SQLAlchemy Core – 选择行

在本文中，我们将看到如何编写一个查询，以在 SQLAlchemy 中针对Python中的 PostgreSQL 数据库，根据特定条件获取所有行。

创建演示表：

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 books 的表，其中包含 book_id 和 book_price 列。如图所示，使用 insert() 和 values()函数将记录插入表中。

Python3

# import necessary packages
import sqlalchemy
from sqlalchemy import create_engine, MetaData,
Table, Column, Numeric, Integer, VARCHAR
from sqlalchemy.engine import result
 
# establish connections
engine = create_engine(
    "database+dialect://username:password0@host:port/databasename")
 
# initialize the Metadata Object
meta = MetaData(bind=engine)
MetaData.reflect(meta)
 
# create a table schema
books = Table(
    'books', meta,
    Column('bookId', Integer, primary_key=True),
    Column('book_price', Numeric),
    Column('genre', VARCHAR),
    Column('book_name', VARCHAR)
)
 
meta.create_all(engine)
 
# insert records into the table
statement1 = books.insert().values(bookId=1, book_price=12.2,
                                   genre='fiction',
                                   book_name='Old age')
statement2 = books.insert().values(bookId=2, book_price=13.2,
                                   genre='non-fiction',
                                   book_name='Saturn rings')
statement3 = books.insert().values(bookId=3, book_price=121.6,
                                   genre='fiction',
                                   book_name='Supernova')
statement4 = books.insert().values(bookId=4, book_price=100,
                                   genre='non-fiction',
                                   book_name='History of the world')
statement5 = books.insert().values(bookId=5, book_price=1112.2,
                                   genre='fiction', book_name='Sun city')
 
# execute the insert records statement
engine.execute(statement1)
engine.execute(statement2)
engine.execute(statement3)
engine.execute(statement4)
engine.execute(statement5)

Python3

# Get the books table from the Metadata object
BOOKS = meta.tables['books']
 
# SQLAlchemy Query to select all rows with
# fiction genre
query = sqlalchemy.select(BOOKS).where(BOOKS.c.genre == 'fiction')
 
# Fetch all the records
result = engine.execute(query).fetchall()
 
# View the records
for record in result:
    print("\n", record)

输出：

样品表

在 SQLAlchemy 中提取完整的行

在表中选择行的过程与常规 SQL 查询的过程略有不同，如下所示

sqlalchemy.select(Tablename).whereTablename.c.column_name == condition)

编程需要懂一点英语

从连接到数据库时初始化的元数据对象获取书籍表。将 SQL 查询传递给 execute()函数并使用 fetchall()函数获取所有结果。使用 for 循环遍历结果。下面代码中显示的 SQLAlchemy 查询会选择该行包含“小说”类型的所有行。

Python3

# Get the books table from the Metadata object
BOOKS = meta.tables['books']
 
# SQLAlchemy Query to select all rows with
# fiction genre
query = sqlalchemy.select(BOOKS).where(BOOKS.c.genre == 'fiction')
 
# Fetch all the records
result = engine.execute(query).fetchall()
 
# View the records
for record in result:
    print("\n", record)

输出：

选择行的输出