cursor

cursor is a database objects to retrieve 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 cursor when we need to update records in a database table in singleton fashion means row by row.

Declare Cursor

A cursor is declared by defining the SQL statement that returns a result set.

Open

A Cursor is opened and populated by executing the SQL statement defined by the cursor.

Fetch

When cursor is opened, rows can be fetched from the cursor one by one or in a block to do data manipulation.

Close

After data manipulation, we should close the cursor explicitly.

Deallocate

Finally, we need to delete the cursor definition and released all the system resources associated with the cursor.

DECLARE @name VARCHAR(50) -- database name
DECLARE @path VARCHAR(256) -- path for backup files
DECLARE @fileName VARCHAR(256) -- filename for backup
DECLARE @fileDate VARCHAR(20) -- used for file name 

SET @path = 'C:\Backup\'

SELECT @fileDate = CONVERT(VARCHAR(20),GETDATE(),112) 

DECLARE db_cursor CURSOR FOR
SELECT name 
FROM MASTER.dbo.sysdatabases 
WHERE name NOT IN ('master','model','msdb','tempdb')

OPEN db_cursor   
FETCH NEXT FROM db_cursor INTO @name   

WHILE @@FETCH_STATUS = 0   
BEGIN   
       SET @fileName = @path + @name + '_' + @fileDate + '.BAK'
       BACKUP DATABASE @name TO DISK = @fileName

       FETCH NEXT FROM db_cursor INTO @name   
END   

CLOSE db_cursor   
DEALLOCATE db_cursor

SQL Server Cursor Components

Based on the example above, cursors include these components:

• DECLARE statements - Declare variables used in the code block
• SET\SELECT statements - Initialize the variables to a specific value
• DECLARE CURSOR statement - Populate the cursor with values that will be evaluated
• NOTE - There are an equal number of variables in the DECLARE <cursor_name> CURSOR FOR statement as there are in the SELECT statement.  This could be 1 or many variables and associated columns.
• OPEN statement - Open the cursor to begin data processing
• FETCH NEXT statements - Assign the specific values from the cursor to the variables
• NOTE - This logic is used for the initial population before the WHILE statement and then again during each loop in the process as a portion of the WHILE statement
• WHILE statement - Condition to begin and continue data processing
• BEGIN...END statement - Start and end of the code block
• NOTE - Based on the data processing multiple BEGIN...END statements can be used
• Data processing - In this example, this logic is to backup a database to a specific path and file name, but this could be just about any DML or administrative logic
• CLOSE statement - Releases the current data and associated locks, but permits the cursor to be re-opened
• DEALLOCATE statement - Destroys the cursor

Syntax to Deallocate Cursor

Deallocate statement delete the cursor definition and free all the system resources associated with the cursor. The basic syntax to close cursor is given below:

 DEALLOCATE cursor_name --after deallocation it can't be reopen 

SQL SERVER – Simple Examples of Cursors

 CREATE TABLE Employee

(

 EmpID int PRIMARY KEY,

 EmpName varchar (50) NOT NULL,

 Salary int NOT NULL,

 Address varchar (200) NOT NULL,

)

GO

INSERT INTO Employee(EmpID,EmpName,Salary,Address) VALUES(1,'Mohan',12000,'Noida')

INSERT INTO Employee(EmpID,EmpName,Salary,Address) VALUES(2,'Pavan',25000,'Delhi')

INSERT INTO Employee(EmpID,EmpName,Salary,Address) VALUES(3,'Amit',22000,'Dehradun')

INSERT INTO Employee(EmpID,EmpName,Salary,Address) VALUES(4,'Sonu',22000,'Noida')

INSERT INTO Employee(EmpID,EmpName,Salary,Address) VALUES(5,'Deepak',28000,'Gurgaon')

GO

SELECT * FROM Employee 

 

 SET NOCOUNT ON

DECLARE @Id int

DECLARE @name varchar(50)

DECLARE @salary int

 DECLARE cur_emp CURSOR

STATIC FOR 

SELECT EmpID,EmpName,Salary from Employee

OPEN cur_emp

IF @@CURSOR_ROWS > 0

 BEGIN 

 FETCH NEXT FROM cur_emp INTO @Id,@name,@salary

 WHILE @@Fetch_status = 0

 BEGIN

 PRINT 'ID : '+ convert(varchar(20),@Id)+', Name : '+@name+ ', Salary : '+convert(varchar(20),@salary)

 FETCH NEXT FROM cur_emp INTO @Id,@name,@salary

 END

1. END

2. CLOSE cur_emp

3. DEALLOCATE cur_emp

4. SET NOCOUNT OFF