《Using Databases with Python》Week3 Data Models and Relational SQL 课堂笔记

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()