Flow control for T-SQL

This topic provides reference information about flow control in SQL Server and Amazon Aurora MySQL, comparing their respective capabilities and syntax differences. You can use this guide to understand how to adapt your existing SQL Server flow control statements when migrating to Aurora MySQL.

SQL Server Usage

Although SQL is a mostly declarative language, it does support flow control commands, which provide run time dynamic changes in script run paths.

SQL Server provides the following flow control keywords.

For more information, see Error Handling.

Examples

Create and populate the OrderItems table.

CREATE TABLE OrderItems
(
    OrderID INT NOT NULL,
    Item VARCHAR(20) NOT NULL,
    Quantity SMALLINT NOT NULL,
    PRIMARY KEY(OrderID, Item)
);

INSERT INTO OrderItems (OrderID, Item, Quantity)
VALUES
(1, 'M8 Bolt', 100),
(2, 'M8 Nut', 100),
(3, 'M8 Washer', 200);

WAITFOR

Use WAITFOR to introduce a one minute delay between background batches purging old data.

SET ROWCOUNT 1000;
WHILE @@ROWCOUNT > 0;
BEGIN;
    DELETE FROM OrderItems
    WHERE OrderDate < '19900101';
    WAITFOR DELAY '00:01:00';
END;

GOTO

Use GOTO to skip a code section based on an input parameter in a stored procedure.

CREATE PROCEDURE ProcessOrderItems
@OrderID INT, @Item VARCHAR(20), @Quantity INT, @UpdateInventory BIT
AS
BEGIN
        INSERT INTO OrderItems (OrderID, Item, Quantity)
        SELECT @OrderID, @item, @Quantity
    IF @UpdateInventory = 0
        GOTO Finish
    UPDATE Inventory
    SET Stock = Stock - @Quantity
    WHERE Item = @Item
    /* Additional Inventory Processing */
finish:
/* Generate Results Log*/
END

Dynamic Procedure Run Path

The following example demonstrates a solution for running different processes based on the number of items in an order.

Declare a cursor for looping through all OrderItems and calculating the total quantity for each order.

DECLARE OrderItemCursor CURSOR FAST_FORWARD
FOR
SELECT OrderID,
    SUM(Quantity) AS NumItems
FROM OrderItems
GROUP BY OrderID
ORDER BY OrderID;
DECLARE @OrderID INT, @NumItems INT;

-- Instantiate the cursor and loop through all orders.
OPEN OrderItemCursor;

FETCH NEXT FROM OrderItemCursor
INTO @OrderID, @NumItems

WHILE @@Fetch_Status = 0
BEGIN;

IF @NumItems > 100
    PRINT 'EXECUTING LogLargeOrder - '
    + CAST(@OrderID AS VARCHAR(5))
    + ' ' + CAST(@NumItems AS VARCHAR(5));
ELSE
    PRINT 'EXECUTING LogSmallOrder - '
    + CAST(@OrderID AS VARCHAR(5))
    + ' ' + CAST(@NumItems AS VARCHAR(5));

FETCH NEXT FROM OrderItemCursor
INTO @OrderID, @NumItems;
END;

-- Close and deallocate the cursor.
CLOSE OrderItemCursor;
DEALLOCATE OrderItemCursor;

For the preceding example, the result looks as shown following.

EXECUTING LogSmallOrder - 1 100
EXECUTING LogSmallOrder - 2 100
EXECUTING LogLargeOrder - 3 200

For more information, see Control-of-Flow in the SQL Server documentation.

MySQL Usage

Amazon Aurora MySQL-Compatible Edition (Aurora MySQL) provides the following flow control constructs:

Examples

Create and populate the OrderItems table.

CREATE TABLE OrderItems
(
    OrderID INT NOT NULL,
    Item VARCHAR(20) NOT NULL,
    Quantity SMALLINT NOT NULL,
    PRIMARY KEY(OrderID, Item)
);

INSERT INTO OrderItems (OrderID, Item, Quantity)
VALUES
(1, 'M8 Bolt', 100),
(2, 'M8 Nut', 100),
(3, 'M8 Washer', 200);

Rewrite of SQL Server WAITFOR delay using SLEEP.

CREATE PROCEDURE P()
BEGIN
    DECLARE RR INT;
    SET RR = (
        SELECT COUNT(*)
        FROM OrderItems
        WHERE OrderDate < '19900101'
        );
    WHILE RR > 0 DO
    DELETE FROM OrderItems
    WHERE OrderDate < '19900101';
        DO SLEEP (60);
    SET RR = (
        SELECT COUNT(*)
        FROM OrderItems
        WHERE OrderDate < '19900101'
        );
    END WHILE;
END;

Rewrite of SQL Server GOTO using nested blocks.

CREATE PROCEDURE ProcessOrderItems
(Var_OrderID INT, Var_Item VARCHAR(20), Var_Quantity INT, UpdateInventory BIT)
BEGIN
        INSERT INTO OrderItems (OrderID, Item, Quantity)
        VALUES(Var_OrderID, Var_Item, Var_Quantity)
    IF @UpdateInventory = 1
    BEGIN
        UPDATE Inventory
        SET Stock = Stock - @Quantity
        WHERE Item = @Item
        /* Additional Inventory Processing...*/
    END
/* Generate Results Log */
END

Dynamic Procedure Run Path

The following example demonstrates a solution for running different processes based on the number of items in an order.

This example provides the same functionality as the example for SQL Server flow control. However, unlike the SQL Server example which you run as a batch script, Aurora MySQL variables can only be used in stored routines such as procedures and functions.

Create a procedure to declare a cursor and loop through the order items.

CREATE PROCEDURE P()
BEGIN
    DECLARE done INT DEFAULT FALSE;
    DECLARE var_OrderID INT;
    DECLARE var_NumItems INT;

    DECLARE OrderItemCursor CURSOR FOR
    SELECT OrderID,
        SUM(Quantity) AS NumItems
    FROM OrderItems
    GROUP BY OrderID
    ORDER BY OrderID;

    DECLARE CONTINUE HANDLER
    FOR NOT FOUND SET done = TRUE;

    OPEN OrderItemCursor;

    CursorStart: LOOP
    FETCH NEXT FROM OrderItemCursor
        INTO var_OrderID, var_NumItems;
    IF done
        THEN LEAVE CursorStart;
    END IF;
    IF var_NumItems > 100
        THEN SELECT CONCAT('EXECUTING LogLargeOrder - ', CAST(var_OrderID AS VARCHAR(5)),' Num Items: ', CAST(var_ NumItems AS VARCHAR(5)))
    ELSE SELECT CONCAT('EXECUTING LogSmallOrder - ', CAST(var_OrderID AS VARCHAR(5)),     ' Num Items: ', CAST(var_NumItems AS VARCHAR(5)))
    END IF;
END LOOP;

CLOSE OrderItemCursor;

END;

Summary

While there are some syntax differences between SQL Server and Aurora MySQL flow control statements, most rewrites should be straightforward. The following table summarizes the differences and identifies how to modify T-SQL code to support similar functionality in Aurora MySQL.

For more information, see Flow Control Statements in the MySQL documentation.