Using the ALTER TABLE statement
When working with SQL Server, there may be times when you need to add new columns to an existing table. This can be done using the ALTER TABLE statement, which allows you to modify the structure of a table. In this article, we will explore the syntax for adding columns in SQL Server.
Syntax for adding a single column
To add a single column to an existing table, you can use the following syntax:
«`sql
ALTER TABLE table_name
ADD column_name data_type;
«`
In this syntax, `table_name` is the name of the table to which you want to add the column, `column_name` is the name of the new column, and `data_type` is the data type of the column.
For example, let’s say we have a table called `Customers` and we want to add a new column called `Email` of type `VARCHAR(100)`. The syntax would be:
«`sql
ALTER TABLE Customers
ADD Email VARCHAR(100);
«`
Syntax for adding multiple columns
If you need to add multiple columns to a table, you can use the following syntax:
«`sql
ALTER TABLE table_name
ADD column1_name data_type,
column2_name data_type,
…
columnN_name data_type;
«`
In this syntax, you can specify multiple columns separated by commas. Each column should have its own name and data type.
For example, let’s say we want to add two columns, `Phone` of type `VARCHAR(20)` and `Address` of type `VARCHAR(200)`, to the `Customers` table. The syntax would be:
«`sql
ALTER TABLE Customers
ADD Phone VARCHAR(20),
Address VARCHAR(200);
«`
Specifying column name, data type, and constraints
When adding a column, you can also specify additional constraints such as NOT NULL, UNIQUE, PRIMARY KEY, and FOREIGN KEY. The syntax for adding a column with constraints is as follows:
«`sql
ALTER TABLE table_name
ADD column_name data_type constraint1,
constraint2,
…
constraintN;
«`
In this syntax, `constraint1`, `constraint2`, etc. represent the constraints you want to apply to the column.
For example, let’s say we want to add a column called `Age` of type `INT` to the `Customers` table, and we want to make it a required field (NOT NULL). The syntax would be:
«`sql
ALTER TABLE Customers
ADD Age INT NOT NULL;
«`
Adding a column with a default value
You can also add a column with a default value using the DEFAULT constraint. The syntax for adding a column with a default value is as follows:
«`sql
ALTER TABLE table_name
ADD column_name data_type DEFAULT default_value;
«`
In this syntax, `default_value` represents the value that will be assigned to the column if no value is specified during an INSERT operation.
For example, let’s say we want to add a column called `Status` of type `VARCHAR(10)` to the `Customers` table, and we want to set the default value to ‘Active’. The syntax would be:
«`sql
ALTER TABLE Customers
ADD Status VARCHAR(10) DEFAULT ‘Active’;
«`
Adding a column with a foreign key constraint
If you want to add a column with a foreign key constraint, you can use the following syntax:
«`sql
ALTER TABLE table_name
ADD column_name data_type,
FOREIGN KEY (column_name) REFERENCES referenced_table (referenced_column);
«`
In this syntax, `referenced_table` is the name of the table that the foreign key references, and `referenced_column` is the column in the referenced table.
For example, let’s say we want to add a column called `OrderID` of type `INT` to the `OrderDetails` table, and we want to make it a foreign key that references the `Orders` table. The syntax would be:
«`sql
ALTER TABLE OrderDetails
ADD OrderID INT,
FOREIGN KEY (OrderID) REFERENCES Orders (OrderID);
«`
Adding a column with an identity property
If you want to add a column with an identity property, you can use the following syntax:
«`sql
ALTER TABLE table_name
ADD column_name data_type IDENTITY(seed, increment);
«`
In this syntax, `seed` represents the starting value for the identity column, and `increment` represents the increment value.
For example, let’s say we want to add a column called `ProductID` of type `INT` to the `Products` table, and we want it to be an identity column with a starting value of 1 and an increment of 1. The syntax would be:
«`sql
ALTER TABLE Products
ADD ProductID INT IDENTITY(1, 1);
«`
Adding a column with a computed column expression
If you want to add a column with a computed column expression, you can use the following syntax:
«`sql
ALTER TABLE table_name
ADD column_name AS computed_column_expression;
«`
In this syntax, `computed_column_expression` represents the expression that will be used to calculate the value of the computed column.
For example, let’s say we want to add a column called `TotalPrice` to the `OrderDetails` table, and we want it to be a computed column that calculates the total price based on the quantity and unit price. The syntax would be:
«`sql
ALTER TABLE OrderDetails
ADD TotalPrice AS Quantity * UnitPrice;
«`
In conclusion, adding columns in SQL Server is a straightforward process that can be done using the ALTER TABLE statement. Whether you need to add a single column or multiple columns, specify data types, constraints, default values, foreign key constraints, identity properties, or computed column expressions, the syntax provided in this article will guide you through the process.
