UNIONALL

Hi Arjun,

UNION query is to combine the results of two queries together. In this respect, UNION is somewhat similar to JOIN in that they are both used to related information from multiple tables. One restriction of UNION is that all corresponding columns need to be of the same data type. Also, when using UNION, only distinct values are selected (similar to SELECT DISTINCT).

Ex:-

Table Store_Information
store_name     Sales     Date
Los Angeles     $1500     Jan-05-1999
San Diego     $250     Jan-07-1999
Los Angeles     $300     Jan-08-1999
Boston     $700     Jan-08-1999

Table Internet_Sales
Date     Sales
Jan-07-1999     $250
Jan-10-1999     $535
Jan-11-1999     $320
Jan-12-1999     $750



SELECT Date FROM Store_Information
UNION
SELECT Date FROM Internet_Sales

Result:
Date
Jan-05-1999
Jan-07-1999
Jan-08-1999
Jan-10-1999
Jan-11-1999
Jan-12-1999


UNION ALL command is also to combine the results of two queries together. The difference between UNION ALL and UNION is that, while UNION only selects distinct values, UNION ALL selects all values.

Ex:-

Table Store_Information
store_name     Sales     Date
Los Angeles     $1500     Jan-05-1999
San Diego     $250     Jan-07-1999
Los Angeles     $300     Jan-08-1999
Boston     $700     Jan-08-1999

Table Internet_Sales
Date     Sales
Jan-07-1999     $250
Jan-10-1999     $535
Jan-11-1999     $320
Jan-12-1999     $750



SELECT Date FROM Store_Information
UNION ALL
SELECT Date FROM Internet_Sales

Result:
Date
Jan-05-1999
Jan-07-1999
Jan-08-1999
Jan-08-1999
Jan-07-1999
Jan-10-1999
Jan-11-1999
Jan-12-1999


Thanks