SQLAlchemy Core – 删除语句

在本文中，我们将看到如何使用 SQLAlchemy 中的 DELETE 语句来处理Python中的 PostgreSQL 数据库。

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
from sqlalchemy.engine import result
import sqlalchemy
from sqlalchemy import create_engine, MetaData,\
Table, Column, Numeric, Integer, VARCHAR, update, delete
 
# 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('book_id', 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(book_id=1, book_price=12.2,
                                   genre='fiction',
                                   book_name='Old age')
statement2 = books.insert().values(book_id=2, book_price=13.2,
                                   genre='non-fiction',
                                   book_name='Saturn rings')
statement3 = books.insert().values(book_id=3, book_price=121.6,
                                   genre='fiction',
                                   book_name='Supernova')
statement4 = books.insert().values(book_id=4, book_price=100,
                                   genre='non-fiction',
                                   book_name='History of the world')
statement5 = books.insert().values(book_id=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)

输出：

样品表

删除 SQLAlchemy 中的表元素

删除表元素的过程与常规 SQL 查询的过程略有不同，如下所示

from sqlalchemy import delete
Tablename.delete().where(Tablename.c.column_name == value)

从连接到数据库时初始化的元数据对象获取书籍表。将删除查询传递给 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 查询删除了“非小说”类型，这将有效地一次删除多行。然后，我们可以编写一个常规的 SQL 查询并使用 fetchall() 打印结果来检查表是否正确更新。

Python3

# delete
dele = BOOKS.delete().where(BOOKS.c.genre == "non-fiction")
 
engine.execute(dele)
 
# write the SQL query inside the
# text() block to fetch all records
sql = text("SELECT * from BOOKS")
 
# Fetch all the records
result = engine.execute(sql).fetchall()
 
# View the records
for record in result:
    print("\n", record)

输出：

删除函数的输出