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# Combine Two Date Fields

Posted on 2013-06-25
Medium Priority
341 Views
Hello:

I have two tables with date fields. Table1 has the date formatted like this: 2013-03-31 13:01:00. Table2 the date is formated like this 2013-03-31. How can I combine the two tables?

SELECT   Table1.ArrivalDateTime, Table2.MeetingDate
FROM         Table1 INNER JOIN
Table2 ON (CONVERT(date, Table1.ArrivalDateTime, 101) = Table2.MeetingDate
0
Question by:RecipeDan
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LVL 16

Expert Comment

ID: 39275489
you can write it as below

SELECT   Table1.ArrivalDateTime, Table2.MeetingDate
FROM         Table1
INNER JOIN   Table2
ON DATEDIFF(dd,Table1.ArrivalDateTime,Table2.MeetingDate) = 0
0

LVL 32

Expert Comment

ID: 39275563
That join is really not good, what if you have multiple arrivals and meetings on the same day.
Your result set will be invalid.
You need to create primary and foreign keys on your tables to implement the join
0

LVL 32

Expert Comment

ID: 39275946
select table2.arrivaldatetime, table2.meetingdate from table1 inner join table2
on cast(floor(cast(table1.arrivaldatetime as float) ) as datetime) = table2.meetingdate
0

LVL 1

Author Comment

ID: 39276138
Awkingoo your suggestion shows as 2013-02-06 00:00:00.000. I need to delete the time in order to join the tables.
0

LVL 41

Expert Comment

ID: 39276502
Do you just want to join on dates? Whats wrong with your query?
0

LVL 49

Accepted Solution

PortletPaul earned 2000 total points
ID: 39276839
SELECT
Table1.ArrivalDateTime
, Table2.MeetingDate
FROM Table1
INNER JOIN Table2 ON CONVERT(DATE, Table1.ArrivalDateTime) = Table2.MeetingDate

You do not need the ,'101'

The only concern here is that applying a function to the datetime field may remove use of indexing you may have on ArrivalDateTime. Opinions differ on this e.g. (see this forum)
Does SQL 2008 also have an optimization such that
Where convert(date, aDateTimeColumn) = '20101020'
will get optimized to an INDEX SEEK rather than an scan?

Yes, it can do a SEEK for above operation.
Tibor Karaszi, SQL Server MVP
I would suggest you inspect the execution plan to see if it is causing table or index scanning.

& some semantics (sorry):

you have 2 fields, one appears to be datetime, the other appears to be date
date and datetime fields are seen in a format - but not stored in a format - hence what they look like doesn't really matter. What does matter is the actual data type of each field.

oh, and an alternative to using convert(date,....) which is efficient is:
SELECT
Table1.ArrivalDateTime
, Table2.MeetingDate
FROM Table1
INNER JOIN Table2 ON dateadd(day, datediff(day,0, Table1.ArrivalDateTime ), 0) = Table2.MeetingDate
the combination of datediff with dateadd strips time to 00:00:00

the notes above regarding indexing still apply of course.
{+2 edits sorry}
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