SQL-将要查找的结果包含在列中，并将所有其他值设置为null

Question

I have two tables, one with orders and another with order comments. I want to join these two tables. They are joined on a column "EID" which exists in both tables. I want all orders. I also want to see all comments with only certain criteria AND all other comments should be set to null. How do I go about this?

Orders Table
Order_Number
1
2
3
4

Comments Table
Comments
Cancelled On
Ordered On
Cancelled On
Cancelled On

In this example I would like to see for my results:

Order_Number | Comments
1 | Cancelled On
2 | Null
3 | Cancelled On
4 | Cancelled On

Thanks!

Answer 1

This seems like a rather trivial left join.

select o.order_number, c.comments
  from orders o
left join comments c
    on o.eid = c.eid
    and (here goes your criteria for comments)

Tested on Oracle, there might be subtle syntax differences for other DB engines.

Answer 2

It depends on one condition: Are you trying to SET the other comments to null? (replace the values in the table) or Are you trying to DISPLAY the other comments as null? (dont display them)

If you want to change the values in the table use

    UPDATE `table` SET `column` = null WHERE condition;

otherwise use:

    SELECT column FROM table JOIN othertable WHERE condition;