SQL Server CURSOR

Cursor is a database object which fetch data row by row basis. Cursor is use for data manipulation and data processing purpose by fetching data by performing loop. Cursor copy data to a temporary table for looping purpose, the temporary table is automatically created on temdb database.Cursor is a part of transaction SQL(T- SQL), here is the step to create a cursor

1)Declare cursor
2)Open cursor
3)Fetch record from cursor
4)Process your statement with the record
5)Fetch next record from the cursor
6)Close cursor
7)Deallocate cursor

Here is the syntax of a cursor

DECLARE cursor_name CURSOR [ LOCAL | GLOBAL ]   
     [ FORWARD_ONLY | SCROLL ]   
     [ STATIC | KEYSET | DYNAMIC | FAST_FORWARD ]   
     [ READ_ONLY | SCROLL_LOCKS | OPTIMISTIC ]   
     [ TYPE_WARNING ]   
     FOR select_statement   
     [ FOR UPDATE [ OF column_name [ ,...n ] ] ]

Declare cursor declared the cursor variable. 

Select statement initialize the variable.Open cursor is opened for storing data retrieved from the result set.

Fetch statement fetch row from the cursor one by one.

Close release data and locking. 

Deallocate destroy the cursor variable and release all the memory.

We have created a table and corresponding data to do some sample example. 

CREATE TABLE student

(

         id INT NOT NULL,

         name VARCHAR (500) NOT NULL,

         age INT NOT NULL,

         adress VARCHAR (500) NOT NULL,

        date_of_birth DATETIME NOT NULL,

        height DECIMAL(10,2) NOT NULL

)

INSERT INTO student(id,name,age,adress,date_of_birth,height)

SELECT 1,'John1',12,'34,student street','01/08/2018',10.5

UNION

SELECT 1,'John2',13,'35,student street','02/08/2018',11

UNION

SELECT 1,'John3',12,'36,student street','03/08/2018',11.5

UNION

SELECT 1,'John4',13,'37,student street','04/08/2018',12

UNION

SELECT 1,'John5',12,'34,student street','05/08/2018',12.5

UNION

SELECT 1,'John6',13,'35,student street','06/08/2018',13.5

UNION

SELECT 1,'John7',15,'36,student street','07/08/2018',14

UNION

SELECT 1,'John8',14,'34,student street','08/08/2018',14.5

UNION

SELECT 1,'John9',12,'35,student street','09/08/2018',15

UNION

SELECT 1,'John10',15,'36,student street','10/08/2018',15.5

UNION

SELECT 1,'John11',16,'34,student street','11/08/2018',16

Here is example of a cursor

DECLARE db_cursor

CURSOR FOR SELECT name, age, date_of_birth FROM student

DECLARE @Name VARCHAR(256);

DECLARE @Age INT;

DECLARE @dateOfBirth DATE;

OPEN db_cursor;

FETCH NEXT FROM db_cursor INTO @Name, @Age, @dateOfBirth;

         WHILE @@FETCH_STATUS = 0

         BEGIN

--your own statment

                    print CONVERT(VARCHAR,@Name)+'           '+CONVERT(VARCHAR,@Age)+CONVERT(VARCHAR,@dateOfBirth)

               FETCH NEXT FROM db_cursor INTO @Name, @Age, @dateOfBirth;

       END;

CLOSE db_cursor;

DEALLOCATE db_cursor;

OUTPUT
John1 122018-01-08
John10 152018-10-08
John11 162018-11-08
John2 132018-02-08
John3 122018-03-08
John4 132018-04-08
John5 122018-05-08
John6 132018-06-08
John7 152018-07-08
John8 142018-08-08
John9 122018-09-08


cursor can be nested,  cursor can be declared under another  cursor. Below is the example of nested cursor.

DECLARE db_cursor

CURSOR FOR SELECT name, age, date_of_birth FROM student

DECLARE @Name VARCHAR(256);

DECLARE @Age INT;

DECLARE @dateOfBirth DATE;

DECLARE @l_Name VARCHAR(256);

DECLARE @l_Age INT;

DECLARE @l_dateOfBirth DATE;

OPEN db_cursor;

FETCH NEXT FROM db_cursor INTO @Name, @Age, @dateOfBirth;

WHILE @@FETCH_STATUS = 0

BEGIN

               Print 'Age:'+CONVERT(VARCHAR,@Age)

               ------------------Nested Cursor---------------------------------

               DECLARE db_nested

               CURSOR FOR SELECT name, age, date_of_birth  

           FROM student2 where age=@Age

               OPEN db_nested;

       FETCH NEXT FROM db_nested INTO @l_Name, @l_Age, @l_dateOfBirth;

              WHILE @@FETCH_STATUS = 0

              BEGIN

                       Print CONVERT(VARCHAR,@l_Name)+' '+CONVERT(VARCHAR,@l_Age)+' '+CONVERT(VARCHAR,@l_dateOfBirth)

             FETCH NEXT FROM db_nested INTO @l_Name, @l_Age, @l_Age;

             END;

     CLOSE db_nested;

          DEALLOCATE db_nested;

--------------------------------------------------------

FETCH NEXT FROM db_cursor INTO @Name, @Age, @dateOfBirth;

END;

CLOSE db_cursor;

DEALLOCATE db_cursor;

Output:

Age:12

Age:15

John4 15 2018-04-08

Age:16

John5 16 2018-05-08

Age:13

John2 13 2018-02-08

Age:12

Age:13

John2 13 2018-02-08

Age:12

Age:13

John2 13 2018-02-08

Age:15

John4 15 2018-04-08

Age:14

John3 14 2018-03-08

Age:12

You can specify some additional parameter during cursor declaration.

• Static Cursors :A static cursor populates the result set at the time of cursor creation and query result is cached for the lifetime of the cursor.

• Dynamic Cursors:A dynamic cursor allows you to see the data up dation, deletion and insertion in the data source while the cursor is open.

• Forward Only Cursors:A forward only cursor is the fastest cursor among the all cursors.Forward Only Cursors doesn't support backward scrolling.

you should keep in mind that a cursor fetch data row by row basis, you can do row by row validation, computation, manipulation of data in each  row. Cursor copy data to a temporary table for looping purpose, if this data is small, that is not an issue, but for the large data it is costly, put an extra burden on CPU is the cursor to manage cursor. That is why cursor is generally used for small record and forth  large record, loop is used.