如何在 SqlAlchemy 中執行“左外連接”

Question

我需要執行這個查詢::

select field11, field12
from Table_1 t1
left outer join Table_2 t2 ON t2.tbl1_id = t1.tbl1_id
where t2.tbl2_id is null

我在 python 中有這些類：

class Table1(Base):
   ....

class Table2(Base):
   table_id =  Column(
        Integer,
        ForeignKey('Table1.id', ondelete='CASCADE'),
    )
    ....

我如何從下面到達上面？

Answer 1

q = session.query(Table1.field1, Table1.field2)\
    .outerjoin(Table2)\ # use in case you have relationship defined
    # .outerjoin(Table2, Table1.id == Table2.table_id)\ # use if you do not have relationship defined
    .filter(Table2.tbl2_id == None)

應該這樣做，假設field1和field2來自Table1 ，並且您定義了一個關系：

class Table2(Base):
    # ...
    table1 = relationship(Table1, backref="table2s")

Answer 2

您也可以僅使用 SQLAlchemy Core 執行此操作：

session.execute(
    select(['field11', 'field12'])
    .select_from(
        Table1.outerjoin(Table2, Table1.tbl1_id == Table2.tbl1_id))
    .where(Table2.tbl2_id.is_(None))
)

PS .outerjoin(table, condition)等價於.join(table, condition, isouter=True) 。