Difference between UNION and UNION ALL in SQL Server

Question

Hi friends I want to know what is the difference between UNION and UNION ALL in SQL Server ? Thank you.

Satyapriya Nayak · Answer

Hi Vipendra SQL 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). The syntax is as follows: [SQL Statement 1] UNION [SQL Statement 2] Say we have the following two tables 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 and we want to find out all the dates where there is a sales transaction. To do so we use the following SQL statement: 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 SQL 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. The syntax for UNION ALL is as follows: [SQL Statement 1] UNION ALL [SQL Statement 2] Let's use the same example as the previous section to illustrate the difference. Assume that we have the following two tables 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 and we want to find out all the dates where there is a sales transaction at a store as well as all the dates where there is a sale over the internet. To do so we use the following SQL statement: 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

Muralidharan Deenathayalan · Answer

When you use union statement it will not return duplicate values. When you use union-all statement it will return duplicate values also. http://blog.sqlauthority.com/2009/03/11/sql-server-difference-between-union-vs-union-all-optimal-performance-comparison/ http://blog.sqlauthority.com/2007/03/10/sql-server-union-vs-union-all-which-is-better-for-performance/