Question

What I am trying to do, using java, is:

• access a database
• read a record from table “Target_stats”
• if the field “threat_level” = 0, doAction1
• if the field “threat_level” > 0, get additional fields from another table “Attacker_stats” and doAction2
• read the next record

Now I have everything I need but a well thought out SQL statement that will allow me to only go through the database only once, if this does not work I suspect I will need to use two separate SQL statements and go through the database a second time. I do not have a clear understanding of case statements, so I will just provide pseudo code using an if statement.

SELECT A.1, A.2, A.3
   if(A.3 > 0){
      SELECT A.1, A.2, A.3, B.1, B.3 
      FROM A 
      JOIN B 
      ON A.1 = B.1
      }
FROM A

Can anyone shed any light on my situation?

EDIT: Thankyou both for your time and effort. I understand both of your comments and I believe that I am headed more towards the right direction however, I’m still having some trouble. I didn’t know about SQLfiddle before so I have now gone ahead and made a sample DB and tried to demonstrate my purpose. Here is the link: http://sqlfiddle.com/#!3/ea108/1 What I want to do here is Select target_stats.server_id, target_stats.target, target_stats.threat_level Where interval_id=3 and if the threat_level>0 I want to retrieve attack_stats.attacker, attack_stats.sig_name Where interval_id=3. Again, thankyou for your time and effort it is very useful to me

EDIT: after some tinkering around, I figured it out. thankyou so much for your help

Answer 1

As @Ocelot20 said, SQL is not procedural code. It is based on set-based operations, not per row operations. One immediate consequence of this is that the SELECT in your pseudo-example is wrong as it relies on rows in the same result set having different column lists.

That said, you can get pretty close to your pseudo-code example, if you can tolerate NULL values where the join is not possible.

Here’s an example that (to me anyway) seems to be close to what your are driving at:

select *
from A 
  left outer join B 
    on A.a = B.d and A.a > 2

You can see it in action in this SQLFiddle, which should show you what sort of output to expect.

Note that what this is actually saying is something like this:

Fetch all the records from table A and also fetch any records from
table B have their d column the same as the a column in table
A, provided the value of A.a is greater than 2.

(This was picked for convenience. In my rather contrived example shifting the conditional column does not effect the output as can be see here).