what is the difference between UNION and UNION ALL ?

Question

what is the difference between UNION and UNION ALL?

Suraj Kumar · Answer

Hello Sonu, Union combine the results of different result set and give the unique result set output only as shown below- SELECT 'RAM' AS NAME UNION SELECT 'SHYAM' AS NAME UNION SELECT 'SONU' AS NAME UNION SELECT 'RAM' AS NAME The output is as below. Here RAM was twice but in result it has come only once i.e., for unique result. NAME RAM SHYAM SONU UNION ALL gives the result set output after combining different result set but it gives all the result with repeated one if the output remains more than one times as below SELECT 'RAM' AS NAME UNION ALL SELECT 'SHYAM' AS NAME UNION ALL SELECT 'SONU' AS NAME UNION ALL SELECT 'RAM' AS NAME Output RAM SHYAM SONU RAM Here RAM has come twice as it was two times.

Code Alone · Answer

For your query you may find detailed solution. https://www.c-sharpcorner.com/blogs/sql-server-union-vs-union-all-which-is-better-for-performance

Jayesh Vaghasiya · 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. A UNION statement effectively does a SELECT DISTINCT on the results set. If you know that all the records returned are unique from your union, use UNION ALL instead, it gives faster results.

Ranjit Powar · Answer

Check this https://www.c-sharpcorner.com/UploadFile/3194c4/set-operators-in-sql-server/

Laxmidhar Sahoo · Answer

Both UNION and UNION ALL clauses combine rows from result sets into a single result set. The UNIONclause removes duplicate rows that exist, while the UNION ALL clause does not. Because the UNION ALL clause does not remove duplicate rows, it processes faster than the UNIONclause.

what is the difference between UNION and UNION ALL ?

Answers (5)