for loop in transact sql - sql

Question

I have a table that contains some data that needs to be updated. Let's assume that the table structure is
(Code, Col1, Col2, Col3)
I need to run an update on specific rows that have the Code value (let's say the values are (1,3,4,5,9,6,30,25,87,100)).
The Col3 value is Col1+Col2 and the Col1 values are (1001,1025,400,25,963,13,432,25,87,100).
I created the following SQL Query:
Declare #Col1 float
Declare #Code nvarchar
set #Col1 = 1001
set #Code = 1
update MyTable set
Col1 = #Col1,
Col3 = #Col1 + Col2
where Code = #Code
So, instead copying all this code after the Declare lines and manually assigning values, is it possible to create two arrays, one for Col1 values other for Code values and iterate through the Code and updating it dynamically?

Answer 1

Usually iteration over rows of data using loops or cursors considered as bad practice in SQL since it is much slower in most cases.
In your particular case there is no need to iterate over some "arrays" to perform your desired update.
Instead you can create temporary table like this:
create table #temp_table (Col1 float, Code nvarchar(10))
fill it with your data like:
insert into #temp_table (Col1, Code)
select 1001, '1'
union all
select 1025, '3'
... and so on
and then perform your update:
update MyTable set
Col1 = T1.Col1,
Col3 = T1.Col1 + Col2
from MyTable as T
inner join #temp_table as T1 on T.Code = T1.Code

Answer 2

You don't need a loop for this, you can just create a Cte (or temp table) with the values you want to update, and JOIN to it in an UPDATE statement:
;With ToUpdate (Code, Col1) As
(
Select 1, 1001 Union All
Select 3, 1025 Union All
Select 4, 400 Union All
Select 5, 25 Union All
Select 9, 963 Union All
Select 6, 13 Union All
Select 30, 432 Union All
Select 25, 25 Union All
Select 87, 87 Union All
Select 100, 100
)
Update T
Set Col1 = U.Col1,
Col3 = U.Col1 + Col2
From MyTable T
Join ToUpdate U On U.Code = T.Code