sqlserver，定义一个变量，但返回NULL，'SET @item_id' 导致出错，但不知为何

Question

DECLARE @user_id bigint
DECLARE @item_ids varchar(1000)
SET @user_id = 7
SET @item_ids = N'536563,562379'
DECLARE @i INT;
DECLARE @count INT;
DECLARE @price BIGINT;
DECLARE @total_price BIGINT;
DECLARE @temp_price BIGINT;
SET @total_price = 1;
SET @i = 0;
SET @count = (SELECT COUNT(VALUE) FROM string_split(@item_ids,',') );
WHILE (@i < @count)
BEGIN
DECLARE @item_id BIGINT;
SET @item_id = CAST(
(
SELECT ID FROM
(
SELECT CAST(VALUE AS BIGINT) AS ID,ROW_NUMBER() OVER(ORDER BY VALUE) AS RowNum
FROM string_split(@item_ids,',')) AS Idx
WHERE RowNum = @i
)
AS BIGINT
) ;
SET @price = (SELECT price FROM tb_item WHERE id = @item_id)
SET @total_price = @total_price + @price
SET @i = @i + 1
END
SELECT @total_price AS TTP

Answer 1

一、SELECT可以在一条语句里对多个变量同时赋值,而SET只能一次对一个变量赋值

二、表达式返回多个值时，用SET将会出错，而SELECT将取最后一个值

三、表达式无返回值时，用SET将置变量值为NULL,用SELECT变量将保持原值

四、使用标量子查询时，如果无返回值，SET和SELECT一样，都将置为NULL