Microsoft SQL Server
Cursori

Sintassi

• DECLARE cursor_name CURSOR [LOCAL | GLOBALE]
  • [FORWARD_ONLY | SCORRERE ]
    [STATICO | KEYSET | DINAMICO | AVANTI VELOCE ]
    [READ_ONLY | SCROLL_LOCKS | OTTIMISTA]
    [TYPE_WARNING]
  • PER select_statement
  • [FOR UPDATE [OF column_name [, ... n]]]

Osservazioni

Cursore di avanzamento di base

Normalmente si vorrebbe evitare di utilizzare i cursori in quanto possono avere un impatto negativo sulle prestazioni. Tuttavia, in alcuni casi speciali potrebbe essere necessario eseguire il loop del record di dati tramite record ed eseguire alcune azioni.

DECLARE @orderId AS INT

-- here we are creating our cursor, as a local cursor and only allowing 
-- forward operations
DECLARE rowCursor CURSOR LOCAL FAST_FORWARD FOR
    -- this is the query that we want to loop through record by record
    SELECT [OrderId]
    FROM [dbo].[Orders]

-- first we need to open the cursor
OPEN rowCursor

-- now we will initialize the cursor by pulling the first row of data, in this example the [OrderId] column,
-- and storing the value into a variable called @orderId
FETCH NEXT FROM rowCursor INTO @orderId

-- start our loop and keep going until we have no more records to loop through
WHILE @@FETCH_STATUS = 0 
BEGIN

    PRINT @orderId
    
    -- this is important, as it tells SQL Server to get the next record and store the [OrderId] column value into the @orderId variable
    FETCH NEXT FROM rowCursor INTO @orderId

END

-- this will release any memory used by the cursor
CLOSE rowCursor
DEALLOCATE rowCursor

Sintassi del cursore rudimentale

Una semplice sintassi del cursore, che funziona su alcune righe di prova di esempio:

/* Prepare test data */
DECLARE @test_table TABLE
(
    Id INT,
    Val VARCHAR(100)
);
INSERT INTO @test_table(Id, Val)
VALUES 
    (1, 'Foo'), 
    (2, 'Bar'), 
    (3, 'Baz');
/* Test data prepared */

/* Iterator variable @myId, for example sake */
DECLARE @myId INT;

/* Cursor to iterate rows and assign values to variables */
DECLARE myCursor CURSOR FOR
    SELECT Id
    FROM @test_table;

/* Start iterating rows */
OPEN myCursor;
FETCH NEXT FROM myCursor INTO @myId;

/* @@FETCH_STATUS global variable will be 1 / true until there are no more rows to fetch */
WHILE @@FETCH_STATUS = 0
BEGIN

    /* Write operations to perform in a loop here. Simple SELECT used for example */
    SELECT Id, Val
    FROM @test_table 
    WHERE Id = @myId;

    /* Set variable(s) to the next value returned from iterator; this is needed otherwise the cursor will loop infinitely. */
    FETCH NEXT FROM myCursor INTO @myId;
END
/* After all is done, clean up */
CLOSE myCursor;
DEALLOCATE myCursor;

Risultati da SSMS. Si noti che queste sono tutte query separate, non sono in alcun modo unificate. Si noti come il motore di query elabora ciascuna iterazione una alla volta anziché come una serie.