CASE Expression in SQL
The CASE statement in SQL allows you to specify a series of conditions and return a different result depending on which condition is true. Here's the basic syntax:
CASE WHEN condition1 THEN result1 WHEN condition2 THEN result2 ELSE result_default END
Here's an example of how you might use the CASE statement in a SELECT statement:
SELECT product_name, price, CASE WHEN price > 100 THEN 'expensive' WHEN price > 50 THEN 'moderate' ELSE 'cheap' END AS price_category FROM products;
This would select the product name, price, and a calculated column called
price_category, which would be 'expensive' for products with a price greater than 100, 'moderate' for products with a price greater than 50, and 'cheap' for all other products.
There are several best practices to keep in mind when using the CASE expression in SQL:
Use the simplest expression that can accomplish the task: The CASE expression should be as simple as possible, as it can become difficult to understand and maintain if it becomes too complex.
Avoid using multiple expressions in the same CASE statement: You can use multiple conditions in the same CASE statement but it is best to use separate CASE statements for readability and maintainability.
Use the ELSE clause: It's a good practice to use the
ELSEclause to return a value when none of the conditions in the
CASEstatement are met.
Use parentheses to group conditions: Parentheses can help to clearly define the conditions that are being evaluated in a
CASEstatement and make the statement more readable.
Test the conditions in order: Make sure that the most selective conditions are tested first in the CASE statement to improve performance.
Avoid using case statements in
SELECTqueries for filtering or join conditions: Since it executes each case statement for each row, it could decrease performance. Consider using a
WHEREclause for filtering or joining on a specific value.
A CASE statement in SQL Server evaluates an expression and returns a value based on the defined conditions.
If none of the conditions are satisfied, we can use an optional ELSE statement to return the default value.
The SQL CASE Statement consists of at least one pair of
CASEstatement must end with the
ELSEargument is optional.
CASE can be used in any statement or clause that allows a valid expression.