SQL LEFT-JOIN on 2 fields for MySQL

问题

I have a view A and a view B.

In A I have a lot of information about some systems, like IP and port which I want to preserve all. In B I have just one information that I want to add at A.

The matching fields between the two views are IP and Port. So I have to match those hosts which has the same IP and Port in both views.

Examples:

View A:

IP | OS     | Hostname | Port | Protocol
1  | Win    | hostONE  | 80   | tcp 
1  | Win    | hostONE  | 443  | tcp 
1  | Win    | hostONE  | 8080 | tcp 
2  | Linux  | hostTWO  | 21   | tcp
2  | Linux  | hostTWO  | 80   | tcp
3  | Linux  | hostTR   | 22   | tcp

View B:

IP | Port | State
1  | 443  | Open
2  | 80   | Closed

OUTPUT

IP | OS     | Hostname | Port | Protocol | State
1  | Win    | hostONE  | 80   | tcp      |
1  | Win    | hostONE  | 443  | tcp      | Open
1  | Win    | hostONE  | 8080 | tcp      |
2  | Linux  | hostTWO  | 21   | tcp      | Closed
2  | Linux  | hostTWO  | 80   | tcp      |
3  | Linux  | hostTR   | 22   | tcp      |

Note: Is possible that some hosts of the view A has no IP/Port related items in View B.

Is also possible that some hosts of the view A has some match in the View B.

I thought that I should be using LEFT JOIN in order to have all the entry of View A and the correct associated entry of View B, but it didn't work. I'm not able to adjust the query with the right WHERE clause and JOIN solution.

Any idea?

回答1:

select a.ip, a.os, a.hostname, a.port, a.protocol,
       b.state
from a
left join b on a.ip = b.ip 
           and a.port = b.port

回答2:

Let's try this way:

select 
    a.ip, 
    a.os, 
    a.hostname, 
    a.port, 
    a.protocol, 
    b.state
from a
left join b 
    on a.ip = b.ip 
        and a.port = b.port /*if you has to filter by columns from right table , then add this condition in ON clause*/
where a.somecolumn = somevalue /*if you have to filter by some column from left table, then add it to where condition*/

So, in where clause you can filter result set by column from right table only on this way:

...
where b.somecolumn <> (=) null

来源：https://stackoverflow.com/questions/18914698/sql-left-join-on-2-fields-for-mysql

标签

mysql

sql

join

left-join