How to put default value in SQL?

In SQL, the default value is used to set a predefined value for a column when no value is explicitly provided during the INSERT statement. This can be quite useful when working with database tables, as it ensures uniformity and avoids the need to provide a value for each record. In this article, we will explore how to put a default value in SQL and answer some related frequently asked questions.

How to Put Default Value in SQL

To put a default value in SQL, you need to specify the default value constraint when creating or altering a table. The syntax for adding a default value to a column is as follows:

ALTER TABLE table_name
ALTER COLUMN column_name SET DEFAULT default_value;

For example, if you want to set the default value of the “age” column in the “employees” table to 30, you can use the following SQL statement:

ALTER TABLE employees
ALTER COLUMN age SET DEFAULT 30;

Once the default value constraint is set, the specified default value will be used whenever no value is provided for that column during an INSERT statement.

Frequently Asked Questions:

1. Can I add a default value to an existing column?

Yes, you can use the ALTER TABLE statement to add a default value to an existing column, as shown in the example above.

2. How can I remove a default value from a column?

To remove a default value from a column, you can use the ALTER TABLE statement with the DROP DEFAULT clause.
ALTER TABLE table_name
ALTER COLUMN column_name DROP DEFAULT;

3. Can the default value be any data type?

Yes, the default value can be of any data type supported by the database system, such as integer, string, date, or boolean.

4. How does the default value affect existing records?

The default value only applies to new records that do not provide a value for the column. Existing records are not affected by the default value.

5. What happens if I explicitly provide a value during INSERT?

If you provide a value during INSERT, the specified value will take precedence over the default value.

6. Can I change the default value of a column in the future?

Yes, you can change the default value of a column by using the ALTER TABLE statement to modify the column and set a new default value.

7. Can I have multiple columns with default values in a table?

Yes, you can have multiple columns with default values in a table. Simply specify the default value constraint for each column you want to have a default value.

8. Is it possible to have a different default value for different rows in a single column?

No, the default value is a fixed value for the column and applies to all rows unless explicitly provided with a different value during INSERT.

9. How can I check if a column has a default value?

You can query the system catalog or information schema of the database to check if a column has a default value.

10. Can I set a default value for a primary key column?

No, primary key columns typically do not have default values as they should be unique and not null.

11. What if the default value violates other constraints like data type or check constraints?

If the default value violates any other constraints defined on the column, such as data type or check constraints, the constraint violation will occur, and the insertion will fail.

12. Is the default value constraint mandatory?

No, the default value constraint is not mandatory. If no default value is specified, the column will be considered as null by default.

In conclusion, setting default values in SQL is a convenient way to ensure consistency and avoid the need to provide a value for every column during database operations. By using the ALTER TABLE statement with the appropriate syntax, you can easily define default values for columns.

Dive into the world of luxury with this video!