How to replace a SQL Server cursor with a different mechanism - json

Question

I am working on SQL Server 2008. I have to create json from two tables, which have a one-to-many relationship. The tables are Customer and Orders.
Each customer may have one or many orders. The json is constructed by first getting data from the customer table and then appending all the purchases they have made.
The following is my query. I have also enclosed the json output from the query. It works and creates valid jsons. The problem is that it's too slow as I am using a cursor to loop through the Customer table. I have managed to avoid cursor to get data from the Orders table by using for xml path. Since I have to handle millions of rows, I have to replace the cursor with some other mechanism.
DECLARE #PaymentType VARCHAR(50),
#Email VARCHAR(100),
#OrderId INT
DECLARE CustomerCursor CURSOR FAST_FORWARD FOR
SELECT TOP 10
PaymentType, Email, OrderId
FROM
CUSTOMER
OPEN CustomerCursor
FETCH NEXT FROM CustomerCursor INTO #PaymentType, #Email, #OrderId
WHILE (##FETCH_STATUS = 0)
BEGIN
DECLARE #customer VARCHAR(MAX)
DECLARE #order VARCHAR(MAX)
DECLARE #customer_with_order VARCHAR(MAX)
-- construct order json
SET #order = '[' + STUFF((SELECT ',{"orderProductID":' + CAST(orderProductID AS VARCHAR) +
',"productType":"' + ProductType + '"' +
',"productName":"' + ProductName + '"' +
',"categoryName":"' + CategoryName + '"' + '}'
FROM ORDERS
WHERE orderid = #OrderId
FOR XML PAT(''), TYPE).value('.', 'VARCHAR(MAX)'), 1, 1, '') + ']'
-- construct customer json
SET #customer = '{"email":"' + CASE WHEN #Email IS NULL THEN '' ELSE
#Email END + '"'
+ ',"eventName": "ChristmasSale", "dataFields": {'
+ '"orderId":' + CAST(CASE WHEN #OrderId IS NULL THEN 0 ELSE
#OrderId END AS VARCHAR)
+ ',"paymentType":"' + CASE WHEN #PaymentType IS NULL THEN
'' ELSE #PaymentType END + '"'
+ ',"products": '
-- combine these two
SET #customer_with_order = #customer + #order + '}}'
-- insert into CUSTOMER_ORDER_DATA
INSERT INTO CUSTOMER_ORDER_DATA(email, order_id, orders)
VALUES (#Email, #OrderId, #customer_with_order)
FETCH NEXT FROM CustomerCursor INTO #PaymentType, #Email, #OrderId
END
CLOSE CustomerCursor
DEALLOCATE CustomerCursor

Answer 1

I can't test this, but I suspect you could rewrite the above as a set based method as below (as I have no way of testing this, there is no way I can be certain this'll work, if it doesn't, you may need to troubleshoot it a little):
INSERT INTO CUSTOMER_ORDER_DATA(email, order_id, orders)
SELECT C.Email,
C.orderid,
'{"email":"' + CASE WHEN #Email IS NULL THEN '' ELSE
#Email END + '"'
+ ',"eventName": "ChristmasSale", "dataFields": {'
+ '"orderId":' + CAST(CASE WHEN #OrderId IS NULL THEN 0 ELSE
#OrderId END AS varchar)
+ ',"paymentType":"' + CASE WHEN #PaymentType IS NULL THEN
'' ELSE #PaymentType END + '"'
+ ',"products": ' +
('[' + STUFF((
SELECT
',{"orderProductID":' + CAST(orderProductID AS varchar)
+ ',"productType":"' + ProductType + '"'
+ ',"productName":"' + ProductName + '"'
+ ',"categoryName":"' + CategoryName + '"'
+'}'
FROM ORDERS AS O
WHERE O.orderid = C.orderid
FOR XML PATH(''),TYPE).value('.', 'varchar(max)'), 1, 1, '') + ']')
FROM CUSTOMER AS C
Considering the OP has 5 millions rows, then this would likely be a bit much for one batch. Seperating it into batching of say 10,000 may be better for performance over all. Unfortunately the OP is still using 2008, so they don't have access to the OFFSET clause.