Question

将我的sql查询转换为sqlalchemy时遇到很多麻烦。我无法找到任何资源来做我想做的事情。

我要转换的查询是：

SELECT 
     COALESCE(d.manager_name, e.name) AS name,
     COALESCE(d.department_name, e.department_name) AS department
FROM employee e
LEFT JOIN department d ON e.id = d.id 
WHERE e.date = '2018-11-05'

在sqlalchemy中，我想到了：

  query = self.session.query(
            func.coalesce(Department.manager_name, Employee.name),
            func.coalesce(Department.department_name, Employee.department_name)).join(Department, 
                    Employee.id == Department.id,
                ).filter(
                    Employee.date == '2018-11-05',
                )

但是请继续获取错误：

sqlalchemy.exc.InvalidRequestError: Can't join table/selectable 'Department' to itself.

为什么？！这些说法是准确的！

Answer 1

由于Department是查询中最左边的项目，因此对其进行联接。要控制联接中的第一个实体或“左”实体，请使用Query.select_from()：

query = self.session.query(
        func.coalesce(Department.manager_name, Employee.name),
        func.coalesce(Department.department_name, Employee.department_name)).\
    select_from(Employee).\
    outerjoin(Department, Employee.id == Department.id).\
    filter(Employee.date == '2018-11-05')

ORM教程的"Querying with Joins"和Query.join(): "Controlling what to Join From"下也对此行为进行了解释。

您的查询构造也使用Query.join()，尽管原始SQL具有LEFT JOIN。在这种情况下，应使用Query.outerjoin()或join(..., isouter=True)。

SQLAlchemy合并并加入

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