📜  SQLAlchemy Core – SQL 表达式

📅  最后修改于: 2022-05-13 01:55:08.491000             🧑  作者: Mango

SQLAlchemy Core – SQL 表达式

在本文中,我们将看到如何使用 SQLAlchmey CORE 使用 SQLAlchemy 中的 text() 针对Python中的 PostgreSQL 数据库编写 SQL 表达式。

为演示创建表

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 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
from sqlalchemy import create_engine, MetaData,\
Table, Column, Numeric, Integer, VARCHAR
from sqlalchemy import text
  
# establish connections
engine = create_engine(
    "dialect+driver://username:password@host:port/database_name")
  
# 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
  
# write the SQL query inside the text() block
sql = text('SELECT * from BOOKS WHERE BOOKS.book_price > 100')
results = engine.execute(sql)
  
# Fetch all the records
result = engine.execute(sql).fetchall()
    
# View the records
for record in result:
    print("\n", record)


Python3
# define a tuple of dictionary of values to be inserted
data = ( { "book_id": 6, "book_price": 400, 
          "genre": "fiction",
          "book_name": "yoga is science" },
         { "book_id": 7, "book_price": 800, 
          "genre": "non-fiction",
          "book_name": "alchemy tutorials" },
)
  
# write the insert statement
statement = text("""INSERT INTO BOOKS\
(book_id, book_price, genre, book_name) \
VALUES(:book_id, :book_price, :genre, :book_name)""")
  
# insert the data one after other using
# execute statement by unpacking dictionary  elements
for line in data:
    engine.execute(statement, **line)
  
# write the SQL query to check
# whether the records are inserted
sql = text("SELECT * FROM BOOKS ")
  
results = engine.execute(sql)
  
# View the records
for record in results:
    print("\n", record)


Python3
# Get the `books` table from the Metadata object
BOOKS = meta.tables['books']
  
# update
stmt = BOOKS.update().where(BOOKS.c.genre == 'non-fiction'
                           ).values(genre='sci-fi')
engine.execute(stmt)
  
# 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)


Python3
# Get the `books` table from the Metadata object
BOOKS = meta.tables['books']
  
# delete
dele = BOOKS.delete().where(BOOKS.c.genre == "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:

样品表

在 SQLAlchemy 中实现查询以执行 SQL 表达式

SQLAlchemy 提供了一个名为 text() 的函数。我们可以在用“”括起来的文本函数中编写任何常规的 SQL 查询。现在,将此 SQL 查询传递给执行函数会将此查询转换为 SQLAlchemy 兼容格式并返回结果。

from sqlalchemy import text
text("YOUR SQL QUERY")

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。

示例 1:执行基本查询

下面代码中显示的 SQLAlchemy 查询选择了书价大于 Rs 的所有行。 100。

Python3

from sqlalchemy import text
  
# write the SQL query inside the text() block
sql = text('SELECT * from BOOKS WHERE BOOKS.book_price > 100')
results = engine.execute(sql)
  
# Fetch all the records
result = engine.execute(sql).fetchall()
    
# View the records
for record in result:
    print("\n", record)

输出:

常规 SQL 表达式的输出

示例 2:执行插入查询

下面的 SQL 表达式将以常规 SQL 方式在创建的表中插入附加记录。

Python3

# define a tuple of dictionary of values to be inserted
data = ( { "book_id": 6, "book_price": 400, 
          "genre": "fiction",
          "book_name": "yoga is science" },
         { "book_id": 7, "book_price": 800, 
          "genre": "non-fiction",
          "book_name": "alchemy tutorials" },
)
  
# write the insert statement
statement = text("""INSERT INTO BOOKS\
(book_id, book_price, genre, book_name) \
VALUES(:book_id, :book_price, :genre, :book_name)""")
  
# insert the data one after other using
# execute statement by unpacking dictionary  elements
for line in data:
    engine.execute(statement, **line)
  
# write the SQL query to check
# whether the records are inserted
sql = text("SELECT * FROM BOOKS ")
  
results = engine.execute(sql)
  
# View the records
for record in results:
    print("\n", record)

输出:

插入查询的输出

示例 3:执行更新查询

让我们看另一个与更新查询相关的示例。

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 查询将类型“非小说”更新为“科幻”,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() 打印结果以检查表是否正确更新。

Python3

# Get the `books` table from the Metadata object
BOOKS = meta.tables['books']
  
# update
stmt = BOOKS.update().where(BOOKS.c.genre == 'non-fiction'
                           ).values(genre='sci-fi')
engine.execute(stmt)
  
# 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:

更新查询的结果

示例 4:执行删除查询

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“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() 打印结果以检查表是否正确更新。

Python3

# Get the `books` table from the Metadata object
BOOKS = meta.tables['books']
  
# delete
dele = BOOKS.delete().where(BOOKS.c.genre == "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:

删除查询的输出