Underforespørsler

Underforespørsler, also known as subqueries, are a fundamental concept in SQL (Structured Query Language), a standard language for managing and manipulating relational databases. A subquery is a query nested within another query. It can be used in various clauses of the SELECT statement, including the SELECT, FROM, WHERE, and HAVING clauses, as well as in the INSERT, UPDATE, and DELETE statements.

Subqueries are particularly useful for performing operations that require multiple steps or conditions. They can be categorized into two main types: scalar subqueries and multi-row subqueries. Scalar subqueries return a single value and are often used in the WHERE clause to filter results based on a condition. Multi-row subqueries, on the other hand, return multiple rows and can be used with operators like IN, ANY, or ALL to compare sets of values.

One of the key advantages of using subqueries is their ability to simplify complex queries by breaking them down into more manageable parts. This can improve readability and maintainability of the code. However, it is important to use subqueries judiciously, as they can sometimes lead to performance issues, especially in large datasets.

In summary, underforespørsler are an essential feature of SQL that allow for the nesting of queries within other queries. They provide a powerful way to handle complex data manipulations and can significantly enhance the functionality and flexibility of SQL queries.