问题
I have a dynamic SQL statement I\'ve created in a stored procedure. I need to iterate over the results using a cursor. I\'m having a hard time figuring out the right syntax. Here\'s what I\'m doing.
SELECT @SQLStatement = \'SELECT userId FROM users\'
DECLARE @UserId
DECLARE users_cursor CURSOR FOR
EXECUTE @SQLStatment --Fails here. Doesn\'t like this
OPEN users_cursor
FETCH NEXT FROM users_cursor
INTO @UserId
WHILE @@FETCH_STATUS = 0
BEGIN
EXEC asp_DoSomethingStoredProc @UserId
END
CLOSE users_cursor
DEALLOCATE users_cursor
What\'s the right way to do this?
回答1:
A cursor will only accept a select statement, so if the SQL really needs to be dynamic make the declare cursor part of the statement you are executing. For the below to work your server will have to be using global cursors.
Declare @UserID varchar(100)
declare @sqlstatement nvarchar(4000)
--move declare cursor into sql to be executed
set @sqlstatement = 'Declare users_cursor CURSOR FOR SELECT userId FROM users'
exec sp_executesql @sqlstatement
OPEN users_cursor
FETCH NEXT FROM users_cursor
INTO @UserId
WHILE @@FETCH_STATUS = 0
BEGIN
Print @UserID
EXEC asp_DoSomethingStoredProc @UserId
FETCH NEXT FROM users_cursor --have to fetch again within loop
INTO @UserId
END
CLOSE users_cursor
DEALLOCATE users_cursor
If you need to avoid using the global cursors, you could also insert the results of your dynamic SQL into a temporary table, and then use that table to populate your cursor.
Declare @UserID varchar(100)
create table #users (UserID varchar(100))
declare @sqlstatement nvarchar(4000)
set @sqlstatement = 'Insert into #users (userID) SELECT userId FROM users'
exec(@sqlstatement)
declare users_cursor cursor for Select UserId from #Users
OPEN users_cursor
FETCH NEXT FROM users_cursor
INTO @UserId
WHILE @@FETCH_STATUS = 0
BEGIN
EXEC asp_DoSomethingStoredProc @UserId
FETCH NEXT FROM users_cursor
INTO @UserId
END
CLOSE users_cursor
DEALLOCATE users_cursor
drop table #users
回答2:
This code is a very good example for a dynamic column with a cursor, since you cannot use '+' in @STATEMENT:
ALTER PROCEDURE dbo.spTEST
AS
SET NOCOUNT ON
DECLARE @query NVARCHAR(4000) = N'' --DATA FILTER
DECLARE @inputList NVARCHAR(4000) = ''
DECLARE @field sysname = '' --COLUMN NAME
DECLARE @my_cur CURSOR
EXECUTE SP_EXECUTESQL
N'SET @my_cur = CURSOR FAST_FORWARD FOR
SELECT
CASE @field
WHEN ''fn'' then fn
WHEN ''n_family_name'' then n_family_name
END
FROM
dbo.vCard
WHERE
CASE @field
WHEN ''fn'' then fn
WHEN ''n_family_name'' then n_family_name
END
LIKE ''%''+@query+''%'';
OPEN @my_cur;',
N'@field sysname, @query NVARCHAR(4000), @my_cur CURSOR OUTPUT',
@field = @field,
@query = @query,
@my_cur = @my_cur OUTPUT
FETCH NEXT FROM @my_cur INTO @inputList
WHILE @@FETCH_STATUS = 0
BEGIN
PRINT @inputList
FETCH NEXT FROM @my_cur INTO @inputList
END
RETURN
回答3:
Working with a non-relational database (IDMS anyone?) over an ODBC connection qualifies as one of those times where cursors and dynamic SQL seems the only route.
select * from a where a=1 and b in (1,2)
takes 45 minutes to respond while re-written to use keysets without the in clause will run in under 1 second:
select * from a where (a=1 and b=1)
union all
select * from a where (a=1 and b=2)
If the in statement for column B contains 1145 rows, using a cursor to create indidivudal statements and execute them as dynamic SQL is far faster than using the in clause. Silly hey?
And yes, there's no time in a relational database that cursor's should be used. I just can't believe I've come across an instance where a cursor loop is several magnitudes quicker.
回答4:
First off, avoid using a cursor if at all possible. Here are some resources for rooting it out when it seems you can't do without:
There Must Be 15 Ways To Lose Your Cursors... part 1, Introduction
Row-By-Row Processing Without Cursor
That said, though, you may be stuck with one after all--I don't know enough from your question to be sure that either of those apply. If that's the case, you've got a different problem--the select statement for your cursor must be an actual SELECT statement, not an EXECUTE statement. You're stuck.
But see the answer from cmsjr (which came in while I was writing) about using a temp table. I'd avoid global cursors even more than "plain" ones....
回答5:
After recently switching from Oracle to SQL Server (employer preference), I notice cursor support in SQL Server is lagging. Cursors are not always evil, sometimes required, sometimes much faster, and sometimes cleaner than trying to tune a complex query by re-arranging or adding optimization hints. The "cursors are evil" opinion is much more prominent in the SQL Server community.
So I guess this answer is to switch to Oracle or give MS a clue.
- Oracle EXECUTE IMMEDIATE into a cursor
- Loop through an implicit cursor (a
forloop implicitly defines/opens/closes the cursor!)
回答6:
There is another example which I would like to share with you
:D
http://www.sommarskog.se/dynamic_sql.html#cursor0
回答7:
Another option in SQL Server is to do all of your dynamic querying into table variable in a stored proc, then use a cursor to query and process that. As to the dreaded cursor debate :), I have seen studies that show that in some situations, a cursor can actually be faster if properly set up. I use them myself when the required query is too complex, or just not humanly (for me ;) ) possible.
回答8:
this code can be useful for you.
example of cursor use in sql server
DECLARE sampleCursor CURSOR FOR
SELECT K.Id FROM TableA K WHERE ....;
OPEN sampleCursor
FETCH NEXT FROM sampleCursor INTO @Id
WHILE @@FETCH_STATUS <> -1
BEGIN
UPDATE TableB
SET
...
来源：https://stackoverflow.com/questions/1045880/using-a-cursor-with-dynamic-sql-in-a-stored-procedure