SQL-使用JOINED表进行SELECT

Question

I'm trying to do a selection from the user tabel. For each user i would like to sum the Timediff for all posts in tblregtime for current user for given date parameters.

Problem is that i need to get information even if there is no registration done at the given date for current user. If no registration i need output that current user has TotalDiff=0 . My current SQL doesn't work in this way. It will just give the fname , lname and TotalDiff if there is a post in tblregtime

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sql:

 select  u.fname,u.lname, sum(cast(TIME_TO_SEC(TIMEDIFF(r.edate,r.sdate)) AS UNSIGNED)-r.break_time) as TotalDiff

from tbluser u 

RIGHT OUTER JOIN tblregtime r on r.userid=u.id where r.projectid=21
and  year(r.sdate)=2013 and month(r.sdate)=10 and day(r.sdate)=7

Answer 1

If you are trying to keep everything in tbluser , then you want a left join instead of a right join . However, you also need to move the where conditions into the on clause. Otherwise, when there is no match, the comparisons will fail (because the r. values will be NULL ):

select u.fname,u.lname,
       sum(cast(TIME_TO_SEC(TIMEDIFF(r.edate,r.sdate)) AS UNSIGNED)-r.break_time) as TotalDiff
from tbluser u LEFT JOIN
     tblregtime r
     on r.userid = u.id and
        r.projectid = 21 and
        year(r.sdate) = 2013 and month(r.sdate) = 10 and day(r.sdate) = 7;

I would also recommend that you change the final date comparison to something like:

r.sdate = '2013-10-07'

This form would allow the use of an index on r.sdate . As you have written it, the SQL engine (at least the SQL engines I am familiar with) would not be smart enough to use the index.