How to Set Default Value in SQL Using ALTER
In SQL, setting a default value for a column is a common practice to ensure that every row has a predefined value when no explicit value is provided during insertion. This can be particularly useful for columns that are not mandatory, but still need to have a default value to maintain data integrity. The ALTER TABLE statement is used to modify the structure of an existing table, and it can also be used to set a default value for a column. In this article, we will discuss how to set a default value in SQL using the ALTER TABLE statement.
Understanding the Basics
Before diving into the process of setting a default value using ALTER, it is essential to understand the basic syntax and structure of the statement. The ALTER TABLE statement is used to add, modify, or delete columns in an existing table. To set a default value for a column, you will need to use the following syntax:
“`sql
ALTER TABLE table_name
ALTER COLUMN column_name SET DEFAULT value;
“`
Here, `table_name` is the name of the table that contains the column you want to modify, `column_name` is the name of the column for which you want to set the default value, and `value` is the default value you want to assign.
Setting a Default Value for a Numeric Column
Let’s consider an example where we have a table named `employees` with a column named `salary`. We want to set the default value for the `salary` column to 5000. To achieve this, we will use the ALTER TABLE statement as follows:
“`sql
ALTER TABLE employees
ALTER COLUMN salary SET DEFAULT 5000;
“`
This statement will update the `salary` column in the `employees` table, setting the default value to 5000 for any new rows inserted without specifying a value for the `salary` column.
Setting a Default Value for a String Column
Similarly, if you have a string column and want to set a default value for it, the process remains the same. For instance, if we have a `department` column in the `employees` table, and we want to set the default value to ‘HR’, we can use the following statement:
“`sql
ALTER TABLE employees
ALTER COLUMN department SET DEFAULT ‘HR’;
“`
This will set the default value for the `department` column to ‘HR’ for any new rows inserted without specifying a value for the `department` column.
Updating Existing Rows
It is important to note that setting a default value using ALTER TABLE does not automatically update existing rows. The default value will only be applied to new rows inserted after the ALTER statement is executed. If you want to update existing rows, you will need to use an UPDATE statement with a WHERE clause to target specific rows or use a more generic approach to update all rows.
Conclusion
In conclusion, setting a default value in SQL using the ALTER TABLE statement is a straightforward process. By following the syntax and understanding the basics, you can easily modify the structure of an existing table and set a default value for a column. This can help maintain data integrity and make your database more robust. Remember that the default value will only be applied to new rows, so if you need to update existing rows, you will need to use additional SQL statements.
