How to Write Alter Query in SQL Server 2008

In SQL Server 2008, the ALTER query is a powerful tool that allows you to modify the structure of existing database objects such as tables, views, stored procedures, and functions. Whether you need to add a new column, change the data type of an existing column, or rename a table, the ALTER query can help you achieve your goals efficiently. In this article, we will guide you through the process of writing an ALTER query in SQL Server 2008.

Firstly, it is essential to understand the basic syntax of an ALTER query. The general structure of an ALTER query is as follows:

“`
ALTER [object type] [object name]
[ALTER COLUMN] [column name] [data type]
[ADD CONSTRAINT] [constraint name] [constraint definition]
[DROP CONSTRAINT] [constraint name]
“`

Let’s take a closer look at each component of this syntax:

1. `[object type]`: This specifies the type of the object you want to alter. Common object types include TABLE, VIEW, PROCEDURE, and FUNCTION.

2. `[object name]`: This is the name of the object you want to alter.

3. `[ALTER COLUMN]`: This clause is used to modify the structure of a column, such as adding a new column, changing the data type, or renaming a column.

4. `[column name]`: This is the name of the column you want to alter.

5. `[data type]`: This specifies the new data type for the column you are altering.

6. `[ADD CONSTRAINT]`: This clause is used to add a new constraint to the object, such as a PRIMARY KEY, FOREIGN KEY, or UNIQUE constraint.

7. `[constraint name]`: This is the name of the new constraint you are adding.

8. `[constraint definition]`: This specifies the definition of the constraint, such as the columns it applies to and any additional conditions.

Now, let’s dive into some practical examples of ALTER queries in SQL Server 2008.

Example 1: Adding a new column to a table

Suppose you have a table named `Employees` with the following columns: `EmployeeID`, `FirstName`, `LastName`, and `Email`. To add a new column named `Department` with the data type `VARCHAR(50)`, you can use the following ALTER query:

“`sql
ALTER TABLE Employees
ADD Department VARCHAR(50);
“`

Example 2: Changing the data type of an existing column

Let’s say you have a table named `Orders` with a column named `OrderDate` of type `DATETIME`. You want to change the data type of `OrderDate` to `DATE`. Here’s the ALTER query to achieve this:

“`sql
ALTER TABLE Orders
ALTER COLUMN OrderDate DATE;
“`

Example 3: Renaming a table

If you need to rename a table, you can use the following ALTER query:

“`sql
EXEC sp_rename ‘OldTableName’, ‘NewTableName’;
“`

Replace `OldTableName` with the current name of the table and `NewTableName` with the desired new name.

Example 4: Adding a PRIMARY KEY constraint to a table

To add a PRIMARY KEY constraint to a table, use the following ALTER query:

“`sql
ALTER TABLE Employees
ADD CONSTRAINT PK_Employees PRIMARY KEY (EmployeeID);
“`

In this example, `EmployeeID` is the column that will be used as the primary key.

By following these examples and understanding the basic syntax of the ALTER query, you should now be able to write ALTER queries in SQL Server 2008 to modify the structure of your database objects as needed. Remember to always backup your database before making any structural changes to ensure that you can revert to a previous state if something goes wrong.