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Home Forums How can I create and use a calculated column in a SQL Server query? – MindStick
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1 Answers

Ravi Vishwakarma
Ravi Vishwakarma 12 Jul 2024
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Creating and using calculated columns in an SQL Server query allows you to generate values based on existing data in your table. These calculated columns can be used in SELECT statements or defined directly in the table schema. 

Here's how to create and use calculated columns in both ways:

1. Using Calculated Columns in a SELECT Statement

You can create calculated columns directly in your SELECT statement by using expressions or functions.

CREATE TABLE CollageStudents (
    StudentID INT PRIMARY KEY, -- Create StudentID with primary key
    FirstName VARCHAR(50),
    LastName VARCHAR(50),
    DOB DATE,
    [Address] VARCHAR(100),
    Age AS DATEDIFF(YEAR, DOB, GETDATE())  -- Calculated column for Age
);

INSERT INTO CollageStudents (StudentID, FirstName, LastName, DOB, [Address])
VALUES
(1, 'John', 'Doe', '2000-05-15', '123 Main St, City'),
(2, 'Jane', 'Smith', '2001-10-20', '456 Oak Ave, Town'),
(3, 'Michael', 'Johnson', '1999-03-08', '789 Elm Rd, Village');

Querying the Table

Now, let's query the CollageStudents table to see how the calculated column Age works:

SELECT StudentID, FirstName, LastName, DOB, Age, Address
FROM CollageStudents;
Ravi Vishwakarma

Explanation

Table Definition:

  • CollageStudents table has columns StudentID, FirstName, LastName, DOB, Address.
  • Age is a calculated column using the AS keyword and the DATEDIFF function to calculate the difference in years between DOB and the current date (GETDATE()).

Querying the Table:

  • The SELECT statement retrieves all columns including Age, which is computed dynamically based on the DOB of each student.

Additional Notes

  1. Updating Calculated Columns: Calculated columns are automatically updated whenever a row is inserted or updated. They provide a convenient way to derive data based on existing columns.
  2. Performance Considerations: While calculated columns simplify querying by dynamically computing values, they should be used judiciously in scenarios where performance implications are minimal.

Modify the Table and Create a new Computed Column

ALTER TABLE CollageStudents
ALTER COLUMN DOB DATETIME -- Change the column datatype
ADD FullName AS (FirstName + ' ' + LastName); -- Add New Computed or Calculated column

Let's see in table

Ravi Vishwakarma

Querying the Table

Now, let's query the CollageStudents table to see how the calculated column Age works:

SELECT * FROM CollageStudents;
Ravi Vishwakarma

You can create a computed column with conditions.

Create ParmanantAddress when IsSameAddress  value is true.

ALTER TABLE CollageStudents
ADD IsSameAddress BIT NOT NULL DEFAULT 0
ADD ParmanantAddress AS (
    CASE 
        WHEN IsSameAddress = 1 THEN [Address] 
        ELSE NULL 
    END
);

-- Update some values 
update CollageStudents
set IsSameAddress = 1
where StudentID = 3
-- Run it
SELECT * FROM CollageStudents;

Output

Ravi Vishwakarma

Read more 

Explain the SQL Server backups and their types

Explain the SQL triggers and their uses

Define the PIVOT Table with examples in SQL server.

Differences between stored procedures and functions in SQL

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