与此 SQLite 语句等效的正确 SQLAlchemy 是什么？
What is the correct SQLAlchemy equivalent to this SQLite statement?
我正在尝试获取每个团队的最新变化。
SQLite 语句
按预期工作。
SELECT * FROM (
SELECT * FROM team_history ORDER BY changed_at DESC
) sub GROUP BY team
SQLAlchemy 实现
无论出于何种原因，我必须使用 asc() 而不是 desc() 进行排序以获得相同的结果，这就是为什么我怀疑我的实现是否正确。
session.query(TeamHistory)\
.select_entity_from(
session.query(TeamHistory).order_by(asc(TeamHistory.changed_at)).subquery()
).group_by(TeamHistory.team)\
.all()
环境
Python: 3.8.0
SQLAlchemy：1.3.23
复制
架构：
CREATE TABLE "team_history" (ID integer PRIMARY KEY, changed_at TEXT, team TEXT);
记录：
[{"ID":1,"changed_at":"2021-03-02 10:00:00","team":"B"},
{"ID":2,"changed_at":"2021-03-02 10:00:00","team":"A"},
{"ID":3,"changed_at":"2021-03-02 10:30:00","team":"B"},
{"ID":4,"changed_at":"2021-03-02 10:00:00","team":"A"},
{"ID":5,"changed_at":"2021-03-02 11:30:00","team":"B"},
{"ID":6,"changed_at":"2021-03-02 10:00:00","team":"A"},
{"ID":7,"changed_at":"2021-03-02 11:00:00","team":"B"},
{"ID":8,"changed_at":"2021-03-02 10:00:00","team":"A"},
{"ID":9,"changed_at":"2021-03-02 10:00:00","team":"B"},
{"ID":10,"changed_at":"2021-03-02 10:00:00","team":"A"},
{"ID":11,"changed_at":"2021-03-02 10:00:00","team":"B"},
{"ID":12,"changed_at":"2021-03-02 10:00:00","team":"A"},
{"ID":13,"changed_at":"2021-03-02 10:00:00","team":"B"},
{"ID":14,"changed_at":"2021-03-02 12:30:00","team":"A"},
{"ID":15,"changed_at":"2021-03-02 10:00:00","team":"B"},
{"ID":16,"changed_at":"2021-03-02 12:00:00","team":"A"},
{"ID":17,"changed_at":"2021-03-02 10:00:00","team":"B"},
{"ID":18,"changed_at":"2021-03-02 13:30:00","team":"A"},
{"ID":19,"changed_at":"2021-03-02 10:00:00","team":"B"},
{"ID":20,"changed_at":"2021-03-02 10:00:00","team":"A"}]
解决方案
谢谢大家！
session.query(TeamHistory)\
.group_by(TeamHistory.team)\
.having(func.max(TeamHistory.changed_at))\
.all()
当您使用时：
SELECT *
FROM tablename
GROUP BY somecolumn
SQLite returns somecolumn 的每个不同值 1 行，但是哪一行？
documentation 表示该行未定义，这意味着它是任意选择的，尽管根据我的经验，似乎将返回属于每个组的结果集中的第一行。
但这并不能保证，应该避免像上面的查询和您的查询。
有多种方法可以为每个 team 获取具有最新 changed_at 的行。
其中之一在 SQLite 中有效（尽管它在其他数据库中不起作用）是：
SELECT * FROM team_history GROUP BY team HAVING MAX(changed_at)
参见demo。
因此，这是您应该转换为 SQLAlchemy 的查询（我无法帮助您）。
还有其他方法，使用 window 函数，或 EXISTS。
基本上您提出的查询应该有效。 SQLite 将日期存储为 ISO-8601 格式的字符串，其中 属性 字典顺序和时间顺序相同。同样使用日期时间列作为 TEXT 具有相同的属性。
因此子查询的排序 desc 应导致以下结果：
ID
DateTime
Team
18
2021-03-02 13:30:00
A
14
2021-03-02 12:30:00
A
16
2021-03-02 12:00:00
A
5
2021-03-02 11:30:00
B
..
..
..
如另一个答案所述，group by 的问题是为每个组任意选择 returned 行，但它看起来总是第一行。知道这一点，我们有不同的解决方案来确定它应该 return:
# Using an aggregate function in select
session.query(TeamHistory.team, func.max(TeamHistory.changed_at)).group_by(TeamHistory.team).all()
# Using an aggregate function with `having`
session.query(TeamHistory).group_by(TeamHistory.team).having(func.max(TeamHistory.changed_at).all()
这导致以下工作示例：
from sqlalchemy import Column, Integer, create_engine, Text, DateTime, func
from sqlalchemy.orm import sessionmaker, deferred, column_property
from sqlalchemy.ext.declarative import declarative_base
import datetime
Base = declarative_base()
class TeamHistory(Base):
__tablename__ = 'team_history'
id = Column(Integer, primary_key=True)
changed_at = Column(DateTime)
team = Column(Text)
if __name__ == '__main__':
engine = create_engine('sqlite://')
Base.metadata.create_all(engine)
Session = sessionmaker(engine)
db = Session()
lst = [{"ID":1,"changed_at":"2021-03-02 10:00:00","team":"B"},{"ID":2,"changed_at":"2021-03-02 10:00:00","team":"A"},{"ID":3,"changed_at":"2021-03-02 10:30:00","team":"B"},{"ID":4,"changed_at":"2021-03-02 10:00:00","team":"A"},{"ID":5,"changed_at":"2021-03-02 11:30:00","team":"B"},{"ID":6,"changed_at":"2021-03-02 10:00:00","team":"A"},{"ID":7,"changed_at":"2021-03-02 11:00:00","team":"B"},{"ID":8,"changed_at":"2021-03-02 10:00:00","team":"A"},{"ID":9,"changed_at":"2021-03-02 10:00:00","team":"B"},{"ID":10,"changed_at":"2021-03-02 10:00:00","team":"A"},{"ID":11,"changed_at":"2021-03-02 10:00:00","team":"B"},{"ID":12,"changed_at":"2021-03-02 10:00:00","team":"A"},{"ID":13,"changed_at":"2021-03-02 10:00:00","team":"B"},{"ID":14,"changed_at":"2021-03-02 12:30:00","team":"A"},{"ID":15,"changed_at":"2021-03-02 10:00:00","team":"B"},{"ID":16,"changed_at":"2021-03-02 12:00:00","team":"A"},{"ID":17,"changed_at":"2021-03-02 10:00:00","team":"B"},{"ID":18,"changed_at":"2021-03-02 13:30:00","team":"A"},{"ID":19,"changed_at":"2021-03-02 10:00:00","team":"B"},{"ID":20,"changed_at":"2021-03-02 10:00:00","team":"A"}]
for dct in lst:
dt = datetime.datetime.strptime(dct.get('changed_at'), '%Y-%m-%d %H:%M:%S')
nth = TeamHistory(id=dct.get('ID'), changed_at=dt, team=dct.get('team'))
db.add(nth)
db.commit()
res = db.query(TeamHistory)\
.select_entity_from(
db.query(TeamHistory).order_by(TeamHistory.changed_at.desc()).subquery()
).group_by(TeamHistory.team)\
.all()
for r in res:
print(r.id, r.changed_at, r.team)
print()
res = db.query(TeamHistory)\
.group_by(TeamHistory.team)\
.having(func.max(TeamHistory.changed_at))\
.all()
for r in res:
print(r.id, r.changed_at, r.team)
print()
res = db.query(TeamHistory.id, func.max(TeamHistory.changed_at), TeamHistory.team)\
.group_by(TeamHistory.team)\
.all()
for r in res:
print(r[0], r[1], r[2])
如果你主要对球队的最后变化感兴趣你也可以使用deferred和column_property只得到MAX(changed_at).
from sqlalchemy import Column, Integer, create_engine, Text, DateTime, func, select
from sqlalchemy.orm import sessionmaker, deferred, column_property
from sqlalchemy.ext.declarative import declarative_base
import datetime
Base = declarative_base()
class TeamHistory(Base):
__tablename__ = 'team_history_def'
id = Column(Integer, primary_key=True)
changed_at = deferred(Column(DateTime))
last_changed = column_property(func.max(changed_at))
team = Column(Text)
if __name__ == '__main__':
engine = create_engine('sqlite://')
Base.metadata.create_all(engine)
Session = sessionmaker(engine)
db = Session()
lst = [{"ID":1,"changed_at":"2021-03-02 10:00:00","team":"B"},{"ID":2,"changed_at":"2021-03-02 10:00:00","team":"A"},{"ID":3,"changed_at":"2021-03-02 10:30:00","team":"B"},{"ID":4,"changed_at":"2021-03-02 10:00:00","team":"A"},{"ID":5,"changed_at":"2021-03-02 11:30:00","team":"B"},{"ID":6,"changed_at":"2021-03-02 10:00:00","team":"A"},{"ID":7,"changed_at":"2021-03-02 11:00:00","team":"B"},{"ID":8,"changed_at":"2021-03-02 10:00:00","team":"A"},{"ID":9,"changed_at":"2021-03-02 10:00:00","team":"B"},{"ID":10,"changed_at":"2021-03-02 10:00:00","team":"A"},{"ID":11,"changed_at":"2021-03-02 10:00:00","team":"B"},{"ID":12,"changed_at":"2021-03-02 10:00:00","team":"A"},{"ID":13,"changed_at":"2021-03-02 10:00:00","team":"B"},{"ID":14,"changed_at":"2021-03-02 12:30:00","team":"A"},{"ID":15,"changed_at":"2021-03-02 10:00:00","team":"B"},{"ID":16,"changed_at":"2021-03-02 12:00:00","team":"A"},{"ID":17,"changed_at":"2021-03-02 10:00:00","team":"B"},{"ID":18,"changed_at":"2021-03-02 13:30:00","team":"A"},{"ID":19,"changed_at":"2021-03-02 10:00:00","team":"B"},{"ID":20,"changed_at":"2021-03-02 10:00:00","team":"A"}]
for dct in lst:
dt = datetime.datetime.strptime(dct.get('changed_at'), '%Y-%m-%d %H:%M:%S')
nth = TeamHistory(id=dct.get('ID'), changed_at=dt, team=dct.get('team'))
db.add(nth)
db.commit()
res = db.query(TeamHistory)\
.group_by(TeamHistory.team)\
.all()
for r in res:
print(r.id, r.changed_at, r.team)
print()
我正在尝试获取每个团队的最新变化。
SQLite 语句
按预期工作。
SELECT * FROM (
SELECT * FROM team_history ORDER BY changed_at DESC
) sub GROUP BY team
SQLAlchemy 实现
无论出于何种原因，我必须使用 asc() 而不是 desc() 进行排序以获得相同的结果，这就是为什么我怀疑我的实现是否正确。
session.query(TeamHistory)\
.select_entity_from(
session.query(TeamHistory).order_by(asc(TeamHistory.changed_at)).subquery()
).group_by(TeamHistory.team)\
.all()
环境
Python: 3.8.0
SQLAlchemy：1.3.23
复制
架构：
CREATE TABLE "team_history" (ID integer PRIMARY KEY, changed_at TEXT, team TEXT);
记录：
[{"ID":1,"changed_at":"2021-03-02 10:00:00","team":"B"},
{"ID":2,"changed_at":"2021-03-02 10:00:00","team":"A"},
{"ID":3,"changed_at":"2021-03-02 10:30:00","team":"B"},
{"ID":4,"changed_at":"2021-03-02 10:00:00","team":"A"},
{"ID":5,"changed_at":"2021-03-02 11:30:00","team":"B"},
{"ID":6,"changed_at":"2021-03-02 10:00:00","team":"A"},
{"ID":7,"changed_at":"2021-03-02 11:00:00","team":"B"},
{"ID":8,"changed_at":"2021-03-02 10:00:00","team":"A"},
{"ID":9,"changed_at":"2021-03-02 10:00:00","team":"B"},
{"ID":10,"changed_at":"2021-03-02 10:00:00","team":"A"},
{"ID":11,"changed_at":"2021-03-02 10:00:00","team":"B"},
{"ID":12,"changed_at":"2021-03-02 10:00:00","team":"A"},
{"ID":13,"changed_at":"2021-03-02 10:00:00","team":"B"},
{"ID":14,"changed_at":"2021-03-02 12:30:00","team":"A"},
{"ID":15,"changed_at":"2021-03-02 10:00:00","team":"B"},
{"ID":16,"changed_at":"2021-03-02 12:00:00","team":"A"},
{"ID":17,"changed_at":"2021-03-02 10:00:00","team":"B"},
{"ID":18,"changed_at":"2021-03-02 13:30:00","team":"A"},
{"ID":19,"changed_at":"2021-03-02 10:00:00","team":"B"},
{"ID":20,"changed_at":"2021-03-02 10:00:00","team":"A"}]
解决方案
谢谢大家！
session.query(TeamHistory)\
.group_by(TeamHistory.team)\
.having(func.max(TeamHistory.changed_at))\
.all()
当您使用时：
SELECT *
FROM tablename
GROUP BY somecolumn
SQLite returns somecolumn 的每个不同值 1 行，但是哪一行？
documentation 表示该行未定义，这意味着它是任意选择的，尽管根据我的经验，似乎将返回属于每个组的结果集中的第一行。
但这并不能保证，应该避免像上面的查询和您的查询。
有多种方法可以为每个 team 获取具有最新 changed_at 的行。
其中之一在 SQLite 中有效（尽管它在其他数据库中不起作用）是：
SELECT * FROM team_history GROUP BY team HAVING MAX(changed_at)
参见demo。
因此，这是您应该转换为 SQLAlchemy 的查询（我无法帮助您）。
还有其他方法，使用 window 函数，或 EXISTS。
基本上您提出的查询应该有效。 SQLite 将日期存储为 ISO-8601 格式的字符串，其中 属性 字典顺序和时间顺序相同。同样使用日期时间列作为 TEXT 具有相同的属性。
因此子查询的排序 desc 应导致以下结果：
| ID | DateTime | Team |
|---|---|---|
| 18 | 2021-03-02 13:30:00 | A |
| 14 | 2021-03-02 12:30:00 | A |
| 16 | 2021-03-02 12:00:00 | A |
| 5 | 2021-03-02 11:30:00 | B |
| .. | .. | .. |
如另一个答案所述，group by 的问题是为每个组任意选择 returned 行，但它看起来总是第一行。知道这一点，我们有不同的解决方案来确定它应该 return:
# Using an aggregate function in select
session.query(TeamHistory.team, func.max(TeamHistory.changed_at)).group_by(TeamHistory.team).all()
# Using an aggregate function with `having`
session.query(TeamHistory).group_by(TeamHistory.team).having(func.max(TeamHistory.changed_at).all()
这导致以下工作示例：
from sqlalchemy import Column, Integer, create_engine, Text, DateTime, func
from sqlalchemy.orm import sessionmaker, deferred, column_property
from sqlalchemy.ext.declarative import declarative_base
import datetime
Base = declarative_base()
class TeamHistory(Base):
__tablename__ = 'team_history'
id = Column(Integer, primary_key=True)
changed_at = Column(DateTime)
team = Column(Text)
if __name__ == '__main__':
engine = create_engine('sqlite://')
Base.metadata.create_all(engine)
Session = sessionmaker(engine)
db = Session()
lst = [{"ID":1,"changed_at":"2021-03-02 10:00:00","team":"B"},{"ID":2,"changed_at":"2021-03-02 10:00:00","team":"A"},{"ID":3,"changed_at":"2021-03-02 10:30:00","team":"B"},{"ID":4,"changed_at":"2021-03-02 10:00:00","team":"A"},{"ID":5,"changed_at":"2021-03-02 11:30:00","team":"B"},{"ID":6,"changed_at":"2021-03-02 10:00:00","team":"A"},{"ID":7,"changed_at":"2021-03-02 11:00:00","team":"B"},{"ID":8,"changed_at":"2021-03-02 10:00:00","team":"A"},{"ID":9,"changed_at":"2021-03-02 10:00:00","team":"B"},{"ID":10,"changed_at":"2021-03-02 10:00:00","team":"A"},{"ID":11,"changed_at":"2021-03-02 10:00:00","team":"B"},{"ID":12,"changed_at":"2021-03-02 10:00:00","team":"A"},{"ID":13,"changed_at":"2021-03-02 10:00:00","team":"B"},{"ID":14,"changed_at":"2021-03-02 12:30:00","team":"A"},{"ID":15,"changed_at":"2021-03-02 10:00:00","team":"B"},{"ID":16,"changed_at":"2021-03-02 12:00:00","team":"A"},{"ID":17,"changed_at":"2021-03-02 10:00:00","team":"B"},{"ID":18,"changed_at":"2021-03-02 13:30:00","team":"A"},{"ID":19,"changed_at":"2021-03-02 10:00:00","team":"B"},{"ID":20,"changed_at":"2021-03-02 10:00:00","team":"A"}]
for dct in lst:
dt = datetime.datetime.strptime(dct.get('changed_at'), '%Y-%m-%d %H:%M:%S')
nth = TeamHistory(id=dct.get('ID'), changed_at=dt, team=dct.get('team'))
db.add(nth)
db.commit()
res = db.query(TeamHistory)\
.select_entity_from(
db.query(TeamHistory).order_by(TeamHistory.changed_at.desc()).subquery()
).group_by(TeamHistory.team)\
.all()
for r in res:
print(r.id, r.changed_at, r.team)
print()
res = db.query(TeamHistory)\
.group_by(TeamHistory.team)\
.having(func.max(TeamHistory.changed_at))\
.all()
for r in res:
print(r.id, r.changed_at, r.team)
print()
res = db.query(TeamHistory.id, func.max(TeamHistory.changed_at), TeamHistory.team)\
.group_by(TeamHistory.team)\
.all()
for r in res:
print(r[0], r[1], r[2])
如果你主要对球队的最后变化感兴趣你也可以使用deferred和column_property只得到MAX(changed_at).
from sqlalchemy import Column, Integer, create_engine, Text, DateTime, func, select
from sqlalchemy.orm import sessionmaker, deferred, column_property
from sqlalchemy.ext.declarative import declarative_base
import datetime
Base = declarative_base()
class TeamHistory(Base):
__tablename__ = 'team_history_def'
id = Column(Integer, primary_key=True)
changed_at = deferred(Column(DateTime))
last_changed = column_property(func.max(changed_at))
team = Column(Text)
if __name__ == '__main__':
engine = create_engine('sqlite://')
Base.metadata.create_all(engine)
Session = sessionmaker(engine)
db = Session()
lst = [{"ID":1,"changed_at":"2021-03-02 10:00:00","team":"B"},{"ID":2,"changed_at":"2021-03-02 10:00:00","team":"A"},{"ID":3,"changed_at":"2021-03-02 10:30:00","team":"B"},{"ID":4,"changed_at":"2021-03-02 10:00:00","team":"A"},{"ID":5,"changed_at":"2021-03-02 11:30:00","team":"B"},{"ID":6,"changed_at":"2021-03-02 10:00:00","team":"A"},{"ID":7,"changed_at":"2021-03-02 11:00:00","team":"B"},{"ID":8,"changed_at":"2021-03-02 10:00:00","team":"A"},{"ID":9,"changed_at":"2021-03-02 10:00:00","team":"B"},{"ID":10,"changed_at":"2021-03-02 10:00:00","team":"A"},{"ID":11,"changed_at":"2021-03-02 10:00:00","team":"B"},{"ID":12,"changed_at":"2021-03-02 10:00:00","team":"A"},{"ID":13,"changed_at":"2021-03-02 10:00:00","team":"B"},{"ID":14,"changed_at":"2021-03-02 12:30:00","team":"A"},{"ID":15,"changed_at":"2021-03-02 10:00:00","team":"B"},{"ID":16,"changed_at":"2021-03-02 12:00:00","team":"A"},{"ID":17,"changed_at":"2021-03-02 10:00:00","team":"B"},{"ID":18,"changed_at":"2021-03-02 13:30:00","team":"A"},{"ID":19,"changed_at":"2021-03-02 10:00:00","team":"B"},{"ID":20,"changed_at":"2021-03-02 10:00:00","team":"A"}]
for dct in lst:
dt = datetime.datetime.strptime(dct.get('changed_at'), '%Y-%m-%d %H:%M:%S')
nth = TeamHistory(id=dct.get('ID'), changed_at=dt, team=dct.get('team'))
db.add(nth)
db.commit()
res = db.query(TeamHistory)\
.group_by(TeamHistory.team)\
.all()
for r in res:
print(r.id, r.changed_at, r.team)
print()