Disabling foreign key constraint, still can't truncate table? (SQL Server 2005)

Question

I have a table called PX_Child that has a foreign key on PX_Parent. I\'d like to temporarily disable this FK constraint so that I can truncate PX_Parent. I\'m not sure how

Answer 1

SQL server will not let you truncate the table while the constraint exists, even if it's disabled. Drop the constraint and re-create it after truncating the table. Or just drop and re-create the tables, whichever is easier to do in your application.

Answer 2

There is an easier-ish way. I was faced with the same problem and found this solution: https://www.mssqltips.com/sqlservertip/3347/drop-and-recreate-all-foreign-key-constraints-in-sql-server/

If you just run this query in your DB, it will generate the T-SQL you need to include before/after your sproc, in order to delete and then restore any foreign key constraints.

Don't worry about trying to understand this query itself.

CREATE TABLE #x -- feel free to use a permanent table
(
  drop_script NVARCHAR(MAX),
  create_script NVARCHAR(MAX)
);

DECLARE @drop   NVARCHAR(MAX) = N'',
        @create NVARCHAR(MAX) = N'';

-- drop is easy, just build a simple concatenated list from sys.foreign_keys:
SELECT @drop += N'
ALTER TABLE ' + QUOTENAME(cs.name) + '.' + QUOTENAME(ct.name) 
    + ' DROP CONSTRAINT ' + QUOTENAME(fk.name) + ';'
FROM sys.foreign_keys AS fk
INNER JOIN sys.tables AS ct
  ON fk.parent_object_id = ct.[object_id]
INNER JOIN sys.schemas AS cs 
  ON ct.[schema_id] = cs.[schema_id];

INSERT #x(drop_script) SELECT @drop;

-- create is a little more complex. We need to generate the list of 
-- columns on both sides of the constraint, even though in most cases
-- there is only one column.
SELECT @create += N'
ALTER TABLE ' 
   + QUOTENAME(cs.name) + '.' + QUOTENAME(ct.name) 
   + ' ADD CONSTRAINT ' + QUOTENAME(fk.name) 
   + ' FOREIGN KEY (' + STUFF((SELECT ',' + QUOTENAME(c.name)
   -- get all the columns in the constraint table
    FROM sys.columns AS c 
    INNER JOIN sys.foreign_key_columns AS fkc 
    ON fkc.parent_column_id = c.column_id
    AND fkc.parent_object_id = c.[object_id]
    WHERE fkc.constraint_object_id = fk.[object_id]
    ORDER BY fkc.constraint_column_id 
    FOR XML PATH(N''), TYPE).value(N'.[1]', N'nvarchar(max)'), 1, 1, N'')
  + ') REFERENCES ' + QUOTENAME(rs.name) + '.' + QUOTENAME(rt.name)
  + '(' + STUFF((SELECT ',' + QUOTENAME(c.name)
   -- get all the referenced columns
    FROM sys.columns AS c 
    INNER JOIN sys.foreign_key_columns AS fkc 
    ON fkc.referenced_column_id = c.column_id
    AND fkc.referenced_object_id = c.[object_id]
    WHERE fkc.constraint_object_id = fk.[object_id]
    ORDER BY fkc.constraint_column_id 
    FOR XML PATH(N''), TYPE).value(N'.[1]', N'nvarchar(max)'), 1, 1, N'') + ');'
FROM sys.foreign_keys AS fk
INNER JOIN sys.tables AS rt -- referenced table
  ON fk.referenced_object_id = rt.[object_id]
INNER JOIN sys.schemas AS rs 
  ON rt.[schema_id] = rs.[schema_id]
INNER JOIN sys.tables AS ct -- constraint table
  ON fk.parent_object_id = ct.[object_id]
INNER JOIN sys.schemas AS cs 
  ON ct.[schema_id] = cs.[schema_id]
WHERE rt.is_ms_shipped = 0 AND ct.is_ms_shipped = 0;

UPDATE #x SET create_script = @create;

PRINT @drop;
PRINT @create;

/*
EXEC sp_executesql @drop
-- clear out data etc. here
EXEC sp_executesql @create;
*/

Generates a bunch of:

ALTER TABLE [dbo].[Whatever] DROP CONSTRAINT....
--
ALTER TABLE [dbo].[Whatever] ADD CONSTRAINT....

Answer 3

There is no such option to truncate table while foreign key constraint but we can use some trick like

 ALTER TABLE [dbo].[table2] DROP CONSTRAINT [FK_table2_table1]
    GO
    truncate table [table1]
GO
    ALTER TABLE [dbo].[table2]  WITH CHECK ADD  CONSTRAINT [FK_table2_table1] FOREIGN KEY([FKId])
    REFERENCES [dbo].[table1] ([ID])
    GO

    ALTER TABLE [dbo].[table2] CHECK CONSTRAINT [FK_table2_table1]
    GO

Answer 4

You can't truncate the table if there is any foreign key referencing it, including disabled constraints. You either need to drop the foreign key constraints or use the DELETE command.