Question

I need help with the following query.

create table #table1
(id int not null primary key identity,
customer_name varchar(25),
usage float,
oldID int null
)


insert into #table1 values('ABC',46.5,null)
insert into #table1 values('ABC',46.5,null)
insert into #table1 values('DEF',36.8,null)
insert into #table1 values('XYZ',50.1,null)
insert into #table1 values('DEF',36.8,null)
insert into #table1 values('XYZ',50.1,null)

select * from #table1

I want my table to be updated like this

id          customer_name             usage                  oldID
----------- ------------------------- ---------------------- -----------
1           ABC                       46.5                   NULL
2           ABC                       46.5                   1
3           DEF                       36.8                   NULL
4           XYZ                       50.1                   NULL
5           DEF                       36.8                   3
6           XYZ                       50.1                   4

1. The two records with the same name and usage means the later record was renewed.
2. In the new record the oldID field should point to its old record (ID).

Although in my actual table, I have a bunch of date fields which I probably can use but this would help me for now.

Answer 1

With cte, without subquerys, updating only customers with several rows:

with cte as (
   select customer_name, min( id ) as id
   from #table1
   group by customer_name
   having count(*) > 1
)
update #table1 
 set oldID = cte.id
 from cte 
 where #table1.customer_name = cte.customer_name
 and #table1.id != cte.id