使用Python的 SQL |第 3 组（处理大数据）

推荐使用Python走SQL |使用Python和 SQLite 设置 1 和 SQL |设置 2

在之前的文章中，数据库的记录仅限于小尺寸和单元组。本文将解释如何使用 SQLite3 模块从数据库中写入和获取大数据，涵盖所有异常。
一种简单的方法是执行查询并使用 fetchall()。这已经在 SET 1 中讨论过了。

• 执行脚本（）
  这是一次执行多个 SQL 语句的便捷方法。它执行作为参数获取的 SQL 脚本。

  Syntax:sqlite3.connect.executescript(script)

  import sqlite3
    
  # Connection with the DataBase
  # 'library.db'
  connection = sqlite3.connect("library.db")
  cursor = connection.cursor()
    
  # SQL piece of code Executed
  # SQL piece of code Executed
  cursor.executescript("""
      CREATE TABLE people(
          firstname,
          lastname,
          age
      );
     
      CREATE TABLE book(
          title,
          author,
          published
      );
     
      INSERT INTO
      book(title, author, published)
      VALUES (
          'Dan Clarke''s GFG Detective Agency',
          'Sean Simpsons',
          1987
      );
      """)
    
  sql = """
  SELECT COUNT(*) FROM book;"""
    
  cursor.execute(sql)
    
  # The output in fetched and returned
  # as a List by fetchall()
  result = cursor.fetchall()
  print(result)
    
  sql = """
  SELECT * FROM book;"""
    
  cursor.execute(sql)
    
  result = cursor.fetchall()
  print(result)
    
  # Changes saved into database
  connection.commit()
    
  # Connection closed(broken) 
  # with DataBase
  connection.close()
  

  输出：

  [(1,)]
  [("Dan Clarke's GFG Detective Agency", 'Sean Simpsons', 1987)]
  

  注意：由于创建/写入数据库的权限，这段代码可能不适用于在线解释器。

• 执行许多（）
  通常情况下，必须将大量数据从数据文件插入数据库（对于更简单的情况，采用列表、数组）。多次迭代代码比每次都写入数据库要简单，每一行都写入数据库。但是在这种情况下不适合使用循环，下面的例子说明了原因。下面解释了 executemany() 的语法和使用，以及如何像循环一样使用它。

  import sqlite3
    
  # Connection with the DataBase
  # 'library.db'
  connection = sqlite3.connect("library.db")
  cursor = connection.cursor()
    
  # SQL piece of code Executed
  cursor.execute("""
        
      CREATE TABLE book(
          title,
          author,
          published);""")
     
  List = [('A', 'B', 2008), ('C', 'D', 2008),
                            ('E', 'F', 2010)]
     
  connection. executemany("""
                     
      INSERT INTO 
      book(title, author, published) 
      VALUES (?, ?, ?)""", List)
    
  sql = """
        SELECT * FROM book;"""
  cursor.execute(sql)
  result = cursor.fetchall()
  for x in result:
      print(x)
    
  # Changes saved into database
  connection.commit()
    
  # Connection closed(broken) 
  # with DataBase
  connection.close()
  

  输出：

  Traceback (most recent call last):
    File "C:/Users/GFG/Desktop/SQLITE3.py", line 16, in 
      List[2][3] =[['A', 'B', 2008], ['C', 'D', 2008], ['E', 'F', 2010]]
  NameError: name 'List' is not defined
  

  使用executemany()，可以使这段代码起作用。

  import sqlite3
     
  # Connection with the DataBase
  # 'library.db'
  connection = sqlite3.connect("library.db")
  cursor = connection.cursor()
     
  # SQL piece of code Executed
  cursor.execute("""
      CREATE TABLE book(
          title,
          author,
          published);""")
     
  List = [('A', 'B', 2008), ('C', 'D', 2008), 
                            ('E', 'F', 2010)]
     
  connection. executemany("""
      INSERT INTO 
      book(title, author, published) 
      VALUES (?, ?, ?)""", List)
     
  sql = """
  SELECT * FROM book;"""
  cursor.execute(sql)
  result = cursor.fetchall()
  for x in result:
      print(x)
     
  # Changes saved into database
  connection.commit()
     
  # Connection closed(broken)
  # with DataBase
  connection.close()
  

  输出：

  ('A', 'B', 2008)
  ('C', 'D', 2008)
  ('E', 'F', 2010)
  

• 获取大数据

  import sqlite3
    
  # Connection created with the
  # database using sqlite3.connect()
  connection = sqlite3.connect("company.db")
  cursor = connection.cursor()
    
  # Create Table command executed
  sql = """
          CREATE TABLE employee ( 
          ID INTEGER PRIMARY KEY, 
          fname VARCHAR(20), 
          lname VARCHAR(30), 
          gender CHAR(1), 
          dob DATE);"""
  cursor.execute(sql)
    
  # Single Tuple inserted
  sql = """
          INSERT INTO employee
          VALUES (1007, "Will", "Olsen", "M", "24-SEP-1865");"""
  cursor.execute(sql)
    
  # Multiple Rows inserted
  List = [(1008, 'Rkb', 'Boss', 'M', "27-NOV-1864"),
          (1098, 'Sak', 'Rose', 'F', "27-DEC-1864"),
          (1908, 'Royal', 'Bassen', "F", "17-NOV-1894")]
    
  connection. executemany(
          "INSERT INTO employee VALUES (?, ?, ?, ?, ?)", List)
    
  print("Method-1\n")
    
  # Multiple Rows fetched from
  # the Database
  for row in connection.execute('SELECT * FROM employee ORDER BY ID'):
          print (row)
    
  print("\nMethod-2\n")
    
  # Method-2 to fetch multiple
  # rows
  sql = """
          SELECT * FROM employee ORDER BY ID;"""
    
  cursor.execute(sql)
  result = cursor.fetchall()
    
  for x in result:
      print(x)
    
  connection.commit()
  connection.close()
  

  输出：

  Method-1
  
  (1007, 'Will', 'Olsen', 'M', '24-SEP-1865')
  (1008, 'Rkb', 'Boss', 'M', '27-NOV-1864')
  (1098, 'Sak', 'Rose', 'F', '27-DEC-1864')
  (1908, 'Royal', 'Bassen', 'F', '17-NOV-1894')
  
  Method-2
  
  (1007, 'Will', 'Olsen', 'M', '24-SEP-1865')
  (1008, 'Rkb', 'Boss', 'M', '27-NOV-1864')
  (1098, 'Sak', 'Rose', 'F', '27-DEC-1864')
  (1908, 'Royal', 'Bassen', 'F', '17-NOV-1894')
  

  注意：由于创建/写入数据库的权限，这段代码可能不适用于在线解释器。