Set Default Value for Column

Use an ALTER TABLE statement:

• Execute the following query, replacing your_table_name with the actual name of your table and default_value with the desired default value:

  ALTER TABLE your_table_name
  ADD DEFAULT default_value FOR column_name;
  

  • This will add a default constraint to the specified column, assigning the specified default value to any new rows inserted into the table without a value provided for that column.

Modify the existing default constraint:

• If the column already has a default constraint, you can modify its value using the following query:

  ALTER TABLE your_table_name
  DROP CONSTRAINT default_constraint_name;
  ALTER TABLE your_table_name
  ADD DEFAULT new_default_value FOR column_name;
  

  • Replace default_constraint_name with the actual name of the existing default constraint and new_default_value with the new desired default value.

Example:

Assuming you have a table named Customers with a column named City, you can set a default value of "Unknown" for this column using the following query:

ALTER TABLE Customers
ADD DEFAULT 'Unknown' FOR City;

Additional notes:

• The default value must be compatible with the data type of the column.
• You can use expressions or functions to calculate the default value.
• If you need to remove the default constraint entirely, use the DROP CONSTRAINT statement without adding a new default value.

Setting a Default Value for an Existing Column in SQL Server

Example 1: Adding a Default Value to a New Column

CREATE TABLE Customers (
    CustomerID INT PRIMARY KEY IDENTITY(1,1),
    FirstName NVARCHAR(50),
    LastName NVARCHAR(50),
    City NVARCHAR(50),
    Country NVARCHAR(50) DEFAULT 'USA'
);

In this example, the Country column is created with a default value of "USA". Whenever a new row is inserted into the Customers table, the Country column will automatically be populated with "USA" unless a different value is explicitly provided.

ALTER TABLE Customers
ADD DEFAULT 'Unknown' FOR City;

This statement adds a default value of "Unknown" to the existing City column in the Customers table. If a new row is inserted without specifying a value for City, it will be automatically set to "Unknown".

Setting a Default Value for a Column When Creating a Table

CREATE TABLE Orders (
    OrderID INT PRIMARY KEY IDENTITY(1,1),
    CustomerID INT,
    OrderDate DATE DEFAULT GETDATE(),
    TotalAmount DECIMAL(10,2)
);

In this example, the OrderDate column is created with a default value of GETDATE(), which will automatically set the order date to the current date and time when a new row is inserted.

• Default Constraints: The DEFAULT clause creates a default constraint. You can drop or modify this constraint using the ALTER TABLE statement.
• Expressions and Functions: You can use expressions and functions to calculate the default value.
• NULL Values: If the column is nullable and you want to enforce the default value for all rows, including existing rows without a value, use the WITH VALUES clause:

ALTER TABLE Customers
ADD DEFAULT 'Unknown' FOR City WITH VALUES;

Alternative Methods for Setting Default Values in SQL Server

While the ALTER TABLE statement is the primary method for setting default values for existing columns in SQL Server, there are a few alternative approaches you can consider:

Using a Trigger:

• Create a trigger: Create an INSERT trigger on the table.
• Check for null values: In the trigger's body, check if the column's value is null.
• Set the default value: If the value is null, set it to the desired default value.

CREATE TRIGGER trg_UpdateCityDefault
ON Customers
AFTER INSERT
AS
BEGIN
    UPDATE Customers
    SET City = 'Unknown'
    WHERE City IS NULL;
END;

Advantages:

• Provides more flexibility for complex default value calculations or business logic.
• Can be used to enforce additional constraints or perform other actions when a new row is inserted.

• Can introduce performance overhead, especially for large tables with frequent inserts.
• May require additional maintenance if the default value or business logic changes.

Using a Stored Procedure:

• Create a stored procedure: Create a stored procedure to insert new rows into the table.
• Set the default value: Within the procedure, check if the column's value is null and set it to the desired default value before inserting the row.

CREATE PROCEDURE sp_InsertCustomer
    @FirstName NVARCHAR(50),
    @LastName NVARCHAR(50),
    @City NVARCHAR(50),
    @Country NVARCHAR(50)
AS
BEGIN
    DECLARE @DefaultCity NVARCHAR(50) = 'Unknown';

    IF @City IS NULL
        SET @City = @DefaultCity;

    INSERT INTO Customers (FirstName, LastName, City, Country)
    VALUES (@FirstName, @LastName, @City, @Country);
END;

• Provides more control over the insertion process and can include additional validation or business logic.
• Can be used to enforce complex default value calculations or business rules.

• Requires additional development and maintenance.
• May introduce performance overhead, especially if the stored procedure is called frequently.