What is the purpose of a database cursor?

A database cursor is used to retrieve rows from a database one at a time.

In more detail, a database cursor is a control structure that enables traversal over the records in a database. Cursors facilitate the processing of rows returned by SQL queries. They are essential when you need to manipulate records on a row-by-row basis rather than as a set.

Cursors are created by database systems like Oracle, MySQL, SQL Server, and PostgreSQL, among others. They are particularly useful when dealing with large datasets that would be impractical to fetch all at once due to memory constraints. Instead, cursors fetch data as needed, reducing memory usage and improving efficiency.

When a cursor is declared, it must be associated with a SELECT statement. This SELECT statement is the query that the cursor will use to fetch data. Once the cursor is opened, it points to a row of data in the result set. You can then fetch rows one at a time, moving the cursor down the result set. After fetching a row, you can perform operations on the data before moving to the next row.

There are different types of cursors, each with its own characteristics. For example, a static cursor displays the result set as it was when the cursor was opened, regardless of changes made to the database in the meantime. A dynamic cursor, on the other hand, reflects changes made to the database after the cursor was opened.

Cursors also have different fetch types. A forward-only cursor can only move forward through the result set, while a scrollable cursor can move both forward and backward.

While cursors can be very useful, they should be used judiciously. Cursors can be resource-intensive and slow compared to set-based operations. Therefore, they are best used when you need to perform complex row-by-row operations that can't be achieved with a single SQL statement.