[sql-server] How to remove foreign key constraint in sql server?

I want to remove foreign key from another table so i can insert values of my choice.

I am new in databases so please tell me correct sql query to drop or remove foreign key value.

Its wrong to do that in refer to referential integrity, because once its broken its not easy to turn it on again without having to go through the records and delete the ones which breaks the constraints.

Anyway the Syntax is as follows:

ALTER TABLE Tablename DROP CONSTRAINT ContName;

See MSDN:

• Delete Primary Keys
• Delete Foreign Key Relationships

You should consider (temporarily) disabling the constraint before you completely delete it.

If you look at the table creation TSQL you will see something like:

ALTER TABLE [dbo].[dbAccounting] CHECK CONSTRAINT [FK_some_FK_constraint]

You can run

ALTER TABLE [dbo].[dbAccounting] NOCHECK CONSTRAINT [FK_some_FK_constraint]

... then insert/update a bunch of values that violate the constraint, and then turn it back on by running the original CHECK statement.

(I have had to do this to cleanup poorly designed systems I've inherited in the past.)

Use those queries to find all FKs:

Declare @SchemaName VarChar(200) = 'Schema Name'
Declare @TableName VarChar(200) = 'Table name'

-- Find FK in This table.
SELECT 
    'IF  EXISTS (SELECT * FROM sys.foreign_keys WHERE object_id = OBJECT_ID(N''' + 
      '[' + OBJECT_SCHEMA_NAME(FK.parent_object_id) + '].[' + FK.name + ']' 
      + ''') AND parent_object_id = OBJECT_ID(N''' + 
      '[' + OBJECT_SCHEMA_NAME(FK.parent_object_id) + '].[' 
      + OBJECT_NAME(FK.parent_object_id) + ']' + ''')) ' +

    'ALTER TABLE ' +  OBJECT_SCHEMA_NAME(FK.parent_object_id) +
    '.[' + OBJECT_NAME(FK.parent_object_id) + 
    '] DROP CONSTRAINT ' + FK.name
    , S.name , O.name, OBJECT_NAME(FK.parent_object_id)
FROM sys.foreign_keys AS FK
INNER JOIN Sys.objects As O 
  ON (O.object_id = FK.parent_object_id )
INNER JOIN SYS.schemas AS S 
  ON (O.schema_id = S.schema_id)  
WHERE 
      O.name = @TableName
      And S.name = @SchemaName


-- Find the FKs in the tables in which this table is used
  SELECT 
    ' IF  EXISTS (SELECT * FROM sys.foreign_keys WHERE object_id = OBJECT_ID(N''' + 
      '[' + OBJECT_SCHEMA_NAME(FK.parent_object_id) + '].[' + FK.name + ']' 
      + ''') AND parent_object_id = OBJECT_ID(N''' + 
      '[' + OBJECT_SCHEMA_NAME(FK.parent_object_id) + '].[' 
      + OBJECT_NAME(FK.parent_object_id) + ']' + ''')) ' +

    ' ALTER TABLE ' +  OBJECT_SCHEMA_NAME(FK.parent_object_id) +
    '.[' + OBJECT_NAME(FK.parent_object_id) + 
    '] DROP CONSTRAINT ' + FK.name
    , S.name , O.name, OBJECT_NAME(FK.parent_object_id)
FROM sys.foreign_keys AS FK
INNER JOIN Sys.objects As O 
  ON (O.object_id = FK.referenced_object_id )
INNER JOIN SYS.schemas AS S 
  ON (O.schema_id = S.schema_id)  
WHERE 
      O.name = @TableName
      And S.name = @SchemaName 

Drop all the foreign keys of a table:

USE [Database_Name]
DECLARE @FOREIGN_KEY_NAME VARCHAR(100)

DECLARE FOREIGN_KEY_CURSOR CURSOR FOR
SELECT name FOREIGN_KEY_NAME FROM sys.foreign_keys WHERE parent_object_id = (SELECT object_id FROM sys.objects WHERE name = 'Table_Name' AND TYPE = 'U')

OPEN FOREIGN_KEY_CURSOR
----------------------------------------------------------
FETCH NEXT FROM FOREIGN_KEY_CURSOR INTO @FOREIGN_KEY_NAME
WHILE @@FETCH_STATUS = 0
    BEGIN
       DECLARE @DROP_COMMAND NVARCHAR(150) = 'ALTER TABLE Table_Name DROP CONSTRAINT' + ' ' + @FOREIGN_KEY_NAME

       EXECUTE Sp_executesql @DROP_COMMAND

       FETCH NEXT FROM FOREIGN_KEY_CURSOR INTO @FOREIGN_KEY_NAME

    END
-----------------------------------------------------------------------------------------------------------------
CLOSE FOREIGN_KEY_CURSOR
DEALLOCATE FOREIGN_KEY_CURSOR

alter table <referenced_table_name> drop  primary key;

Foreign key constraint will be removed.

Alternatively, you can also delete a Foreign Key Constraint from the SQL Server Management Studio itself. You can try it if the commands do not work.

1. Expand your database view.
2. Right Click on Table which has foreign key constraint. Choose Design. A tab with the information about table columns will open.
3. Right click on the column which has the foreign key reference. Or you can right click on any column. Choose Relationships.
4. A list of relationships will appear (if you have one) in a pop up window.
5. From there you can delete the foreign key constraint.

I hope that helps

To remove all the constraints from the DB:

SELECT 'ALTER TABLE ' + Table_Name  +' DROP CONSTRAINT ' + Constraint_Name
FROM Information_Schema.CONSTRAINT_TABLE_USAGE

To be on the safer side, just name all your constraints and take note of them in the comment section.

ALTER TABLE[table_name]
DROP CONSTRAINT Constraint_name

ALTER TABLE table
DROP FOREIGN KEY fk_key

EDIT: didn't notice you were using sql-server, my bad

ALTER TABLE table
DROP CONSTRAINT fk_key

ALTER TABLE [TableName] DROP CONSTRAINT [CONSTRAINT_NAME]

But, be careful man, once you do that, you may never get a chance back, and you should read some basic database book see why we need foreign key

Depending on the DB you are using there's a syntax or another.

If you're using Oracle you have to put what the other users told you:

ALTER TABLE table_name DROP CONSTRAINT fk_name;

But if you use MySQL then this will give you a syntax error, instead you can type:

ALTER TABLE table_name DROP INDEX fk_name;

firstly use

show create table table_name;

to see the descriptive structure of your table.

There you may see constraints respective to foreign keys you used in that table. First delete the respective constraint with

alter table table_name drop constraint constraint_name;

and then delete the respective foreign keys or column you wanted...GoodLuck!!

If you find yourself in a situation where the FK name of a table has been auto-generated and you aren't able to view what it exactly is (in the case of not having rights to a database for instance) you could try something like this:

DECLARE @table NVARCHAR(512), @sql NVARCHAR(MAX);
SELECT @table = N'dbo.Table';
SELECT @sql = 'ALTER TABLE ' + @table
    + ' DROP CONSTRAINT ' + NAME + ';'
    FROM sys.foreign_keys
    WHERE [type] = 'F'
    AND [parent_object_id] = OBJECT_ID(@table);
EXEC sp_executeSQL @sql;

Build up a stored proc which drops the constraint of the specified table without specifying the actual FK name. It drops the constraint where the object [type] is equal to F (Foreign Key constraint).

Note: if there are multiple FK's in the table it will drop them all. So this solution works best if the table you are targeting has just one FK.

If you don't know foreign key constraint name then try this to find it.

sp_help 'TableName'   

additionally for different schema

sp_help 'schemaName.TableName'   

then

ALTER TABLE <TABLE_NAME> DROP CONSTRAINT <FOREIGN_KEY_NAME>