如何使用触发器更新新插入元组的列

Question

I have a table BillRecords

bill_id        buy_amt        sell_amt        profit
----------------------------------------------------
0              200            300             NULL
1              1000           1200            NULL

Let's say I want to insert following record

INSERT INTO BillRecords(bill_id, buy_amt, sell_amt) 
VALUES (2, 2000, 2500)

I'm expecting that as soon as I insert record, trigger should only update profit column for newly inserted record (ie WHERE bill_id = 2) and not for all the rows in profit column. What should I do?

Note: I'm using SQL Server

Answer 1

No need for a trigger to do what you want. It is much simpler, and more efficient, to use a computed column:

create table billrecords (
    bill_id  int primary key,
    buy_amt  int,
    sell_amt int,
    profit as (buy_amt - sell_amt)
);

Or if you want to add it to an already-existing table:

alter table billrecords
    add profit as (buy_amt - sell_amt);

This gives you an always up-to-date value, that you can access just like any other column. For better performance, you can use option persisted , so the value column is physically stored, and recomputed only when needed instead of every time it is accessed.

Answer 2

Since you only want to update the profit amount of the transaction having bill_id = 2, you can use the below after trigger this in combination with a derived column "profit" which fetches the profit amount.

CREATE  TRIGGER trigger_name   
    ON billrecords      
    After INSERT   
    AS
    IF EXISTS (SELECT * from inserted)
     begin
     
        update  m
        set m.profit = m.sell_amt - m.bill_amt, m.bill_amt = null, m.sell_amt = null
        from billrecords m join inserted i
        on i.bill_id = m.bill_id
        where i.bill_id = 2 
     
     end;