如果满足条件，是否可以选择一个表并加入另一个表？

Question

is it possible to select a table and join another table if the condition was met?

like select * from table if (condition, join a table, nothing to do)

action log table

id  |  user_id |  action                            |   log_time       
    |          | 0=login,1=logout,2=register_client |   
 1  |   001    |             0                      |   2014-07-20 03:10:20 
 2  |   001    |             3                      |   2014-07-20 03:10:34
 3  |   001    |             3                      |   2014-07-20 03:11:58

registration log table

id | user_id | client_id |  date_reg
1  |  001    | 186       |   2014-07-20 03:10:34
2  |  001    | 187       |   2014-07-20 03:11:58

the result that i got is redundant. maybe there's is something wrong with my table. i need to display the first table joined with the 2nd table in which the result should 3 rows only showing 3 activites which is login and registers 2 times based on the 1st table.

Answer 1

select t1.id, t1.type,t2.details 
from table1 t1 
left outer join table2 t2
where 
t1.id=t2.id and 
t1.type=t2.type

Answer 2

据我所知，这是最好的方法：SELECT table1.id，table1。[用户ID]，table1.type，table2.details from table1左联接table2 ON table1.type = table2.type WHERE（条件）

Answer 3

You are looking for left join. You don't need a condition. It will give null when there is no corresponding line in table b.

select t1.*, t2.details
from table1 t1
left join table2 t2 on t1.id = t2.id and t1.type = t2.type