sqlserver中关于sql语句

上图分别为a，b两列的数据让c的数据为b-a=？。有-符号则c=-。怎么写sql语句。
c的数据类型为什么？

update table set c = a-b

update table set c = DateDiff(MINUTE,a,b)

update table set c = DateDiff(MINUTE,a,b)

update table_name set c=datediff(minute,a,b)

update table set datediff(minute,a,b)

SELECT a,b,DATEDIFF(mi,a,b) FROM dbo.Table