Python Can'；t将此SQL字符串（具有值…AS）转换为SQLAlchemy代码

我使用的SQL查询可以从连接在一起的3个表中识别

Max

编辑时间：

Select Identity.SSN, Schedule.First_Class, Students.Last_Name,
(SELECT Max(v)
    FROM (VALUES (Students.Edit_DtTm), (Schedule.Edit_DtTm),
    (Identity.Edit_DtTm)) AS value(v)) as [MaxEditDate]
FROM Schedule
LEFT JOIN Students ON Schedule.stdnt_id=Students.Student_Id
LEFT JOIN Identity ON Schedule.std_id=Identity.std_id

我需要将其放在SQLAlchemy中，以便可以引用代码中其他地方使用的列。下面是我试图做的最简单的版本，但它不起作用。我试着改变查询它的方式，但我要么得到一个SQL错误，因为我使用的

值不正确

，要么它没有正确连接，并在没有将其与外部查询匹配的情况下得到这些列中的实际最高值

max_edit_subquery = sa.func.values(Students.Edit_DtTm, Schedule.Edit_DtTm, Identity.Edit_DtTm)
base_query = (sa.select([Identity.SSN, Schedule.First_Class, Students.Last_Name,
                            (sa.select([sa.func.max(self.max_edit_subquery)]))]).
                            select_from(Schedule.__table__.join(Students, Schedule.stdnt_id == Students.stdnt_id).
                            join(Ident, Schedule.std_id == Identity.std_id)))

---

我不是SQLAlchemy的专家，但您可以与UNION ALL交换

值

：

Select Identity.SSN, Schedule.First_Class, Students.Last_Name,
(SELECT Max(v)
    FROM (SELECT Students.Edit_DtTm AS v
         UNION ALL SELECT Schedule.Edit_DtTm
         UNION ALL SELECT Identity.Edit_DtTm) s
   ) as [MaxEditDate]
FROM Schedule
LEFT JOIN Students ON Schedule.stdnt_id=Students.Student_Id
LEFT JOIN Identity ON Schedule.std_id=Identity.std_id;

另一种方法是使用

magest

函数（在T-SQL中不可用）：

---

我希望它能帮助您将其翻译成ORM版本。

我遇到了类似的问题，我使用以下方法解决了这个问题。我已经添加了完整的代码和结果查询。代码在MSSQL服务器上执行。在下面的代码片段中，我使用了不同的表，并用您的需求中使用的表和列进行了屏蔽

from sqlalchemy import *
from sqlalchemy.ext.compiler import compiles
from sqlalchemy.types import String
from sqlalchemy.sql.expression import FromClause


class values(FromClause):
    def __init__(self, *args):
        self.column_names = args


@compiles(values)
def compile_values(element, compiler, asfrom=False, **kwrgs):
    values = "VALUES %s" % ", ".join("(%s)" % compiler.render_literal_value(elem, String()) for elem in element.column_names)
    if asfrom:
        values = "(%s)" % values
    return values


base_query = self.db_session.query(Schedule.Edit_DtTm.label("Schedule_Edit_DtTm"),
                                 Identity.Edit_DtTm.label("Identity_Edit_DtTm"),        
                                 Students.Edit_DtTm.label("Students_Edit_DtTm"),
                                 Identity.SSN                            
                                 ).outerjoin(Students, Schedule.stdnt_id==Students.Student_Id
                                 ).outerjoin(Identity, Schedule.std_id==Identity.std_id).subquery()

values_at_from_clause = values(("Students_Edit_DtTm"), ("Schedule_Edit_DtTm"), ("Identity_Edit_DtTm")
                                ).alias('values(MaxEditDate)')

get_max_from_values = self.db_session.query(func.max(text('MaxEditDate'))
                                            ).select_from(values_at_from_clause)

output_query = self.db_session.query(get_max_from_values.subquery()
                                     ).label("MaxEditDate")

**print output_query**


SELECT
    anon_1.Schedule_Edit_DtTm AS anon_1_Schedule_Edit_DtTm,
    anon_1.Students_Edit_DtTm AS anon_1_Students_Edit_DtTm,
    anon_1.Identity_Edit_DtTm AS anon_1_Identity_Edit_DtTm,
    anon_1.SSN AS anon_1_SSN
    (
        SELECT
            anon_2.max_1
        FROM
            (
                SELECT
                    max( MaxEditDate ) AS max_1
                FROM
                    (
                    VALUES (Students_Edit_DtTm),
                           (Schedule_Edit_DtTm),
                           (Identity_Edit_DtTm)
                       ) AS values(MaxEditDate)
            ) AS anon_2
    ) AS MaxEditDate
FROM
    (
        SELECT
            Schedule.Edit_DtTm AS Schedule_Edit_DtTm,
            Students.Edit_DtTm AS Students_Edit_DtTm,
            Identity.Edit_DtTm AS Identity_Edit_DtTm,
            Identity.SSN AS SSN
        FROM
            Schedule WITH(NOLOCK)
        LEFT JOIN Students WITH(NOLOCK) ON
            Schedule.stdnt_id==Students.Student_Id
        LEFT JOIN Identity WITH(NOLOCK) ON
            Schedule.std_id==Identity.std_id
    ) AS anon_1

---

我建议您添加一个示例数据，用于检索最大值，以及您在代码段中给出的当前解决方案的错误之处，并添加一些额外的调整（处理非文本），您应该能够从中使用配方。其中一个答案实际上与此类实施有关。我最初投票认为这是一个骗局，但另一方面，使用相关列而不是文字有区别。感谢@IljaEverilä，我以前看过那篇文章，希望不必使用配方，但这似乎是我唯一的选择。更改查询可能是我最好的选择，但是如果能够使用我的原始版本（出于好奇），那就太好了。现在我正在尝试这个，我仍然很难将它转换成SQL炼金术代码。上面的解决方案是从

from sqlalchemy import *
from sqlalchemy.ext.compiler import compiles
from sqlalchemy.types import String
from sqlalchemy.sql.expression import FromClause


class values(FromClause):
    def __init__(self, *args):
        self.column_names = args


@compiles(values)
def compile_values(element, compiler, asfrom=False, **kwrgs):
    values = "VALUES %s" % ", ".join("(%s)" % compiler.render_literal_value(elem, String()) for elem in element.column_names)
    if asfrom:
        values = "(%s)" % values
    return values


base_query = self.db_session.query(Schedule.Edit_DtTm.label("Schedule_Edit_DtTm"),
                                 Identity.Edit_DtTm.label("Identity_Edit_DtTm"),        
                                 Students.Edit_DtTm.label("Students_Edit_DtTm"),
                                 Identity.SSN                            
                                 ).outerjoin(Students, Schedule.stdnt_id==Students.Student_Id
                                 ).outerjoin(Identity, Schedule.std_id==Identity.std_id).subquery()

values_at_from_clause = values(("Students_Edit_DtTm"), ("Schedule_Edit_DtTm"), ("Identity_Edit_DtTm")
                                ).alias('values(MaxEditDate)')

get_max_from_values = self.db_session.query(func.max(text('MaxEditDate'))
                                            ).select_from(values_at_from_clause)

output_query = self.db_session.query(get_max_from_values.subquery()
                                     ).label("MaxEditDate")

**print output_query**


SELECT
    anon_1.Schedule_Edit_DtTm AS anon_1_Schedule_Edit_DtTm,
    anon_1.Students_Edit_DtTm AS anon_1_Students_Edit_DtTm,
    anon_1.Identity_Edit_DtTm AS anon_1_Identity_Edit_DtTm,
    anon_1.SSN AS anon_1_SSN
    (
        SELECT
            anon_2.max_1
        FROM
            (
                SELECT
                    max( MaxEditDate ) AS max_1
                FROM
                    (
                    VALUES (Students_Edit_DtTm),
                           (Schedule_Edit_DtTm),
                           (Identity_Edit_DtTm)
                       ) AS values(MaxEditDate)
            ) AS anon_2
    ) AS MaxEditDate
FROM
    (
        SELECT
            Schedule.Edit_DtTm AS Schedule_Edit_DtTm,
            Students.Edit_DtTm AS Students_Edit_DtTm,
            Identity.Edit_DtTm AS Identity_Edit_DtTm,
            Identity.SSN AS SSN
        FROM
            Schedule WITH(NOLOCK)
        LEFT JOIN Students WITH(NOLOCK) ON
            Schedule.stdnt_id==Students.Student_Id
        LEFT JOIN Identity WITH(NOLOCK) ON
            Schedule.std_id==Identity.std_id
    ) AS anon_1