SQL Server How to Select * from table_name where in (@parameter )

Question

i have data @P = (000000000111,000000000222) from table_B

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this code can exec complete

select * from table_A where id in (000000000111,000000000222)

but this code can not exec complete

declare @P nvarchar(100)
set @P = (select id from table_B)
select @P
select * from merchant where id in (@P)

how to this code can exec complete

Answer 1

in is a set operation, it works with sets, not with strings are you are assuming.

There are multiple ways to select rows from one table where the id exists in another, for example

select * from merchant m
where exists (select * from table_B b where b.id=m.id)

or in line with your thinking

select * from merchant
where id in (select id from table_B)

Here, the in operates on a set not a string.

Answer 2

First, you don't need a parameter:

select *
from merchant
where id in (select id from table_b);

I would suggest exists instead, but that is a nuance.

If you did want to store the values, you would use a table:

declare @p table (id nvarchar(100));

insert into @p (id)
    select id
    from table_b;

select *
from merchant m
where exists (select 1 from @p p where p.id = m.id);

Of course, this is just a more verbose version of the first query.