How can I use SQL CASE to return multiple columns?

You can use SQL CASE statements to return multiple columns by embedding them within your SELECT clause. Each CASE statement will effectively define a column, and you can have as many as your query needs.

The basic idea is that each CASE statement evaluates a condition and returns a result based on that evaluation. This allows you to populate different values across various columns based on the same data conditions.

Here's how you can achieve this:

1. Basic Syntax

The fundamental approach involves structuring your query to have multiple CASE statements within the SELECT clause. Each CASE will generate its own derived column based on the specified conditions.

2. Example with a Sample Table

Let's assume you have a table named employees with columns like employee_id, department, and salary. We can generate new columns based on these using CASE.

3. Sample SQL Query

Here’s an SQL query showing how you might utilize CASE statements to return multiple columns:

SELECT
  employee_id,
  CASE
    WHEN department = 'IT' THEN 'Technical'
    WHEN department = 'HR' THEN 'Administration'
    ELSE 'Other'
  END AS department_category,
  CASE
    WHEN salary > 70000 THEN 'High Salary'
    WHEN salary > 50000 THEN 'Medium Salary'
    ELSE 'Low Salary'
  END AS salary_level
FROM
  employees;

In this example:

- The first CASE statement classifies departments as either 'Technical', 'Administration', or 'Other'. This becomes a new column called department_category.

- The second CASE statement categorizes salaries into 'High Salary', 'Medium Salary', or 'Low Salary'. This becomes the column salary_level.

4. Explanation

The query retrieves employee_id along with two new dynamically generated columns. department_category provides a simplified classification based on department, while salary_level categorizes each employee based on their income. Both columns are created using CASE statements.

5. Use Cases

- Data Categorization: You can use it for categorizing values into groups based on defined criteria.

- Conditional Logic: Allows for the execution of specific logic on different values, creating useful flags or indicators.

- Dynamic Summaries: Useful in generating a variety of summaries from the same base data.

By employing CASE statements in your SQL queries, you can derive multiple columns based on the same conditions, resulting in more expressive and insightful data sets.

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