Working with SQL: Using ALTER TABLE... SET DEFAULT

The SQL language offers different commands for interacting with databases, including setting default values for database columns. According to the quiz question above, the correct statement for setting a default value for a column in SQL is ALTER TABLE ... SET DEFAULT.

The ALTER TABLE command in SQL is a powerful statement used to add, delete/drop, or modify columns in an existing table. Also, you can use ALTER TABLE to add and drop various constraints on an existing table.

When setting the default values, the ALTER TABLE ... SET DEFAULT clause comes into play. This clause sets a default value for a column when no value is specified. If you don't enter a value for that column when inserting rows, the column takes the default value that you set.

Here is a basic example:

ALTER TABLE Employees
ALTER COLUMN Department SET DEFAULT 'Human Resources';

In this example, if new rows are inserted into the Employees table without specifying a value for the Department column, SQL will automatically fill it with the default value 'Human Resources'.

However, keep in mind that this default value will only apply to future inserts. It doesn't change any existing data in the column. So, setting a default value is especially good if you repeatedly add similar kinds of data to your database, as it can save effort and reduce errors.

It's a best practice to ensure the default value you set suits the context of your tables and aligns perfectly with the type of data inserted into that particular column. Plus, bear in mind that not all columns should have default values. For instance, unique ID columns should never be set to have a default value, as each ID needs to be unique.

Additionally, it's important to mention that the SET DEFAULT clause is SQL standard, but not all SQL databases support it. Some databases, like MySQL, use a different syntax to set a default value to a column:

ALTER TABLE Employees 
MODIFY Department varchar(255) DEFAULT 'Human Resources';

In summary, the ALTER TABLE ... SET DEFAULT statement is an efficient tool to manage your database system, setting default values to minimize data entry errors and save time. However, the command and syntax may vary depending on the SQL database system you are using.