MySQL左连接并选择

Question

I have two tables, mytable1 and mytable2. I get results from mytable1 and left join with mytable2.

CREATE TABLE mytable1 
    ( userid int, 
     type int, 
     datetime1 DATETIME );
CREATE TABLE mytable2
    ( userid int,
     name varchar(20),
     day2 DATE,
     time2 TIME );

SELECT x.*, d.name, d.day2, d.time2
  FROM ( SELECT * FROM mytable1 WHERE TYPE=1 ORDER BY userid ASC LIMIT 0,50) x
   LEFT JOIN mytable2 d
   ON x.userid = d.userid

But I need to filter the results, so I will get only results if "mytable2.day2 mytable2.time2" is bigger than 2 days ago. For example let datetime be "2014.03.27 21:00:00". I will get results if mytable2 record is newer than "2014.03.27 21:00:00"

You can check sql fiddle here inital: http://sqlfiddle.com/#!2/a5364/2
My tryout: http://sqlfiddle.com/#!2/a5364/5

Answer 1

Here is what the query will look like :-

select x.*, d.name, d.day2, d.time2
from ( SELECT * FROM mytable1 WHERE type=1 ORDER BY userid ASC LIMIT 0,50) x
left join mytable2 d
on x.userid = d.userid
where ( convert(CONCAT(d.day2,' ',d.time2),datetime) >= SUBDATE(curdate(),INTERVAL 2 DAY))

Use convert() to get the datetime and then use subdate() to find the date before 2 days.

Verify it on the SQL Fiddle you provided.

Answer 2

The question is: if there are no records at all in mytable2 that match a record in mytable1 , do you still want to see the mytable1 record? If so, move the condition to the on clause:

  select x.*, d.name, d.day2, d.time2
  from (SELECT *
        FROM mytable1
        WHERE type = 1
        ORDER BY userid ASC
        LIMIT 0,  50
       ) x left join
       mytable2 d
       on x.userid = d.userid and
          (d.day2 + d.time2) >= DATE_SUB(NOW(), INTERVAL 2 DAY);

Otherwise, you can change the join to an inner join :

  select x.*, d.name, d.day2, d.time2
  from (SELECT *
        FROM mytable1
        WHERE type = 1
        ORDER BY userid ASC
        LIMIT 0, 50
       ) x join
       mytable2 d
       on x.userid = d.userid
  where (d.day2 + d.time2) >= DATE_SUB(NOW(), INTERVAL 2 DAY);