SQL SERVER cursor usage

       When we process data, there is often a need to process data in a loop. If we can use CTE or other statements to process it, there is no problem, but sometimes it is difficult to process. At this time, we can choose to use cursor processing, choose Which form to use depends on efficiency. Generally, cursors are not efficient, but there are also suitable scenarios.

       The cursor is divided into static cursor and dynamic cursor. The data of the static cursor is fixed and will not change due to the change of the data table; the data of the dynamic cursor changes with the change of the data table. The cursor is a dynamic cursor by default STATIC settings, OK, on ​​the test data:

--Test Data
if not object_id(N'Tempdb..#T') is null
	drop table #T
Go
Create table #T([id] int,[name] nvarchar(22))
Insert #T
select 1,N'Zhang San' union all
select 2,N'Li Si' union all
select 3,N'Wang Wu' union all
select 4,N'Zhao Liu'
Go
-- end of test data

       Let's first look at the use of static cursors:

DECLARE @id INT , @name NVARCHAR(50) -- declare variables, data to be read
DECLARE cur CURSOR STATIC -- declare a static cursor
FOR
    SELECT  * FROM    #T
OPEN cur -- open the cursor
FETCH NEXT FROM cur INTO @id, @name --fetch data
WHILE ( @@fetch_status = 0 ) -- determine if there is still data
    BEGIN
        SELECT 'Data: ' + RTRIM(@id) + @name
		UPDATE #T SET name='test' WHERE id=4 -- test static and dynamic
        FETCH NEXT FROM cur INTO @id, @name -- must write the next piece of data here
    END
CLOSE cur -- close the cursor
DEALLOCATE cur

       The results are as follows, we can see that the data whose ID is 4 has not changed, it is still Zhao Liu, not the test after UPDATE:

       Let's take a look at the dynamic cursor, just remove the STATIC keyword:

DECLARE @id INT , @name NVARCHAR(50) -- declare variables, data to be read
DECLARE cur CURSOR -- remove the STATIC keyword
FOR
    SELECT  * FROM    #T
OPEN cur -- open the cursor
FETCH NEXT FROM cur INTO @id, @name --fetch data
WHILE ( @@fetch_status = 0 ) -- determine if there is still data
    BEGIN
        SELECT 'Data: ' + RTRIM(@id) + @name
		UPDATE #T SET name='test' WHERE id=4 -- test static and dynamic
        FETCH NEXT FROM cur INTO @id, @name -- must write the next piece of data here
    END
CLOSE cur -- close the cursor
DEALLOCATE cur

       We look at the results and we can see that the data with ID 4 has been changed to test:

       The above is the usage of cursors, as well as the introduction and use of dynamic and static cursors.