Schema Management

SQL Default Constraints

Using DEFAULT Constraints

DEFAULT constraints set default column values, simplifying data entry.

Introduction to SQL Default Constraints

SQL DEFAULT constraints are used to automatically insert a default value into a column if no value is specified during an insert operation. This can simplify data entry and ensure data integrity by providing consistent default values for specific columns.

How DEFAULT Constraints Work

When creating a table, you can specify a DEFAULT constraint on one or more columns. This constraint ensures that a predefined value is used if no value is explicitly provided for that column during an INSERT operation. The default value can be a constant, a function, or a NULL value, depending on the column's data type and the database system being used.

Creating a Table with DEFAULT Constraints

To create a table with DEFAULT constraints, you specify the default value in the column definition. Here is an example of how to create a table with DEFAULT constraints:

Inserting Data with DEFAULT Constraints

When inserting data into a table with DEFAULT constraints, you can omit the columns with default values from the INSERT statement. The database will automatically apply the default values. Here's an example:

In the example above, the StartDate will automatically be set to the current date, and the Salary will be set to 50000.00.

Modifying Existing Tables to Add DEFAULT Constraints

You can add DEFAULT constraints to existing tables using the ALTER TABLE statement. Here's how you can add a DEFAULT constraint to an existing column:

Removing DEFAULT Constraints

If you need to remove a DEFAULT constraint from a column, you can also use the ALTER TABLE statement. Here's an example:

Benefits of Using DEFAULT Constraints

Consistency: Ensures that all entries have a value, either specified or default, maintaining data integrity.
Simplification: Reduces the need for specifying values for every column during data insertion.
Efficiency: Minimizes manual data entry errors and improves the speed of data insertion.

Schema Management

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