We need to use existing sql to create a bunch of tables and this sql includes ALTER TABLE statements to add in foreign keys. When we run this block of SQL, however, errors are thrown because tables needed for the foreign keys haven't been created yet (they are later on in the block of SQL).
Is there a way to turn off foreign key constraints at database level? (versus at table level) We just need to create all of the tables and foreign keys and then turn it back on.
Thanks!
To disable foreign key constraints when you want to truncate a table:
Use FOREIGN_KEY_CHECKS
SET FOREIGN_KEY_CHECKS=0;
and remember to enable it when you’re done:
SET FOREIGN_KEY_CHECKS=1;
Or you can use DISABLE KEYS:
ALTER TABLE table_name DISABLE KEYS;
Again, remember to enable if thereafter:
ALTER TABLE table_name ENABLE KEYS;
Note that DISABLE KEYS does not work on InnoDB tables as it works properly for MyISAM.
Use ON DELETE SET NULL
If you don’t want to turn key checking on and off, you can permanently modify it to ON DELETE SET NULL:
--Delete the current foreign key first:
ALTER TABLE table_name1 DROP FOREIGN KEY fk_name1;
ALTER TABLE table_name2 DROP FOREIGN KEY fk_name2;
--Then add the foreign key constraints back
ALTER TABLE table_name1
ADD FOREIGN KEY (table2_id)
REFERENCES table2(id)
ON DELETE SET NULL;
ALTER TABLE tablename2
ADD FOREIGN KEY (table1_id)
REFERENCES table1(id)
ON DELETE SET NULL;
In SSMS, if I open a table with constraints, right click on one and select "Script Constraint As" > "CREATE TO", these are the lines that are generated automatically:
ALTER TABLE [dbo].[table] DROP CONSTRAINT [CK_table_col]
GO
ALTER TABLE [dbo].[table] WITH CHECK ADD CONSTRAINT [CK_table_col] CHECK (([col] IN('SUP', 'ADM')))
GO
ALTER TABLE [dbo].[table] CHECK CONSTRAINT [CK_table_col]
GO
I get what the first two lines do, that's not a problem. However, the CHECK CONSTRAINT at the end seems unclear. If I remove it, it still checks that the existing records match the new constraint. If I run the CHECK CONSTRAINT on it's own, it just tells me that the query succeeded... Why is it needed? What does it do?
A CONSTRAINT can be disabled, using NOCHECK. This cannot be done when you CREATE the CONSTRAINT, but has to be done explicitly afterwards, such as with the following:
ALTER TABLE dbo.MyTable NOCHECK CONSTRAINT chk_MyTable;
If you disable a constraint and then use the CREATE TO script option, then that would be reflected too. As such SSMS explicitly defines whether the constraint is to be checked or not afterwards. Although yes enforcing the CHECK is redundant, it creates consistent scripts.
I have a SQLServer database which I want to drop a table from. The table has FK constraints, but in this case it doesn't matter because when I repopulate the table, the FKs will be replaced correctly.
I've done EXEC sp_msforeachtable "ALTER TABLE ? NOCHECK CONSTRAINT all" which gives a message: sp_msforeachtable "ALTER TABLE ? NOCHECK CONSTRAINTS all" succeded, but when I try to drop my table I get the following SQL error: SQL Error: Could not drop object 'myTable' because ti is referenced by a FOREIGN KEY constraint.
Should the command not have affected my entire database and allowed me to drop the table without an issue? I've also tried doing EXEC sp_msforeachtable "ALTER TABLE myTable NOCHECK CONSTRAINT all" which results in the same error.
NOCheck disable the constraints so they won't be enforced. This would allow you to delete the data without violating the constraint.
Dropping the table would make the constraint definition invalid. You can't have a constraint that references a table that doesn't exist. You wouldn't be able to drop the referenced column from the table either while the constraint exists.
If you will be repopulating the table, just delete the table data and reload it. If you absolutely must drop and recreate the table, you'll need to include the drop & create statements for your foreign key constraints as well.
Note that if you disable the constraint, you'll need to enable using WITH CHECK CHECK (yes twice). The first check turns the constraint on for new data, the second tells SQL to validate the existing data. If you only do one, new data will be checked, but the existing data will not be 'TRUSTED', which can affect how SQL will leverage the FK reference in queries.
Should the command not have affected my entire database and allowed me
to drop the table without an issue?
No. It doesn't matter what the NOCHECK state is; if there are FK constraints that reference a target table, that target table cannot be dropped.
The only way to drop the table is to first drop the FK constraints that reference it.
I have a table in my database and I did a: Script table as > Create to > query editor and some of my columns have this code:
ALTER TABLE [dbo].[TableName] WITH CHECK ADD CONSTRAINT [ConstraintName] FOREIGN KEY([ColumnName])
REFERENCES [dbo].[ForeignKeyTable] ([ColumnName])
GO
ALTER TABLE [dbo].[TableName] CHECK CONSTRAINT [ConstraintName2]
GO
What does the second constraint do? Other columns in the table only have the first constraint.
The second constraint turns on (activates) the check constraint. It is just template code from SSMS
It is possible to generate the create check constraints snippet with or without activating it (e.g. nocheck) - so that is itself one of 2 snippets. Because you opted for the constraints to end up activated, SSMS follows with the activation, not really caring if it was activated already or not. It doesn't really hurt and is just something SSMS does
I have a row in a table. This row has an ID column referenced in a few other tables with millions of rows. The SQL statement to delete the row always times out. From my design, I know the row I wish to delete is never referenced any where else. Hence I would like SQL to ignore having to check all other tables for a foreign key reference to this row and delete the row immediately. Is there a quick way to do this in SQL 2008?
Perhaps something along the lines of:
DELETE FROM myTable where myTable.ID = 6850 IGNORE CONSTRAINTS
Or something along those lines.
You can set the constraints on that table / column to not check temporarily, then re-enable the constraints. General form would be:
ALTER TABLE TableName NOCHECK CONSTRAINT ConstraintName
Then re-enable all constraints with
ALTER TABLE TableName CHECK CONSTRAINT ConstraintName
I assume that this would be temporary though? You obviously wouldn't want to do this consistently.
Yes, simply run
DELETE FROM myTable where myTable.ID = 6850
AND LET ENGINE VERIFY THE CONSTRAINTS.
If you're trying to be 'clever' and disable constraints, you'll pay a huge price: enabling back the constraints has to verify every row instead of the one you just deleted. There are internal flags SQL keeps to know that a constraint is 'trusted' or not. You're 'optimization' would result in either changing these flags to 'false' (meaning SQL no longer trusts the constraints) or it has to re-verify them from scratch.
See Guidelines for Disabling Indexes and Constraints and Non-trusted constraints and performance.
Unless you did some solid measurements that demonstrated that the constraint verification of the DELETE operation are a performance bottleneck, let the engine do its work.
Do not under any circumstances disable the constraints. This is an extremely stupid practice. You cannot maintain data integrity if you do things like this. Data integrity is the first consideration of a database because without it, you have nothing.
The correct method is to delete from the child tables before trying to delete the parent record. You are probably timing out because you have set up cascading deltes which is another bad practice in a large database.
I know this is an old thread, but I landed here when my row deletes were blocked by foreign key constraints. In my case, my table design permitted "NULL" values in the constrained column. In the rows to be deleted, I changed the constrained column value to "NULL" (which does not violate the Foreign Key Constraint) and then deleted all the rows.
I wanted to delete all records from both tables because it was all test data. I used SSMS GUI to temporarily disable a FK constraint, then I ran a DELETE query on both tables, and finally I re-enabled the FK constraint.
To disable the FK constraint:
expand the database object [1]
expand the dependant table object [2]
expand the 'Keys' folder
right click on the foreign key
choose the 'Modify' option
change the 'Enforce Foreign Key Constraint' option to 'No'
close the 'Foreign Key Relationships' window
close the table designer tab
when prompted confirm save changes
run necessary delete queries
re-enable foreign key constraint the same way you just disabled it.
[1] in the 'Object Explorer' pane, can be accessed via the 'View' menu option, or key F8
[2] if you're not sure which table is the dependant one, you can check by right clicking the table in question and selecting the 'View Dependencies' option.
This is the way to disable foreign key checks in MySQL. Not relevant to OP's question since they use MS SQL Server, but google search results do turn this up so here's for reference:
SET FOREIGN_KEY_CHECKS = 0;
/ Run your script /
SET FOREIGN_KEY_CHECKS = 1;
See if this helps, This is for ignoring the foreign key checks.
But deleting disabling this is very bad practice.
On all tables with foreign keys pointing to this one, use:
ALTER TABLE MyOtherTable NOCHECK CONSTRAINT fk_name
You can disable all of the constaints on your database by the following line of code:
EXEC sp_MSforeachtable "ALTER TABLE ? NOCHECK CONSTRAINT all"
and after the runing your update/delete command, you can enable it again as the following:
EXEC sp_MSforeachtable "ALTER TABLE ? WITH CHECK CHECK CONSTRAINT all"
You could maybe disable and re-enable constraints:
http://sqlforums.windowsitpro.com/web/forum/messageview.aspx?catid=60&threadid=48410&enterthread=y
For the testing purpose only, I used the following command in MySQL to delete only one record from a table that has foreign key references.
SET FOREIGN_KEY_CHECKS = 0; -- Disabling foreign key checks before running the following query.
DELETE FROM table_name WHERE id = id_to_delete; -- Deleting a record from a table that has foreign key reference.
SET FOREIGN_KEY_CHECKS = 1; -- Enabling foreign key checks after running the above query.
Temporarily disable constraints on a table T-SQL, SQL Server
MSSQL
ALTER TABLE TableName NOCHECK CONSTRAINT ALL
ALTER TABLE TableName CHECK CONSTRAINT ALL
ALTER TABLE TableName NOCHECK CONSTRAINT FK_Table_RefTable
ALTER TABLE TableName CHECK CONSTRAINT FK_Table_RefTable
ref
DELETE FROM TableName
DBCC CHECKIDENT ('TableName', RESEED, 0)
MySql
SET FOREIGN_KEY_CHECKS = 0; -- Disable foreign key checking.
TRUNCATE TABLE [YOUR TABLE];
SET FOREIGN_KEY_CHECKS = 1;