Question

Do Foreign Key constraints get checked on an SQL update statement that doesn’t update the columns with the Constraint? (In MS SQL Server)

Say I have a couple of tables with the following columns:

OrderItems

    - OrderItemID
    - OrderItemTypeID (FK to a OrderItemTypeID column on another table called OrderItemTypes) 
    - ItemName

If I just update

update [dbo].[OrderItems]
set    [ItemName] = 'Product 3'
where  [OrderItemID] = 2508 

Will the FK constraint do it’s lookup/check with the update statement above? (even thought the update is not change the value of that column?)

Answer 1

No, the foreign key is not checked. This is pretty easy to see by examining the execution plans of two different updates.

create table a (
    id int primary key
)

create table b (
    id int, 
    fkid int
)

alter table b add foreign key (fkid) references a(id)

insert into a values (1)
insert into a values (2)

insert into b values (5,1) -- Seek on table a's PK

update b set id = 6 where id = 5 -- No seek on table a's PK

update b set fkid = 2 where id = 6 -- Seek on table a's PK

drop table b
drop table a