Beginning Microsoft SQL Server 2008 ... - S3 Tech Training

Chapter 6: Constraints
The INSERT statement conflicted with the CHECK constraint
“CN_CustomerDateInSystem”. The conflict occurred in database “Accounting”, table
“dbo.Customers”, column ‘DateInSystem’.
The statement has been terminated.
Now if we change things to use a DateInSystem that meets the criterion used in the CHECK (anything
with today’s date or earlier), the INSERT works fine.
DEFAULT Constraints
This will be the first of two different types of data integrity tools that will be called something to do with
“default.” This is, unfortunately, very confusing, but I’ll do my best to make it clear (and I think it will
become so).
We’ll see the other type of default when we look at rules and defaults later in the chapter.
A DEFAULT constraint, like all constraints, becomes an integral part of the table definition. It defines
what to do when a new row is inserted that doesn’t include data for the column on which you have
defined the default constraint. You can either define it as a literal value (say, setting a default salary to
zero or “UNKNOWN” for a string column) or as one of several system values such as GETDATE().
The main things to understand about a DEFAULT constraint are that:
❑ Defaults are only used in INSERT statements. They are ignored for UPDATE and DELETE
statements.
❑ If any value is supplied in the INSERT, then the default is not used.
❑ If no value is supplied, the default will always be used.
Defaults are only made use of in INSERT statements. I cannot express enough how much this confuses
many SQL Server beginners. Think about it this way: When you are first inserting the record, SQL Server
doesn’t have any kind of value for your column except what you supplied (if anything) or the default. If
neither of these is supplied, then SQL Server will either insert a NULL (essentially amounting to “I don’t
know”), or if your column definition says NOT NULL, then SQL Server will reject the record. After that
first insert, however, SQL Server already has some value for that column. If you are updating that column,
then it has your new value. If the column in question isn’t part of an UPDATE statement, then SQL
Server just leaves what is already in the column.
If a value was provided for the column, then there is no reason to use the default. The supplied value
is used.
If no value is supplied, then the default will always be used. Now this seems simple enough until you
think about the circumstance where a NULL value is what you actually want to go into that column for a
record. If you don’t supply a value on a column that has a default defined, then the default will be used.
What do you do if you really wanted it to be NULL? Say so — insert NULL as part of your INSERT statement.
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