从同一表中另一列中的最小日期更新日期列

Question

I need to update a field called "FirstPayment" with the date from another field in the same table.

Here is the code:

UPDATE R1
SET R1.FirstPayment = (select min(r1.effective_date) where R1.amount>0)
from CTTC_RentalCarPayments R1, CTTC_RentalCarPayments R2
where r1.id=r2.id

This results in "An aggregate may not appear in the set list of an UPDATE statement"

I know I am missing something simple here.

I also know the FirstPayment field isn't necessary because I could get that date in a query but the application I am feeding this data too needs it in this column.

Answer 1

Use window functions:

with toupdate as (
      select r.*,
             min(case when r1.amount > 0 then r.effective_date end) over (partition by id) as new_effective_date
      from CTTC_RentalCarPayments r
     )
update toupdate
    set FirstPayment = new_effective_date;

Answer 2

You could set value of the field you want to get to a variable and then update the field to that variable:

Declare @firstPayment datetime;
Set @firstPayment = (select min(r1.effective_date) from r1 where R1.amount>0); 
Declare @firstPaymentID int;
Set @firstPaymentID = (select id from r1 where @firstPayment = FirstPayment);

UPDATE r1
SET FirstPayment = @firstPayment where Id = @firstPaymentID;

Let me know if this helps. It's not a recursive CTE but it might work too.