How is the case expression used in SQL?

In SQL, the CASE expression is used to select different results based on conditions.

In general, CASE expressions come in two forms: simple CASE expressions and searched CASE expressions.

1. Simple CASE expression:
   A simple CASE expression selects a result based on the value of a specific expression. Its syntax is as follows:
2. The CASE expression checks for a specific value and returns a corresponding result. If none of the specified values are found, it returns a default result.
3. The expression value will be compared with each value in the WHEN clauses, and if a match is found, the corresponding result will be returned. If no match is found in any WHEN clause, the result from the ELSE clause will be returned. If no ELSE clause is provided, NULL will be returned.
4. Search CASE Expression:
   Search CASE expression allows the selection of results based on multiple conditions. Its syntax is as follows:
5. If condition1 is met, return result1. If condition2 is met, return result2. Otherwise, return the specified result.
6. In each WHEN clause, conditions are evaluated one by one. If a condition is true, the corresponding result is returned. If no condition is true, the result from the ELSE clause is returned. If no ELSE clause is provided, NULL is returned.

You can also nest CASE expressions, by using another CASE expression within the WHEN or ELSE clause to achieve more complex condition evaluation.

By using a CASE expression, it is possible to perform conditional logic in a query, allowing for the selection of different results based on varying conditions, thereby enhancing the outcome of the query.