Когда То Иначе SQL: Объяснение синтаксиса и примеры использования

When-then-else statements in SQL are used to perform conditional logic. They allow you to execute different code blocks based on specified conditions.

Here's an example of a when-then-else statement in SQL:

SELECT CASE 
    WHEN condition1 THEN result1
    WHEN condition2 THEN result2
    ELSE result3
END AS output
FROM your_table;
    

In this example, the conditions (condition1, condition2) are evaluated one by one. When a condition is met, the corresponding result is returned. If none of the conditions are met, the result after the ELSE keyword is returned.

You can have multiple WHEN statements and only one ELSE statement. Make sure to end the statement with the keyword END. The result of the when-then-else statement is aliased as 'output' in this example.

Understanding the "when then else" SQL Clause

SQL, or Structured Query Language, is a programming language that is widely used for managing and manipulating data in relational databases. The "when then else" clause is a powerful feature in SQL that allows you to specify conditional logic in your queries. In this article, we will explore how the "when then else" clause works and how you can use it in your SQL statements.

The Syntax of the "when then else" Clause

The basic syntax of the "when then else" clause in SQL is as follows:

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

The "CASE" keyword indicates the start of the "when then else" clause. After that, you can specify one or more conditions using the "WHEN" keyword. If a condition is true, the corresponding result will be returned. If none of the conditions are true, the default result specified in the "ELSE" clause will be returned. The "END" keyword marks the end of the "when then else" clause.

Using the "when then else" Clause in SQL

Let's consider a scenario where we have a table called "employees" with the following columns:

• id (integer)
• name (varchar)
• salary (decimal)
• department (varchar)

We want to retrieve the names and salaries of the employees, but with an additional "bonus" column that is computed based on their salaries. If the salary is less than 5000, the bonus should be 500. If the salary is between 5000 and 10000, the bonus should be 1000. Otherwise, the bonus should be 1500.

To achieve this, we can use the "when then else" clause in our SQL query:

SELECT name, salary,
       CASE 
           WHEN salary < 5000 THEN 500
           WHEN salary >= 5000 AND salary < 10000 THEN 1000
           ELSE 1500
       END AS bonus
FROM employees;

In this example, we are using the "CASE" statement to compute the bonus based on the salary of each employee. If the salary is less than 5000, the result will be 500. If the salary is between 5000 and 10000, the result will be 1000. Otherwise, the result will be 1500. The "AS" keyword is used to alias the computed column as "bonus" in the result set.

It is important to note that the "when then else" clause is not limited to simple value assignments. You can also use it to perform more complex computations or even perform subqueries within the clause.

Conclusion

The "when then else" clause in SQL provides a powerful way to handle conditional logic in your queries. By using the "when then else" clause, you can specify multiple conditions and outcomes, allowing for more flexible and dynamic data retrieval and manipulation. Whether you need to compute values based on certain conditions or perform more complex computations, the "when then else" clause is a valuable tool in your SQL arsenal.