What is Cursors in SQL

 

Cursors in SQL

In SQL, a cursor is a database object used to traverse through a result set and retrieve data from it. Cursors are commonly used in situations where a large amount of data needs to be processed, but it is not practical to load all the data into memory at once. Cursors provide a way to fetch data from the result set one row at a time, making it possible to process large amounts of data efficiently.

Cursor Types:

SQL supports different types of cursors. Below are some of the most commonly used cursor types:

1. Static Cursor:
• A static cursor creates a temporary table containing a snapshot of the result set. Any changes to the underlying data will not be reflected in the cursor. This type of cursor is read-only and is not suitable for situations where data needs to be updated.
2. Dynamic Cursor:
• A dynamic cursor is a read/write cursor that reflects changes to the underlying data. This type of cursor is suitable for situations where data needs to be updated.
3. Keyset Cursor:
• A keyset cursor creates a temporary table containing only the unique identifiers (keys) of the rows in the result set. Any changes to the data that affect the keys will be reflected in the cursor. This type of cursor is read-only.
4. Forward-Only Cursor:
• A forward-only cursor can only move forward through the result set. This type of cursor is the fastest and uses the least amount of memory.

Syntax:

The syntax for declaring and using a cursor in SQL is as follows:

 

1. Declare the cursor:

DECLARE cursor_name CURSOR [LOCAL | GLOBAL] [SCROLL] [FORWARD_ONLY | SCROLL_LOCKS | OPTIMISTIC] FOR select_statement;

2. Open the cursor:

OPEN cursor_name;

3. Fetch data from the cursor:

FETCH NEXT FROM cursor_name INTO variable_name1, variable_name2, ...;

4. Close the cursor:

CLOSE cursor_name;

5. Deallocate the cursor:

DEALLOCATE cursor_name;

Example Code and Use Case:

Let's say we have a table named "employees" with columns "employee_id", "first_name", "last_name", and "salary". We want to use a cursor to calculate the total salary of all employees.

DECLARE @employee_id INT, @first_name VARCHAR(50), @last_name VARCHAR(50), @salary INT, @total_salary INT;

DECLARE employee_cursor CURSOR FOR SELECT employee_id, first_name, last_name, salary FROM employees;

SET @total_salary = 0;

OPEN employee_cursor;

FETCH NEXT FROM employee_cursor INTO @employee_id, @first_name, @last_name, @salary;

WHILE @@FETCH_STATUS = 0 BEGIN SET @total_salary = @total_salary + @salary; FETCH NEXT FROM employee_cursor INTO @employee_id, @first_name, @last_name, @salary; END

CLOSE employee_cursor; DEALLOCATE employee_cursor;

SELECT @total_salary AS total_salary;

In this example,:

• We declare a cursor named "employee_cursor" that selects all columns from the "employees" table. We then set the initial value of the variable "@total_salary" to 0.
• Next, we open the cursor and fetch the first row of data into the variables "@employee_id", "@first_name", "@last_name", and "@salary". We then enter a while loop that continues to fetch data from the cursor until there is no more data to fetch.
• Inside the while loop, we add the value of "@salary" to "@total_salary". Finally, we close the cursor and deallocate it.
• The output of this example code is the total salary of all employees.

In conclusion, cursors are a powerful tool in SQL that allow us to process large amounts of data efficiently. By selecting the appropriate cursor type and using the correct syntax