Когда использовать CASE в SQL

В языке SQL оператор CASE используется для выполнения условных операций в запросах. Он позволяет нам выбирать различные пути выполнения в зависимости от заданных условий.

Оператор CASE имеет две формы: простую и поисковую.

Простая форма оператора CASE:

      
         SELECT column_name,
            CASE
               WHEN condition1 THEN value1
               WHEN condition2 THEN value2
               ELSE value
            END
         FROM table_name;
      
   

В этом примере мы выбираем столбец column_name и для каждой строки проверяем несколько условий. Если условие 1 выполняется, возвращается value1. Если условие 1 не выполняется, но выполняется условие 2, возвращается value2. В противном случае возвращается значение value.

Поисковая форма оператора CASE:

      
         SELECT column_name,
            CASE column_name
               WHEN value1 THEN result1
               WHEN value2 THEN result2
               ELSE result
            END
         FROM table_name;
      
   

В этом примере оператор CASE сравнивает значение в столбце column_name с заданными значениями и возвращает соответствующий результат.

Оператор CASE SQL очень полезен для выполнения условных операций и принятия решений на основе данных в таблицах.

When Case SQL: A Comprehensive Explanation

Welcome to this comprehensive guide on the WHEN CASE statement in SQL!

The WHEN CASE statement is a powerful tool that allows you to conditionally perform actions within your SQL queries. It is useful when you want to perform different actions based on different conditions. Let's dive into the details and explore how it works.

Basic Syntax

The basic syntax of the WHEN CASE statement is as follows:

CASE
    WHEN condition1 THEN result1
    WHEN condition2 THEN result2
    ...
    ELSE result
END

The statement starts with the CASE keyword and is followed by multiple WHEN conditions. Each WHEN condition consists of a condition to be evaluated, followed by the THEN keyword and the result or action to be taken if the condition is true.

The ELSE keyword is optional and is used to define a default result or action when none of the previous conditions evaluate to true. The END keyword marks the end of the WHEN CASE statement.

Using WHEN CASE in SQL Queries

Let's consider a scenario where we have a table called "Employees" with columns "EmployeeID", "FirstName", "LastName", and "Salary". We want to categorize the employees based on their salary range into three categories: "Low", "Medium", and "High". We can achieve this using the WHEN CASE statement. Let's see how it's done:

SELECT FirstName, LastName, Salary,
    CASE
        WHEN Salary < 50000 THEN 'Low'
        WHEN Salary >= 50000 AND Salary < 100000 THEN 'Medium'
        ELSE 'High'
    END AS SalaryCategory
    FROM Employees;

In the above example, we used the WHEN CASE statement to evaluate the salary of each employee. If the salary is less than 50000, we categorize it as 'Low'. If the salary is between 50000 and 100000, we categorize it as 'Medium'. For any other salary amount, we categorize it as 'High'.

Nesting WHEN CASE Statements

The WHEN CASE statement can also be nested within other WHEN CASE statements to handle more complex conditions. Let's see an example:

SELECT FirstName, LastName,
    CASE
        WHEN Salary >= 100000 THEN 'High Earner'
        WHEN Salary >= 50000 THEN 
            CASE
                WHEN DepartmentID = 1 THEN 'Marketing'
                WHEN DepartmentID = 2 THEN 'Finance'
                ELSE 'Other'
            END
        ELSE 'Low Earner'
    END AS EmployeeCategory
    FROM Employees;

In this example, we categorized employees as 'High Earner' if their salary is 100000 or more. For employees with a salary between 50000 and 99999, we further categorized them based on their department. If the department ID is 1, we assign them to the 'Marketing' category. If the department ID is 2, we assign them to the 'Finance' category. For any other department ID, we assign them to the 'Other' category. Employees with a salary less than 50000 are categorized as 'Low Earner'.

Conclusion

The WHEN CASE statement is a powerful tool in SQL that allows you to perform conditional actions within your queries. It provides a flexible way to categorize data based on specific conditions. By using the WHEN CASE statement, you can easily handle complex logic and achieve the desired results in your SQL queries.

Now that you have a comprehensive understanding of the WHEN CASE statement, you can start using it in your own SQL queries to handle conditional actions efficiently. Happy coding!