DEFAULT: Setting Default Values in SQL: Harnessing the DEFAULT Constraint

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In SQL, the DEFAULT constraint provides a default value for a column whenever there’s an insertion of a new record and no specific value has been assigned to this column. The main purpose is to maintain data integrity and prevent null or erroneous entries from occurring, especially when no value has been specified during a record’s creation.

1. Understanding the DEFAULT constraint

The syntax of the DEFAULT constraint in SQL is fairly straightforward:

Typically, the datatype could be INT, VARCHAR, DATE, etc, and value is the default value you want to set for this column.

2. Setting the DEFAULT constraint after table creation

There might be a scenario where you have already created a table and you want to add a DEFAULT constraint. In such cases, you can use the ALTER TABLE statement:

3. A Practical Example

Let’s create a table named ‘Employees’ with a DEFAULT constraint on ‘JoiningDate’ column:

This means that if you do not provide a specific joining date when inserting a new record, the current date will be used as a default.

4. Inserting Without a Specific Value for ‘JoiningDate’

Now if we insert a new record without a specific ‘JoiningDate’, SQL Server will automatically use the current date:

5. Dropping a DEFAULT constraint

If we want to remove the DEFAULT constraint from a table, we can use the DROP CONSTRAINT command:

In conclusion, the DEFAULT constraint in SQL offers a measure of control over how data is inserted into tables. It assures data integrity and adds another useful tool to the data architect’s toolbox.