A CASE statement in SQL is a control flow structure that allows you to execute conditional logic directly within your SQL queries. It functions similarly to an IF-THEN-ELSE statement found in many programming languages. By using a CASE statement, you can evaluate a set of conditions and return different values based on which condition is true. This is particularly useful when you want to categorize or convert data in a query result without having to create multiple query statements or algorithms.
The syntax of a CASE statement is straightforward. It starts with the keyword CASE, followed by a series of conditions defined with WHEN, each of which is paired with a corresponding result defined using THEN. You can terminate the block using the keyword END. Additionally, there’s an optional ELSE clause that allows you to specify a default value when none of the conditions are met. For example, if you want to classify sales amounts into categories, you could write a query like this:
SELECT
sale_amount,
CASE
WHEN sale_amount < 100 THEN 'Low'
WHEN sale_amount BETWEEN 100 AND 500 THEN 'Medium'
ELSE 'High'
END AS sale_category
FROM sales;
In this example, the query checks the value of sale_amount and categorizes it as 'Low', 'Medium', or 'High' based on the specified thresholds. Using a CASE statement this way avoids the need for complex joins or subqueries, enhancing both the readability of your SQL code and its performance. Overall, the CASE statement is an essential tool in SQL, enabling developers to incorporate conditional logic efficiently and directly within their data retrieval processes.