[英]Select specific columns from table in SQLAlchemy
我正在尝试从表中选择特定的列,如下所示:
users = Table('users', metadata, autoload=True)
s = users.select([users.c.email])
results = s.execute()
print results
我收到这个错误:
> Traceback (most recent call last): File "my_mailer.py", line 35, in
> <module>
> s = users.select([users.c.email]) File "/task/__pips__/sqlalchemy/sql/selectable.py", line 175, in select
> return Select([self], whereclause, **params) File "/task/__pips__/sqlalchemy/sql/selectable.py", line 2082, in __init__
> self._whereclause = _literal_as_text(whereclause) File "/task/__pips__/sqlalchemy/sql/elements.py", line 2745, in
> _literal_as_text
> "SQL expression object or string expected." sqlalchemy.exc.ArgumentError: SQL expression object or string
> expected.
所以我尝试了这个:
users = Table('users', metadata, autoload=True)
s = users.select('email')
results = s.execute()
print results
得到这个回应:
> Traceback (most recent call last): File "my_mailer.py", line 36, in
> <module>
> results = s.execute() File "/task/__pips__/sqlalchemy/sql/base.py", line 124, in execute
> return e._execute_clauseelement(self, multiparams, params) File "/task/__pips__/sqlalchemy/engine/base.py", line 1605, in
> _execute_clauseelement
> return connection._execute_clauseelement(elem, multiparams, params) File "/task/__pips__/sqlalchemy/engine/base.py", line 761,
> in _execute_clauseelement
> compiled_sql, distilled_params File "/task/__pips__/sqlalchemy/engine/base.py", line 874, in
> _execute_context
> context) File "/task/__pips__/sqlalchemy/engine/base.py", line 1023, in _handle_dbapi_exception
> exc_info File "/task/__pips__/sqlalchemy/util/compat.py", line 185, in raise_from_cause
> reraise(type(exception), exception, tb=exc_tb) File "/task/__pips__/sqlalchemy/engine/base.py", line 867, in
> _execute_context
> context) File "/task/__pips__/sqlalchemy/engine/default.py", line 388, in do_execute
> cursor.execute(statement, parameters) sqlalchemy.exc.ProgrammingError: (ProgrammingError) argument of WHERE
> must be type boolean, not type character varying LINE 3: WHERE email
果然,这里的第一个参数是“whereclause”,而不是像其他地方一样的“列”,这反映在文档中 :
表中没有select()形式的参数。
问题:如何使用表格中的select仅选择特定列? 通常,为什么地球上的列参数不能在表格中选择 ? 我无法理解为什么有人决定让它与标准选择不同。
使用通用select
而不是Table.select
:
stmt = select([users.c.email])
result = conn.execute(stmt)
使用元数据,您可以这样做:
metadata = MetaData(bind=engine)
tblusers = metadata.tables['users']
tblproducts = metadata.tables['products']
# example 1: only columns you want.
data_users = tblusers.select().with_only_columns([tblusers.c.id, tblusers.c.name]).execute()
# example 2: w/ where & order_by
data_products = tblproducts.select().with_only_columns([tblproducts.c.id, tblproducts.c.price, tblproductos.c.description]).where(tblproducts.c.stock > 0).order_by(tblproducts.c.brand).execute()
好吧......
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