Does UPDATE happen column by column in SQL Server

Question

I have following query -

 UPDATE THINGS

SET Col 1 = CASE when 'A' then 'APPLE'
                 when 'B' then 'BALL'
                 when 'C' then 'CARROT'
                 else NULL
                 end,
    Col 2 = Case Col 1 when 'APPLE' then 'FRUIT'
                       when 'BALL'  then 'TOY'
                       when 'CARROT' then 'SALAD'
             else NULL
             end

My question is will the update happen on column by column basis so that I can get updated values in my Col 2 successfully? If it is not possible by above query, is there any other way to update it within a single query? I cannot write two separate queries.

Answer 1

You can use this approach by using variable to store data and then use it foe next column.

DECLARE @val VARCHAR(20) -- type of Col1
UPDATE THINGS
SET @val = [Col 1] = CASE WHEN 'A' THEN 'APPLE'
                          WHEN 'B' THEN 'BALL'
                          WHEN 'C' THEN 'CARROT'
                     END,
       [Col 2] = CASE @val WHEN 'APPLE'  THEN 'FRUIT'
                           WHEN 'BALL'   THEN 'TOY'
                           WHEN 'CARROT' THEN 'SALAD'
                 END

Answer 2

You can't rely upon the order of execution of assignments in a query. Most likely, the DBMS will pull the row, check the values, then write the changes. In SQL Server, I'd write it like this:

UPDATE THINGS
SET [Col 1] = CASE WHEN [Col 1] = 'A' then 'APPLE'
             WHEN [Col 1] = 'B' then 'BALL'
             WHEN [Col 1] = 'C' then 'CARROT'
             ELSE NULL
             END,
    [Col 2] = CASE WHEN [Col 1] IN ('A','APPLE') then 'FRUIT'
             WHEN [Col 1] IN ('B', 'BALL')  then 'TOY'
             WHEN [Col 1] IN ('C','CARROT') then 'SALAD'
             ELSE NULL
             END

Answer 3

Is there a reason you cannot write your SQL this way:

UPDATE THINGS
SET Col1 = CASE col1
             when 'A' then 'APPLE'
             when 'B' then 'BALL'
             when 'C' then 'CARROT'
             else NULL
             end,
Col2 = Case Col1
            when 'A' then 'FRUIT'
            when 'B' then 'TOY'
            when 'C' then 'SALAD'
         else NULL
         end

Answer 4

Actually, the columns are logically updated in their order in the update statement.

Here's a little sample to demonstrate it:

create table #t (i int, j int);
go
insert #t values (1,10);
go

update #t 
set j = i,
i = 5;

select * from #t;
go

drop table #t;
go