Question

I have a database table that records movements of users in a game. Each time they move I record their user id, the move_index (increments each row) and the id of the zone they are in. I have a special move_index value of -1 to indicate the last movement of that user.

id   user_id  move_index  zone_id   
----------------------------------------------------------------
0    11        0          0  
1    11        1          0 
2    11        2          0   
3    11       -1          3    

4    22        0          0   
5    22        1          1   
6    22        2          1    
7    22       -1          3    

I want to do two things with sql:

• Detect all the users who started and finished in specific zones (e.g. started in zone 0 and finished in zone 3)
• Extending the above, detect all users who started and finished in specific zones AND passed through a specific zone.

I know how to do this with multiple SQL statements & java – but I don’t know how to do this in a single SQL statement. Do I need to do a select and then a select on the results of this select ?

Answer 1

You can simply carry out a SUBQUERY to achieve this within a “single” query.

e.g.: SELECT * FROM t1 WHERE column1 = (SELECT column1 FROM t2);

In essence, you’re using the results of the “inner” SELECT as the working data set for the “outer” SELECT.