Pooling fields from two tables into one view

eoffshore · July 30th, 2004, 11:22 AM

I have two tables that record different date/time events. Think of one table for repairs and another table for installs. Both table have a createDate and createTime as well as several other common fields..and some unique fields.

I want to create a view that combines the two tables so that I can have a date/time ordered timeline that combines the two table. There is no relationship between the records in the repair and the installs table so I suspect the view I want will be a full outer join. But how can I have one column in the view called createDate that is populated with the createdate data from both the repairs and the installs tables?

The Meek Shall Inherit the Gulag

Justin Cave · July 30th, 2004, 11:35 AM

A UNION would probably be easier here...

Code:

SELECT *
  FROM (SELECT <<column list>>, createDate, createTime
          FROM table1
        UNION ALL
        SELECT <<column list>>, createDate, createTime
          FROM table2)
 ORDER BY createDate, createTime;

Justin
Distributed Database Consulting, Inc.
http://www.ddbcinc.com/askDDBC

owain · August 2nd, 2004, 03:30 AM

Justin, why have you nested the UNION query in another query? The ORDER BY caluse is a perfectely valid statement in a UNION query, as long as it appears after all the queries have been UNIONed together. Therefore a simpler query would be:

Code:

   SELECT <<column list>>, createDate, createTime
     FROM repairs

UNION ALL

   SELECT <<column list>>, createDate, createTime
     FROM installs

 ORDER BY createDate, createTime

Regards
Owain Williams

Justin Cave · August 2nd, 2004, 03:52 AM

In this case, I find the parenthesis to be helpful in reading the query. Without the parenthesis and outer SELECT statement, it might not be obvious to a reader whether the ORDER BY applied to the result of the UNION operation or whether it applied to the last SELECT statement in the UNION.

Under the covers, Oracle will treat both queries identically, so whichever formulation is easier to read ought to be chosen. To my eye, the outer SELECT makes the query more obvious, but that is purely a matter of taste.

Justin
Distributed Database Consulting, Inc.
http://www.ddbcinc.com/askDDBC

owain · August 2nd, 2004, 05:42 AM

I see your point, I would like to add that I personally find the nested approach adds complexity to the statement, however I can see how others may find it easier to read.

ANSI SQL states that you can not order the individual queries in a UNION query, you can only ORDER the entire UNIONed result set, again I am assuming that the author knows this and using a nested query probably makes this clearer.

Finally, using the nested query approach, is there a possibility that this will create a higher overhead for the server? For example, the server might create the UNIONed query in a temporary table and then query this table to order and return the result set. Could this be possible or is the server cleaver enough to work out that the 2 queries are the same?

Regards
Owain Williams