Inserting Rows into a Table with Default Column Values with SQL

Inserting Rows into a Table with Default Column Values

Overview

When inserting rows into a table, some columns may have default values assigned to them. These default values are used when you do not explicitly provide values for those columns in your INSERT statement. This mechanism helps maintain data integrity and simplifies data insertion.

Default Column Values

Default column values are predefined values that are automatically used by the database when no explicit value is provided for a column during an INSERT operation. Default values can be constants, expressions, or database functions.

Syntax for Inserting Data

Inserting with Default Values for Some Columns

If a column has a default value and you omit it in your INSERT statement, the default value will be used: 

INSERT INTO table_name (column1, column2, column3)
VALUES (value1, value2, DEFAULT);

In this syntax:

• table_name: The name of the table where data is being inserted.
• column1, column2, column3: Columns in the table.
• value1, value2: Values to be inserted into column1 and column2.
• DEFAULT: Indicates that the default value for column3 should be used.

Omitting Columns with Default Values

If you do not include a column with a default value in the INSERT statement, the default value is automatically used: 

INSERT INTO table_name (column1, column2)
VALUES (value1, value2);

Here, if column3 has a default value, that value will be automatically applied.

Examples

Table Definition with Default Values

Consider a table Products with the following definition: 

CREATE TABLE Products (
    ProductID INT PRIMARY KEY,
    ProductName VARCHAR(50),
    Quantity INT DEFAULT 0,
    DateAdded DATE DEFAULT CURRENT_DATE
);

In this table:

• Quantity has a default value of 0.
• DateAdded has a default value of the current date.

Inserting Rows with Default Values

Omitting Columns with Default Values 

INSERT INTO Products (ProductID, ProductName)
VALUES (1, 'Widget');

In this example:

• Quantity will default to 0.
• DateAdded will default to the current date.

Using Default Keyword

You can explicitly specify the DEFAULT keyword for columns with default values: 

INSERT INTO Products (ProductID, ProductName, Quantity, DateAdded)
VALUES (2, 'Gadget', DEFAULT, DEFAULT);

In this example:

• Quantity will use its default value of 0.
• DateAdded will use the current date as its default value.

Considerations

Data Type Compatibility

Ensure that default values are compatible with the column’s data type. For example, a default value for a column of type VARCHAR must be a string, and for a column of type INTEGER, it must be an integer.

Constraints and Defaults

Default values must respect constraints defined on the column. For example:

• If a column is defined with a UNIQUE constraint, the default value must not conflict with existing values.
• Default values must comply with NOT NULL constraints, meaning they must provide a valid value for the column.

Performance Impact

Using default values typically has minimal performance impact. However, if complex expressions or functions are used as default values, they might affect performance. Test and optimize as needed.

Updating Default Values

If the default value of a column is updated, it affects future INSERT operations but does not change existing rows. For example: 

ALTER TABLE Products
ALTER COLUMN Quantity SET DEFAULT 10;

Future inserts will use 10 as the default value for Quantity, but existing rows will remain unchanged unless explicitly updated.

Conclusion

Inserting rows into a table with default column values simplifies data management by ensuring columns are populated with appropriate values even when not explicitly provided. Understanding and using default values correctly helps maintain data integrity and streamline data insertion processes.