Databassammenføjning

Databassammenføjning, also known as database join, is a fundamental operation in relational database management systems. It is used to combine rows from two or more tables based on a related column between them. The primary purpose of a join is to retrieve data from multiple tables in a single query, which can be more efficient and easier to manage than performing multiple separate queries.

There are several types of joins, each serving different purposes:

1. Inner Join: Returns only the rows that have matching values in both tables. This is the most common type of join and is used to combine rows from two tables based on a related column.

2. Left (Outer) Join: Returns all rows from the left table and the matched rows from the right table. If there is no match, the result is NULL on the side of the right table.

3. Right (Outer) Join: Returns all rows from the right table and the matched rows from the left table. If there is no match, the result is NULL on the side of the left table.

4. Full (Outer) Join: Returns all rows when there is a match in either left or right table. Rows for which there is no match are filled with NULL values.

5. Self Join: A regular join, but the table is joined with itself. This is useful when you need to compare rows within the same table.

Joins are specified in SQL (Structured Query Language) using the JOIN keyword, followed by the type of join and the conditions for the join. Proper use of joins can significantly enhance the efficiency and readability of database queries, making it a crucial skill for database administrators and developers.