In SQL Server, you can use the CASE statement to perform IF…THEN logic within a SELECT statement. The CASE statement evaluates a list of conditions and returns one of multiple possible result expressions.

Here’s the basic syntax for using a CASE statement:

SELECT
    column1,
    column2,
    CASE
        WHEN condition1 THEN result1
        WHEN condition2 THEN result2
        ...
        ELSE default_result
    END AS alias_name
FROM
    table_name;

• condition1, condition2, … are the conditions you want to test.
• result1, result2, … are the values returned if the corresponding condition is true.
• default_result is the value returned if none of the conditions are true. The ELSE part is optional.
• alias_name is the name you want to give to the result of the CASE statement (this is also optional).

Example

Suppose you have a table named Employees with a column Salary. You want to categorize each employee as ‘Low’, ‘Medium’, or ‘High’ based on their salary:

SELECT
    EmployeeID,
    Name,
    Salary,
    CASE
        WHEN Salary < 30000 THEN 'Low'
        WHEN Salary BETWEEN 30000 AND 60000 THEN 'Medium'
        WHEN Salary > 60000 THEN 'High'
        ELSE 'Not Specified' -- This line is optional
    END AS SalaryCategory
FROM
    Employees;

In this example, for each row in the Employees table, the CASE statement checks the Salary column and assigns a SalaryCategory based on the specified conditions.