如何从一个表中选择所有行，并根据另一表计算字段值

Question

I have a table merchandises and merchandise_groups_merchandise . The latter table is the middle table for storing which merchandise is selected inside which merchandise groups - a many to many. There's a relation from merchandises.id to merchandise_groups_merchandise.merchandise_id .

I'm trying to put together a query to produce the following results, listing all the merchandise from the merchandises table and where is_selected is a boolean decided by whether the merchandise_id is in the merchandise_groups_merchandise table for a specific merchandise_group. And, the merchandise_groups_merchandise.group_id is specified by the user.

I've tried a LEFT join, between the two tables, but of course, that only returns the actual merchandise that's in the group middle table.

How can I do this?

-------------------------------
|id  |title  |is_selected
---------------------------
|1   |Tree   |1
|2   |Log    |0
|3   |Toy    |1

This is the SQL of the query that I've tried:

PARAMETERS group_id Short;
SELECT IIf(IsNull([merchandise_groups_merchandise].[merchandise_id]),False,True) AS selected, merchandises.id, merchandises.title
FROM merchandises LEFT JOIN merchandise_groups_merchandise ON merchandises.id = merchandise_groups_merchandise.merchandise_id
WHERE (((merchandise_groups_merchandise.merchandise_group_id)=[group_id]));

Answer 1

One method is with iif() and an exists subquery:

select m.*,
       iif(exists (select 1
                   from merchandise_groups_merchandise as mgm
                   where m.id = mgm.merchandise_id and
                         mgm.merchandise_group_id = [group_id]
                  )), 1, 0) as IsInGroupFlag
from merchandizes as m