Question

我想优化我的数据库查询：

link_list = select(
    columns=[link_table.c.rating, link_table.c.url, link_table.c.donations_in],
    whereclause=and_(
        not_(link_table.c.id.in_(
            select(
                columns=[request_table.c.recipient],
                whereclause=request_table.c.donator==donator.id
            ).as_scalar()
        )),
        link_table.c.id!=donator.id,
    ),
    limit=20,
).execute().fetchall()

并尝试在一个查询中合并这两个选择：

link_list = select(
    columns=[link_table.c.rating, link_table.c.url, link_table.c.donations_in],
    whereclause=and_(
        link_table.c.active==True,
        link_table.c.id!=donator.id,
        request_table.c.donator==donator.id,
        link_table.c.id!=request_table.c.recipient,
    ),
    limit=20,
    order_by=[link_table.c.rating.desc()]
).execute().fetchall()

数据库架构如下所示：

link_table = Table('links', metadata,
    Column('id', Integer, primary_key=True, autoincrement=True),
    Column('url', Unicode(250), index=True, unique=True),
    Column('registration_date', DateTime),
    Column('donations_in', Integer),
    Column('active', Boolean),
)
request_table = Table('requests', metadata,
    Column('id', Integer, primary_key=True, autoincrement=True),
    Column('recipient', Integer, ForeignKey('links.id')),
    Column('donator', Integer, ForeignKey('links.id')),
    Column('date', DateTime),
)

request_table中有几个链接（donator）指向link_table中的一个链接。我希望从link_table获得链接，这些链接尚未“请求”。

但这不起作用。它真的可能，我正在尝试做什么？如果是这样，你会怎么做？

非常感谢你！

Answer 1

您可能正在寻找SQL NOT EXISTS构造：

http://www.sqlalchemy.org/docs/orm/tutorial.html#using-exists

Answer 2

重新masida's answer：

首先，原始查询：

>>> print select(
...     columns=[link_table.c.url, link_table.c.donations_in],
...     whereclause=and_(
...         not_(link_table.c.id.in_(
...             select(
...                 columns=[request_table.c.recipient],
...                 whereclause=request_table.c.donator==5
...             ).as_scalar()
...         )),
...         link_table.c.id!=5,
...     ),
...     limit=20,
... )
SELECT links.url, links.donations_in 
FROM links 
WHERE links.id NOT IN (SELECT requests.recipient 
FROM requests 
WHERE requests.donator = :donator_1) AND links.id != :id_1 
 LIMIT 20

用exists（）重写：

>>> print select(
...     columns=[link_table.c.url, link_table.c.donations_in],
...     whereclause=and_(
...     not_(exists().where(request_table.c.donator==5)),
...     #    ^^^^^^^^^^^^^^
...         link_table.c.id!=5,
...     ),
...     limit=20,
... )
SELECT links.url, links.donations_in 
FROM links 
WHERE NOT (EXISTS (SELECT * 
FROM requests 
WHERE requests.donator = :donator_1)) AND links.id != :id_1 
 LIMIT 20

SQLAlchemy：选择多个表

2 个答案: