SQL UNION

Question

Hi Explain the SQL UNION statement. Write SQL syntax for the SQL UNION statement along with an example. What is the difference between UNION and UNION ALL ?

Satyapriya Nayak · Answer

Hi Arun UNION The purpose of the 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 UNION ALL The purpose of the 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 UNION is used to select information from two tables.But it selects only distinct records from both the table.  while UNION ALL selects all records from both the tables. Please refer the below links http://www.1keydata.com/sql/sqlunion.html http://www.1keydata.com/sql/sqlunionall.html Thanks

Senthilkumar · Answer

SQL Union is used to consolidate the records in the two table into single result and both columns list and names should be sale to combine the records. It will not filter the duplicate records. It combines the rows rather than columns. For example have two tables. SELECT idnameaddress FROM Employee1 UNION SELECT idnameaddress FROM Employee2 It will combine the two table results.

Muralidharan Deenathayalan · Answer

UNION The UNION command is used to select related information from two tables much like the JOIN command. However when using the UNION command all selected columns need to be of the same data type. With UNION only distinct values are selected. UNION ALL The UNION ALL command is equal to the UNION command except that UNION ALL selects all values. The difference between Union and Union all is that Union all will not eliminate duplicate rows instead it just pulls all rows from all tables fitting your query specifics and combines them into a table. http://blog.sqlauthority.com/2008/10/15/sql-server-introduction-and-example-of-union-and-union-all/