Liittymäkyselyt

Liittymäkyselyt refers to connection queries or join queries in the context of database systems. These are essential operations used to retrieve data from multiple tables simultaneously. The fundamental principle behind a liittymäkysely is to combine rows from two or more tables based on a related column between them. This allows for the retrieval of more comprehensive information than would be possible by querying individual tables.

The most common type of liittymäkysely is the INNER JOIN, which returns only those rows where the join condition is met in both tables. Other types include LEFT JOIN (or LEFT OUTER JOIN), which returns all rows from the left table and the matched rows from the right table; RIGHT JOIN (or RIGHT OUTER JOIN), which returns all rows from the right table and the matched rows from the left table; and FULL JOIN (or FULL OUTER JOIN), which returns all rows when there is a match in either the left or the right table.

Liittymäkyselyt are typically expressed using SQL (Structured Query Language) statements. The syntax involves specifying the tables to be joined and the condition that links them, usually through a WHERE clause or an explicit JOIN clause. The performance of liittymäkyselyt can be significantly impacted by factors such as the size of the tables, the presence of indexes on the join columns, and the complexity of the query. Database optimizers play a crucial role in determining the most efficient execution plan for these queries.