Cursors In SQL Server: An OverviewIn this SQL Server Tutorial, the cursor is a database object that retrieves data from a result set one row at a time, instead of the T-SQL commands that operate on all the rows in the result set at one time.We use a cursor when we need to update records in a database table in a singleton fashion, which means row by row in this SQL Server Course.
Cursor in SQL Server
In SQL Server, a cursor is a database object that allows us to retrieve and manipulate each row one at a time. A cursor is nothing more than a row pointer. It is always used alongside a SELECT command in SQL. It is typically a set of SQL logic that loops through a set number of rows one by one.
Life Cycle of Cursor
The life cycle of a cursor can be divided into five phases, as follows:
- Declare Cursor
- Open Cursor
- Fetch Cursor
- Close Cursor
- Deallocate Cursor
1. Declare Cursor
Declares and associates a cursor variable with a certain SELECT statement.
DECLARE cursor_name CURSOR FOR
SELECT column1, column2, ...
2. Open Cursor
Opens the cursor, making it possible to retrieve rows.
3. Fetch Cursor
Retrieves the next row from the cursor's result set.
FETCH cursor_name INTO variable1, variable2, ...;
4. Close Cursor
The cursor is closed, freeing the resources connected with it.
5. Deallocate Cursor
The cursor definition is removed and its resources are released.
Types of Cursors in SQL Server
The different types of cursors in SQL Server are given below:
- Static Cursors
- Dynamic Cursors
- Forward-Only Cursors
- Keyset Cursors
1. Static Cursors
SQL Server static cursors retrieve a snapshot of the result set at the moment of cursor formation and keep that snapshot for the duration of the cursor's life. Changes made to the underlying data after the cursor is created do not affect the result set displayed by the cursor.
2. Dynamic Cursors
SQL Server dynamic cursors are more flexible because they reflect changes in the underlying data, updating the result set dynamically as changes occur. Because of the dynamic nature of the result set, this form of cursor allows for real-time updates, although it may incur additional expense.
3. Forward-Only Cursors
Forward-only cursors in SQL Server allow for only forward traversal of the result set. They are designed for forward scrolling, making them more efficient for one-way data access and consuming less overall resources than other cursor kinds.
4. Keyset Cursors
In SQL Server, keyset cursors keep a static set of SQL keys from the result set, enabling changes to the data but prohibiting changes to the key values. This strikes a balance between the fixed nature of static cursors and the dynamic adaptability of dynamic cursors, allowing for quick navigation and change.
Limitations of SQL Server Cursor
A cursor has some limitations, thus it should only be used when there are no other options. The limitations are as follows:
- Cursor uses network resources by requiring a network roundtrip for every record it retrieves.
- A cursor is a memory resident set of pointers, which implies it consumes memory that other processes on our machine could use.
- When processing data, it puts locks on a section of the database or the full table.
- The cursor's performance and speed are slower since it updates table records one row at a time.
- While cursors are faster than loops, they have additional overhead.
- Another factor influencing cursor speed is the number of columns and rows brought into the cursor. It is the amount of time required to open your cursor and perform a fetch statement.
Cursors provide row-by-row control in SQL Server, allowing operations that set-based commands cannot. Understand their lifecycle, types, and restrictions so that you may employ them efficiently as needed, balancing their benefits against any performance drawbacks.
Q1. What is a cursor and what are the different types of cursor?
- Implicit Cursors
- Explicit Cursors