Question

I have the following tables

Actions: actionID, time, type
States: stateID, time, type

I need to find out what was the current state at the time the actions were performed.

So I started up with something like

SELECT actions.actionID, states.stateID
FROM actions, states
WHERE states.time < actions.time

This gives me a result table with ALL the states that existed before a given action took place. But I need only the LAST state before the action.

So I need some kind of GROUP BY based on max(states.stateID) but I honestly can’t figure out how to solve this one !

EDIT

So if actions table contains this:

actionID  time                   type
1         '2012-10-30 02:05:00'  100
2         '2012-10-30 02:11:00'  200

And states table contains this:

stateID  time                   type
11       '2012-10-30 02:00:00'  A
12       '2012-10-30 02:10:00'  B
13       '2012-10-30 02:15:00'  A

I would like the following result:

actionID  stateID
1         11          // Because before action 1, the state that prevailed was 11
2         12          // Because before action 2, the state that prevailed was 12

Answer 1

try this:

select t.actionID,t.stateID,a.type action_type,s.type state_type  from 

(SELECT actions.actionID,max(states.stateID) stateID
FROM actions, states
WHERE states.time < actions.time
group by actions.actionID)as t

join 

actions a on t.actionID=a.actionID

join

states s on t.stateID=s.stateID

SQL FIDDLE DEMO