CURSOR

Translation reference to convert Transact-SQL CURSOR statement to Snowflake Scripting

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Applies to

• SQL Server

• Azure Synapse Analytics

Some parts in the output code are omitted for clarity reasons.

Description

Transact-SQL statements produce a complete result set, but there are times when the results are best processed one row at a time. Opening a cursor on a result set allows processing the result set one row at a time. You can assign a cursor to a variable or parameter with a cursor data type. For more information check here.

//ISO Syntax  
DECLARE cursor_name [ INSENSITIVE ] [ SCROLL ] CURSOR   
     FOR select_statement   
     [ FOR { READ ONLY | UPDATE [ OF column_name [ ,...n ] ] } ]  
[;]  

//Transact-SQL Extended Syntax  
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 ] ] ]  
[;]  

FETCH   
          [ [ NEXT | PRIOR | FIRST | LAST   
                    | ABSOLUTE { n | @nvar }   
                    | RELATIVE { n | @nvar }   
               ]   
               FROM   
          ]   
{ { [ GLOBAL ] cursor_name } | @cursor_variable_name }   
[ INTO @variable_name [ ,...n ] ] 

OPEN { { [ GLOBAL ] cursor_name } | cursor_variable_name }  

CLOSE { { [ GLOBAL ] cursor_name } | cursor_variable_name }

DEALLOCATE { { [ GLOBAL ] cursor_name } | @cursor_variable_name }  

Sample Source Patterns

Transact-SQL

Notice that the following parameters are inherently supported by Snowflake Scripting.

• [LOCAL].

• [FORWARD_ONLY].

• [FAST_FORWARD] Specifies a FORWARD_ONLY (FETCH NEXT only) and READ_ONLY

• [READ_ONLY] the WHERE CURRENT OF does not exist in Snowflake Scripting.

Cursor

IN -> SqlServer_01.sql

CREATE TABLE vEmployee   (
    PersonID INT,
    LastName VARCHAR(255),
    FirstName VARCHAR(255),
);

INSERT INTO vEmployee(PersonID, LastName, FirstName) 
VALUES
    (1, 'AA', 'A'),
    (2, 'BB', 'B'),
    (3, 'CC', 'C'),
    (4, 'DD', 'D'),
    (5, 'EE', 'E'),
    (6, 'FF', 'F'),
    (7, 'GG', 'G');
	
CREATE OR ALTER PROCEDURE CursorExample
AS
    DECLARE 
        @CursorVar CURSOR, 
	@firstName VARCHAR;

    SET @CursorVar = CURSOR LOCAL FORWARD_ONLY STATIC READ_ONLY 
	FOR  
	SELECT FirstName
	FROM vEmployee;  

    OPEN @CursorVar;

    FETCH NEXT FROM @CursorVar INTO @firstName;
    FETCH NEXT FROM @CursorVar INTO @firstName;

    CLOSE @CursorVar;

    SELECT @firstName;
GO

Result

Snowflake Scripting

Cursor

OUT -> SqlServer_01.sql

CREATE OR REPLACE TABLE vEmployee (
	PersonID INT,
	LastName VARCHAR(255),
	FirstName VARCHAR(255)
)
COMMENT = '{"origin":"sf_sc","name":"snowconvert","version":{"major":1, "minor":0},{"attributes":{"component":"transact"}}'
;

INSERT INTO vEmployee (PersonID, LastName, FirstName)
VALUES
    (1, 'AA', 'A'),
    (2, 'BB', 'B'),
    (3, 'CC', 'C'),
    (4, 'DD', 'D'),
    (5, 'EE', 'E'),
    (6, 'FF', 'F'),
    (7, 'GG', 'G');

CREATE OR REPLACE PROCEDURE CursorExample ()
RETURNS TABLE()
LANGUAGE SQL
COMMENT = '{"origin":"sf_sc","name":"snowconvert","version":{"major":1, "minor":0},{"attributes":{"component":"transact"}}'
EXECUTE AS CALLER
AS
$$
	DECLARE
		CURSORVAR CURSOR
		FOR
			SELECT FirstName
			FROM vEmployee;
		FIRSTNAME VARCHAR;
		ProcedureResultSet RESULTSET;
	BEGIN
		 
		 
		OPEN CURSORVAR;
		FETCH
			CURSORVAR
		INTO
			:FIRSTNAME;
		FETCH
			CURSORVAR
		INTO
			:FIRSTNAME;
		CLOSE CURSORVAR;
		ProcedureResultSet := (
		SELECT
			:FIRSTNAME);
		RETURN TABLE(ProcedureResultSet);
	END;
$$;

Result

Known Issues

The following parameters are not supported:

DECLARE CURSOR

• [ GLOBAL ] Allows referencing the cursor name in any stored procedure or batch executed by the connection. Snowflake Scripting only allows the use of the cursor locally.

• [ SCROLL ] Snowflake Scripting only support FETCH NEXT.

• [ KEYSET | DYNAMIC ] If after opening a cursor and update to the table is made, these options may display some of the changes when fetching the cursor, Snowflake scripting only supports STATIC, in other words, after the cursor is opened the changes to the table are not detected by the cursor.

• [SCROLL_LOCKS] Specifies that positioned updates or deletes made through the cursor are guaranteed to succeed, Snowflake Scripting cannot guarantee it.

• [OPTIMISTIC] When an update or delete is made through the cursor it uses comparisons of timestamp column values, or a checksum value if the table has no timestamp column, to determine whether the row was modified after it was read into the cursor. Snowflake Scripting does not have an internal process to replicate it.

• [TYPE_WARNING]

FETCH

• [PRIOR | FIRST | LAST] Snowscripting only support NEXT.

• [ABSOLUTE] Snowflake Scripting only supports NEXT but the behavior can be replicated.

• [RELATIVE] Snowflake Scripting but the behavior can be replicated.

• [ GLOBAL ] Allows referencing the cursor name in any stored procedure or batch executed by the connection. Snowflake Scripting only allows the use of the cursor locally.

• FETCH without INTO is not supported.

• When the FETCH statement is located inside a loop it is considered a complex pattern as it may have an impact on the Snowflake translated code performance. Check the related issues section for more information.

Fetch inside loop sample

Input

IN -> SqlServer_02.sql

CREATE OR ALTER PROCEDURE cursor_procedure1
AS
BEGIN
DECLARE cursor1 CURSOR FOR SELECT col1 FROM my_table;
WHILE 1=0
   BEGIN
      FETCH NEXT FROM @cursor1 INTO @variable1;
   END
END;

Output

OPEN

• [ GLOBAL ] Allows referencing the cursor name in any stored procedure or batch executed by the connection. Snowflake Scripting only allows the use of the cursor locally.

CLOSE

• [ GLOBAL ] Allows referencing the cursor name in any stored procedure or batch executed by the connection. Snowflake Scripting only allows the use of the cursor locally.

DEALLOCATED Removes a cursor reference and there is no equivalent in Snowflake Scripting.

WHERE CURRENT OF the use of this statement is not supported, for example:

CREATE OR ALTER PROCEDURE CursorWithCurrent
AS
    DECLARE 
        @CursorVar CURSOR;

    SET @CursorVar = CURSOR 
	FOR  
	SELECT FirstName  
	FROM vEmployee;  

    OPEN @CursorVar;

    FETCH NEXT FROM @CursorVar;
    FETCH NEXT FROM @CursorVar;

    UPDATE vEmployee SET LastName = 'Changed' WHERE CURRENT OF @CursorVar;

    CLOSE @CursorVar;
GO

Environment variables

• @@CURSOR_ROWS

• @@FETCH_STATUS

Related EWIs

1. SSC-FDM-TS0013: Snowflake Scripting cursor rows are not modifiable.

2. SSC-PRF-0003: Fetch inside a loop is considered a complex pattern, this could degrade Snowflake performance.