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    <p>It would be interesting to know which version of SQL Server you are using, because in SQL Server 2005 and 2008, the query plan shows all deletes against the clustered index are completed before the referential integrity checks are done. Constraint checking <em>is</em> deferred!</p> <p>The groupId values deleted are captured by an Eager Spool, after the delete operator. A sequence operator at the root of the plan then plays the second branch, which reads all rows from the spool and checks for other rows that reference the deleted rows using a semi-join. The last operation in the plan is an Assert, which will raise an error if referential integrity has been compromised.</p> <p>Reproduction script, using the data shown in the MPTT article referenced in the original question:</p> <pre><code> USE tempdb ; IF OBJECT_ID(N'dbo.Test', N'U') IS NOT NULL DROP TABLE dbo.Test ; CREATE TABLE dbo.Test ( groupId UNIQUEIDENTIFIER NOT NULL PRIMARY KEY NONCLUSTERED, parentId UNIQUEIDENTIFIER REFERENCES dbo.Test(groupId) NULL, name NVARCHAR(50) NOT NULL, lft INTEGER NOT NULL, rgt INTEGER NOT NULL, ) ; CREATE NONCLUSTERED INDEX nc1 ON dbo.Test (parentId) ; INSERT dbo.Test (groupId, parentId, name, lft, rgt) VALUES ({guid '00000000-0000-0000-0000-000000000000'}, NULL, N'Food', 1, 18), ({guid '99163693-B535-47A1-9F5B-E5F235CEB8E2'}, {guid '00000000-0000-0000-0000-000000000000'}, N'Fruit', 2, 11), ({guid '6A3FFED1-0230-4909-A7D4-DD9CBA674A1D'}, {guid '99163693-B535-47A1-9F5B-E5F235CEB8E2'}, N'Red', 3, 6), ({guid '2E7A25D8-8555-4FEC-A0D3-8B8E60501F6A'}, {guid '6A3FFED1-0230-4909-A7D4-DD9CBA674A1D'}, N'Cherry', 4, 5), ({guid '20613684-1081-4A42-AEAE-F21250D56703'}, {guid '99163693-B535-47A1-9F5B-E5F235CEB8E2'}, N'Yellow', 7, 10), ({guid 'B6D5A4AB-1A10-4069-842D-AAE09E73B3CD'}, {guid '20613684-1081-4A42-AEAE-F21250D56703'}, N'Banana', 8, 9), ({guid '174FF0C3-BAE0-4023-8819-EC89E080834C'}, {guid '00000000-0000-0000-0000-000000000000'}, N'Meat', 12, 17), ({guid '3CA5356E-343B-461C-8602-B9ECAEA1304D'}, {guid '174FF0C3-BAE0-4023-8819-EC89E080834C'}, N'Beef', 13, 14), ({guid '79664130-ACDF-48CE-A33D-7473153819E8'}, {guid '174FF0C3-BAE0-4023-8819-EC89E080834C'}, N'Pork', 15, 16) ; DELETE dbo.Test WHERE groupId > {guid '00000000-0000-0000-0000-000000000000'} ; </code></pre>
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    1. POSQL: DELETE data from self-referencing table in specific order
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    1. USPaul White
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    1. COHow is the "{guid '00000000-0000-0000-0000-000000000000'}" syntax different from using '00000000-0000-0000-0000-000000000000'? I've just seen the {guid '00000000-0000-0000-0000-000000000000'} syntax for the first time recently and have not been able to find documentation for it.
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    2. CO@firedfly - This is documented here http://msdn.microsoft.com/en-us/library/ms712494%28v=VS.85%29.aspx
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      1. USMartin Smith
    3. COThanks, Martin. @firedfly: the {guid ''} syntax returns a value of type UNIQUEIDENTIFIER. When you write it the other way, it is a value of type VARCHAR(36). It often doesn't matter much, but I prefer to be explicit about types where possible. The alternative is to write e.g. CONVERT(UNIQUEIDENTIFIER, '00000000-0000-0000-0000-000000000000').
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