如何在Python中更新 SQLAlchemy 中的现有表行?
在本文中,我们将了解如何使用 SQLAlchemy 中的 UPDATE 语句对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
# 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)
Python3
from sqlalchemy import text
# Get the `books` table from the Metadata object
BOOKS = meta.tables['books']
# update
u = update(BOOKS)
u = u.values({"genre": "sci-fi"})
u = u.where(BOOKS.c.genre == "fiction")
engine.execute(u)
# 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:
更新 SQLAlchemy 中的表元素
更新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 update
upd = update(tablename)
val = upd.values({"column_name":"value"})
cond = val.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到表。将更新查询传递给 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 查询将“小说”类型更新为“科幻”类型,这将有效地一次更新多行。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
from sqlalchemy import text
# Get the `books` table from the Metadata object
BOOKS = meta.tables['books']
# update
u = update(BOOKS)
u = u.values({"genre": "sci-fi"})
u = u.where(BOOKS.c.genre == "fiction")
engine.execute(u)
# 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: