Обсуждение: Join help, please
I am confused. given this view:
AS
SELECT
employee.id ,
work_type.type ,
permit ,
work_type.overhead ,
work_type.descrip
from
permitted_work
inner join employee on
employee.employee_key = permitted_work.employee_key
inner join work_type on
work_type.work_type_key = work_type.work_type_key
;
Why do I have 38475 rows, when the base table only has 855?
My thinking was that the inner joins would constrain this view to the rows
that exist in the base (permitted_work) table.
Clearly I am misunderstanding something basic here.
--
"They that would give up essential liberty for temporary safety deserve
neither liberty nor safety."
-- Benjamin Franklin
work_type.work_type_key = work_type.work_type_key
You've done a cross join.
The three types of thing (permitted_work; employee; work_type) don’t stand in a 1:1 relationship with each other. You might have multiple work_types or permitted_work for each employee, I’m guessing.
Each existing combination produces one row in the result. So an employee with three permitted_works and 4 work types will produce 12 rows in the joined result.
If you want one row per employee, you might consider using array_agg with group_by to collapse the multiple work_types or permitted_works into arrays alongside the employee information.
On Mar 18, 2020, 11:51 -0700, stan <stanb@panix.com>, wrote:
I am confused. given this view:
AS
SELECT
employee.id ,
work_type.type ,
permit ,
work_type.overhead ,
work_type.descrip
from
permitted_work
inner join employee on
employee.employee_key = permitted_work.employee_key
inner join work_type on
work_type.work_type_key = work_type.work_type_key
;
Why do I have 38475 rows, when the base table only has 855?
My thinking was that the inner joins would constrain this view to the rows
that exist in the base (permitted_work) table.
Clearly I am misunderstanding something basic here.
--
"They that would give up essential liberty for temporary safety deserve
neither liberty nor safety."
-- Benjamin Franklin
On Wed, Mar 18, 2020, 12:55 PM Michael Lewis <mlewis@entrata.com> wrote:
work_type.work_type_key = work_type.work_type_keyYou've done a cross join.
You meant to do permitted_work.work_type_key = work_type.work_type_key I expect