How to list out all Foreign Keys with "WITH NOCHECK" in SQL Server

Solution 1:

The following will return the name of the foreign keys in the current database that are disabled i.e. WITH NOCHECK

For SQL Server 2005/2008:

select * from sys.foreign_keys where is_disabled=1

-- drop table t1
-- drop table t2
create table t1(i int not null, fk int not null)
create table t2(i int not null)
-- create primary key on t2
alter table t2
add constraint pk_1 primary key (i)
-- create foriegn key on t1
alter table t1
add constraint fk_1 foreign key (fk)
    references t2 (i)
--insert some records
insert t2 values(100)
insert t2 values(200)
insert t2 values(300)
insert t2 values(400)
insert t2 values(500)
insert t1 values(1,100)
insert t1 values(2,100)
insert t1 values(3,500)
insert t1 values(4,500)
----------------------------
-- 1. enabled and trusted
select name,is_disabled,is_not_trusted from sys.foreign_keys
GO

-- 2. disable the constraint
alter table t1 NOCHECK CONSTRAINT fk_1
select name,is_disabled,is_not_trusted from sys.foreign_keys
GO

-- 3. re-enable constraint, data isnt checked, so not trusted.
-- this means the optimizer will still have to check the column
alter table  t1 CHECK CONSTRAINT fk_1 
select name,is_disabled,is_not_trusted from sys.foreign_keys
GO

--4. drop the foreign key constraint & re-add 
-- it making sure its checked
-- constraint is then enabled and trusted
alter table t1  DROP CONSTRAINT fk_1
alter table t1 WITH CHECK 
add constraint fk_1 foreign key (fk)
    references t2 (i)
select name,is_disabled,is_not_trusted from sys.foreign_keys
GO


--5. drop the foreign key constraint & add but dont check
-- constraint is then enabled, but not trusted
alter table t1  DROP CONSTRAINT fk_1
alter table t1 WITH NOCHECK 
add constraint fk_1 foreign key (fk)
    references t2 (i)
select name,is_disabled,is_not_trusted from sys.foreign_keys
GO

is_disabled means the constraint is disabled

isnottrusted means that SQL Server does not trust that the column has been checked against the foreign key table.

Thus it cannot be assumed that re-enabling the foreign key constraint will be optimized. To ensure the optimizer trusts the column, it's best to drop the foreign key constraint & re-create it with the WITH CHECK option (4.)

Solution 2:

SELECT * FROM sys.foreign_keys AS f Where Is_Not_Trusted = 1

Solution 3:

The following script will generate the alter statements that will both check existing data and prevent any new violations for foreign keys that are not currently trusted ('with nocheck').

Execute it in SQL Server Management Studio to generate the scripts and then copy them into a query window to execute them.

select
    'alter table ' + quotename(s.name) + '.' + quotename(t.name) + ' with check check constraint ' + fk.name +';'
from 
    sys.foreign_keys fk
inner join
    sys.tables t
on
    fk.parent_object_id = t.object_id
inner join
    sys.schemas s
on
    t.schema_id = s.schema_id
where 
    fk.is_not_trusted = 1