Foreign key)，作用是保持数据一致性，完整性，主要目的是控制存储在外键表中的数据。 使两张表形成关联，外键只能引用外表中的列的值。
如果我们要构建上面概念模型所表示的数据库，那么我们用到的一些SQL语句有：
CREATE TABLE Genre(
id INTEGER NOT NULL PRIMARY KEY AUTOINCREMENT UNIQUE,
name TEXT
)
CREATE TABLE Album(
id INTEGER NOT NULL PRIMARY KEY AUTOINCREMENT UNIQUE,
artist_id INTEGER
title TEXT
)
CREATE TABLE Track(
id INTEGER NOT NULL PRIMARY KEY
AUTOINCREMENT UNIQUE,
title TEXT,
album_id INTEGER,
genre_id INTEGER,
len INTEGER,
rating INTEGER,
count INTEGER
)
15.6 Inserting Relational Data
插入数据
insert into Artist(name) values ('Led Zepplin')
insert into Artist(name) values ('AC/DC')
像上面这样就往Artist表中加入了两行数据。
而对于Album表来说，它连接了Artist表，有两列数据要插入，那么这样。
insert into Album(title,artist_id) values ('Who Made Who',2)
insert into Album(title,artist_id) values ('IV',1)
所以这样之后，我们就建立起了数据之间的关系。
15.7 Reconstructing Data with JOIN
JOIN操作像是在几个表之间的SELECT操作。
而我们告诉JOIN怎么使用这些key则需要用到ON语句。有点像WHERE语句。
select Album.title, Artist.name from Album join Artist on Album.artist_id = Artist.id
如果把事情变复杂一些……
Work Example: Tracks.py
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 = 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)
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()
使用python脚本建立数据库的过程，注意其中的关键字IGNORE，它的作用是如果当期数据存在，那就不插入，否则插入。在这个地方十分有用，因为索引不能随意变化。
作业代码
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;
DROP TABLE IF EXISTS Genre;
CREATE TABLE Artist (
id INTEGER NOT NULL PRIMARY KEY AUTOINCREMENT UNIQUE,
name TEXT UNIQUE
);
CREATE TABLE Genre (
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,
genre_id INTEGER,
len INTEGER, rating INTEGER, count INTEGER
);
''')
fname = 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)
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')
genre = lookup(entry,'Genre')
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 or genre is None:
continue
print(name, artist, album, genre, 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 IGNORE INTO Genre (name)
VALUES ( ? )''', ( genre, ) )
cur.execute('SELECT id FROM Genre WHERE name = ? ', (genre, ))
genre_id = cur.fetchone()[0]
cur.execute('''INSERT OR REPLACE INTO Track
(title, album_id, genre_id, len, rating, count)
VALUES ( ?, ?, ?, ?, ? ,?)''',
( name, album_id, genre_id, length, rating, count ) )
conn.commit()