Strange constraints in generated SQL

Note: This question is not related to Visual Paradigm. Anyone that knows SQL could answer it.

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I am using Visual Paradigm to model a database in our project (using ER diagrams). When Visual Paradigm generates the SQL equivalent for the database and I import it in MSSQL it works pretty.

I took a look in generated SQL code to make sure anything is right and I saw something strange!:

For tblContracts I defined a constraint named EndAfterStart to make sure the value of endDate is always bigger than startDate. The generated SQL code for this constraint is here:

IF NOT EXISTS (SELECT * FROM sys.check_constraints WHERE object_id=OBJECT_ID(N'[dbo].[EndAfterStart]'))
ALTER TABLE [dbo].[tblContracts] WITH CHECK ADD CONSTRAINT [EndAfterStart] CHECK (([startDate]<=[endDate]))
GO
ALTER TABLE [dbo].[tblContracts] CHECK CONSTRAINT [EndAfterStart]
GO

And the questions:

1. Why tblContracts is altered twice to add this开发者_Go百科 constraint?!
2. Isn't first two lines enough?
3. What is different between second line and forth line?

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First and second lines create EndAfterStart constraint if it doesn't exist. Fourth line enables EndAfterStart constraint.

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The second line adds the constraint to the table; the fourth line enables the constraint.