Exploring the fetch statement in SQL cursor operations

19 Sep 2023

In SQL, cursors allow us to process records one by one, providing a way to retrieve and manipulate data stored in tables. One of the key components of working with cursors is the FETCH statement, which enables us to retrieve and work with specific records from the cursor result.

Understanding Cursors

Before diving into the FETCH statement, let’s have a quick overview of cursors. A cursor is a database object that allows us to traverse the records returned by a SELECT statement. It provides a way to iterate over a result set in a controlled manner, typically used within stored procedures or scripts.

Cursors typically consist of the following four components:

DECLARE: This step declares and initializes the cursor using the desired SELECT statement.
OPEN: The cursor is opened, making it ready to process the result set.
FETCH: This step retrieves a single record from the cursor result set for processing.
CLOSE: The cursor is closed when we have finished processing the result set.

Using FETCH Statement

The FETCH statement plays a crucial role in retrieving specific records from a cursor result set. It allows us to control which records we want to process based on conditions or position within the result set. Here’s the basic syntax for the FETCH statement:

FETCH {NEXT | PRIOR | FIRST | LAST | ABSOLUTE n | RELATIVE n} FROM <cursor_name>

Let’s break down the different options we can use with the FETCH statement:

NEXT: Retrieves the next record in the result set. This is often used as the default option when fetching records.
PRIOR: Points the cursor to the previous record in the result set. This is useful when traversing the result set backward.
FIRST: Moves the cursor to the first record in the result set. This is handy if we want to start processing data from the beginning.
LAST: Positions the cursor at the last record in the result set. This is beneficial when working with large result sets and wanting to process data from the end.
ABSOLUTE n: Retrieves the record at a specific position, where ‘n’ represents the absolute position of the record in the result set.
RELATIVE n: Fetches a record relative to the current position of the cursor, where ‘n’ represents the number of records we want to go forward or backward from the current position.

Examples

Let’s have a look at a few examples to understand how we can use the FETCH statement effectively:

Example 1: Fetching the Next Record

DECLARE myCursor CURSOR FOR SELECT * FROM customers
OPEN myCursor

FETCH NEXT FROM myCursor

In this example, we declare a cursor named myCursor and open it using a SELECT statement that retrieves all records from the customers table. We then use the FETCH statement with the NEXT option to retrieve the first record from the cursor result set.

Example 2: Fetching Records by Position

DECLARE myCursor CURSOR FOR SELECT * FROM customers
OPEN myCursor

FETCH ABSOLUTE 5 FROM myCursor

In this example, we use the ABSOLUTE option with the FETCH statement to retrieve the record at position 5 from the cursor result set. This can be helpful in scenarios where we want to process specific records without iterating through all of them.

Conclusion

The FETCH statement is a powerful tool when working with cursors in SQL. It allows us to retrieve specific records from the cursor result set, giving us finer control over the data we want to process. By understanding and utilizing the various options available with the FETCH statement, we can efficiently manipulate data and perform operations on a targeted subset.

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