How to List Out All Foreign Keys With "WITH NOCHECK" Applied in SQL Server

Understanding Foreign Keys in SQL Server and How to List Them Out With NOCHECK

SQL Server is a powerful relational database management system that uses foreign keys to establish relationships between tables. A foreign key is a field or set of fields in one table that refers to the primary key of another table. In this article, we will explore how to list out all foreign keys with “WITH NOCHECK” applied to it in SQL Server.

Introduction

Foreign keys are an essential concept in database design and are used to enforce referential integrity between tables. They ensure that data is consistent across related tables by preventing the insertion of invalid or inconsistent data. In this article, we will focus on how to list out all foreign keys with “WITH NOCHECK” applied to it.

What Does “WITH NOCHECK” Mean?

In SQL Server, “WITH NOCHECK” is an option that can be applied to foreign key constraints. It disables the constraint, allowing you to make changes to your data without enforcing referential integrity. When a foreign key constraint is disabled with “WITH NOCHECK”, any inserts or updates made to the parent table do not trigger checks against the dependent table.

Listing Out Foreign Keys With “WITH NOCHECK”

To list out all foreign keys in a database that are applied with “WITH NOCHECK”, you can use the following T-SQL query:

SELECT name, is_disabled, is_not_trusted FROM sys.foreign_keys WHERE is_disabled = 1;

This query will return a list of all foreign key constraints that are currently disabled.

Understanding the Difference Between Disabled and Not Trusted

There was some discussion in the original Stack Overflow post about the difference between disabled and not trusted. In this section, we’ll explore the distinction between these two concepts.

Disabled Foreign Key Constraint

A disabled foreign key constraint means that SQL Server does not enforce referential integrity for a particular relationship between tables. When a foreign key constraint is disabled, any inserts or updates made to the parent table do not trigger checks against the dependent table. However, re-enabling the foreign key constraint will allow SQL Server to trust the column and optimize queries accordingly.

Not Trusted Foreign Key Constraint

A not trusted foreign key constraint means that SQL Server does not trust that a particular column has been checked against the foreign key table. This can occur when a foreign key constraint is disabled with “WITH NOCHECK” or when a column’s values are not consistent with the corresponding values in the dependent table. When a foreign key constraint is not trusted, re-enabling it will require SQL Server to perform additional checks to ensure data consistency.

Example Scenario

To illustrate the difference between disabled and not trusted foreign key constraints, let’s consider an example scenario:

Suppose we have two tables: t1 and t2. The t1 table has a foreign key constraint on column fk that references the primary key of t2.

-- create tables
CREATE TABLE t1 (i INT NOT NULL, fk INT NOT NULL);
CREATE TABLE t2 (i INT NOT NULL PRIMARY KEY);

-- insert some records into t2
INSERT INTO t2 VALUES(100), (200), (300), (400), (500);

-- insert some records into t1 with valid foreign key values
INSERT INTO t1 VALUES(1, 100), (2, 100), (3, 500);

In this scenario, the foreign key constraint on t1 is enabled and trusted. If we make any changes to the data in t1, SQL Server will enforce referential integrity by checking the values against the corresponding values in t2.

Now, let’s disable the foreign key constraint with “WITH NOCHECK”:

-- disable foreign key constraint with WITH NOCHECK
ALTER TABLE t1 NOCHECK CONSTRAINT fk_1;

-- insert some records into t1 with invalid foreign key values
INSERT INTO t1 VALUES(4, 100), (5, 500);

In this scenario, the foreign key constraint is disabled with “WITH NOCHECK”. Any inserts or updates made to t1 do not trigger checks against t2. However, if we re-enable the foreign key constraint without specifying the “WITH CHECK” option, SQL Server will not trust the column and may perform additional checks to ensure data consistency.

-- re-enable foreign key constraint
ALTER TABLE t1 WITH NOCHECK CONSTRAINT fk_1;

-- insert some records into t1 with invalid foreign key values (this should trigger an error)
INSERT INTO t1 VALUES(6, 100);

In this scenario, the foreign key constraint is enabled without specifying the “WITH CHECK” option. Any inserts or updates made to t1 will trigger checks against t2, and SQL Server may perform additional checks to ensure data consistency.

Conclusion

Foreign keys are an essential concept in database design that help enforce referential integrity between tables. In this article, we explored how to list out all foreign keys with “WITH NOCHECK” applied to it in SQL Server. We also discussed the difference between disabled and not trusted foreign key constraints and provided an example scenario to illustrate the distinction.

Last modified on 2023-07-05