在 SQLAlchemy 中选择 NULL 值

在本文中，我们将看到如何使用Python中的 SQLAlchemy 将 NULL 值选择到 PostgreSQL 数据库中。

首先出于演示目的，让我们在 PostgreSQL 中使用 SQLAlchemy 创建一个示例表，如下所示

在 PostgreSQL 中使用 SQLAlchemy 创建表：

从 SQLAlchemy 包中导入必要的函数。
使用 create_engine()函数与 PostgreSQL 数据库建立连接，如下所示
创建一个名为 book_publisher 的表，其中包含 publisher_id、publisher_name 和 publisher_estd 列
如图所示，使用 insert() 和 values()函数将记录插入表中。

Syntax: engine = create_engine(dialect+driver://username:password@host:port/database_name)

Parameters:

dialect – Name of the DBMS. The dialect is the system SQLAlchemy uses to communicate with various types of DBAPIs and databases like PostgreSQL, MySQL, MS SQL, etc.
driver – Name of the DB API that moves information between SQLAlchemy and the database.
Username – Name of the admin
Password – Password of the admin
host – Name of the host in which the database is hosted
port – port number through which the database can be accessed
database_name– Name of the database

编程需要懂一点英语

创建演示表：

Python3

# import necessary packages
from sqlalchemy import create_engine, MetaData,/
Table, Column, Integer, String
 
# establish connection
engine = create_engine(
    "postgresql+psycopg2://\
    postgres:Saibaba97%40@127.0.0.1:5432/test")
 
# store engine objects
meta = MetaData()
 
# create a table
book_publisher = Table(
    'book_publisher', meta,
    Column('publisherId', Integer, primary_key=True),
    Column('publisherName', String),
    Column('publisherEstd', Integer),
)
# use create_all() function to create a
# table using objects stored in meta.
meta.create_all(engine)
 
# insert values
statement1 = book_publisher.insert().values(
    publisherId=1, publisherName="Oxford", publisherEstd=1900)
 
statement2 = book_publisher.insert().values(
    publisherId=2, publisherName='Stanford', publisherEstd=1910)
 
statement3 = book_publisher.insert().values(
    publisherId=3, publisherName="MIT", publisherEstd=1920)
 
statement4 = book_publisher.insert().values(
    publisherId=4, publisherName="Springer", publisherEstd=1930)
 
statement5 = book_publisher.insert().values(
    publisherId=5, publisherName="Packt", publisherEstd=1940)
 
statement6 = book_publisher.insert().values(
    publisherId=6, publisherName=None, publisherEstd=None)
 
engine.execute(statement1)
engine.execute(statement2)
engine.execute(statement3)
engine.execute(statement4)
engine.execute(statement5)
engine.execute(statement6)

Python3

# write a conventional SQL query
# with NULL equivalent as None
s = book_publisher.select().where(
  book_publisher.c.publisherName == None)
 
# output get stored in result object
result = engine.execute(s)
 
# iteratte through the result object
# to get all rows of the output
for row in result:
    print(row)

输出：

使用 SQLAlchemy 创建的示例表

在 PostgreSQL 中使用 SQLAlchemy 选择 NULL 值

使用 select() 和 where函数，我们可以从数据库中查询数据。 Python中的NULL等效项是None 。

其他事情保持不变，例如传统的 SQL 查询。查询被传递给execute()语句，输出存储在结果对象中，并且可以迭代行以获取所有行

代码：

Python3

# write a conventional SQL query
# with NULL equivalent as None
s = book_publisher.select().where(
  book_publisher.c.publisherName == None)
 
# output get stored in result object
result = engine.execute(s)
 
# iteratte through the result object
# to get all rows of the output
for row in result:
    print(row)

输出：

正如我们从上面显示的输出中知道的那样，第 6 行有 None 值，并且在下面的输出中正确返回

(6, None, None)