我们可以插入一张表并通过查看其他表的值来更新插入行的一列吗？

Question

How to write a trigger which upon insert in table3 looks at table1 if the code and id has a price there first and updates the price of table3 accordingly. But if the id + code cannot be found then it looks in table2 for the id and uses that price to calculate the total price and updates the price in table3 for that insert.

table1

| id | code | price |
|----|------|-------|
| 1  | fire | 3     |
| 2  | ice  | 4     |
| 3  | air  | 5     |

table2

| id | price |
|----|-------|
| 1  | 8     |
| 2  | 9     |

table3

| id | code | qty | price |
|----|------|-----|-------|
| 1  | fire | 2   |       | -- price = 2 * 3 = 6
| 2  | none | 3   |       | -- price = 3 * 9 = 27

Answer 1

In your trigger function you should use if exists to check it. You can write your trigger function like below:

CREATE FUNCTION update_price()
RETURNS trigger AS $$
  BEGIN

if exists (select price from table1 where code=new.code and id=new.id) then
new.price= new.qty * (select price from table1 where code=new.code and id=new.id);
else
new.price= new.qty * (select price from table2 where id=new.id);
end if;
  
    return new;
  END;
$$ LANGUAGE plpgsql;

Then you can write your trigger on table3 as below:

create trigger update_price 
before insert
on table3
for each row 
execute procedure update_price();

DEMO