# referencing data from another table without using a join

• Hi all so I have a homework assignment that I am having difficulty understanding. the problem I have been given is this:

ix. Referring to the customers and shipping_details tables on the ER, display all data from the customers table but only where the province on the shipping_details table is equal to NS. Do not use a join for this requirement.

So I can complete the question easily enough using a join but I am having difficulty understanding how one can reference another table without using a join. I am trying to use a vector aggregate function but I am very unclear on how to reference a table and a value within a column of that table without using a join

• Since you don't need to return any data from the shipping_details table, you can use a https://en.wikipedia.org/wiki/Relational_algebra#Semijoin_(%E2%8B%89)(%E2%8B%8A) , which can be written with either `IN` and a subquery or `EXISTS` and a correlated subquery.

Examples:

``````-- Using IN --
select c.*
from customers as c
where c.shipping_detail_id IN
( select sd.shipping_detail_id
from shipping_details as sd
where sd.province = 'NS'
) ;
-- Using EXISTS --
select c.*
from customers as c
where EXISTS
( select 1
from shipping_details as sd
where sd.province = 'NS'
and sd.shipping_detail_id = c.shipping_detail_id    -- correlation
) ;
``````

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