Difference Between ‘UNION ALL’ and ‘UNION’

‘UNION ALL’ vs ‘UNION’

The importance of databases and database management systems is increasing with the use of information technology in the daily life of common people. Every organization is converting their manual records and data to digital databases. There are several terms and functions in a database that play a very vital role in  database management. Database management itself is a significant element in an organization with large amounts, and sometimes sensitive, data to be created and managed on a regular basis. The databases store data in the forms of tables, which in turn contain rows and columns to maintain records.

The tables in a database need very efficient management to make the best use of the resources as well as get the query results as quickly as possible. The tables store records with different data types as per the requirement and they may sometimes have duplicate data stored in a single table. The records of one table may also be present in other tables within the same database. A database management system, like SQL Server or Oracle, has several commands to manage tables. UNION and UNION ALL are two such commands that manage table data and optimize the query results.

The UNION command is used to select related information, having the same data type, from multiple tables. Furthermore, the union command only selects the distinct records. This necessitates a relatively lengthier processing time and greater use of system resources as it performs search and match operations to find out the distinct information out of the combined records of the tables. Moreover, UNION command is effective with tables having a large number of records with lots of duplications because the query results would be very specific. This is applicable if very high system resources are available. However, it is more effective with tables having most of the unique data since less duplication would easily be processed with systems having limited resources.

UNION ALL command selects all the records from the tables. Unlike UNION, UNION ALL does it in a very efficient way since it does not check for the redundant data and fetches all the results. The search result is a combined table having all data including duplicate rows. UNION ALL is fast because it does not involve sorting of data. Further to this, UNION ALL command is most effective when used with tables containing fewer records even if the table contains a lot of duplicates. Although, tables with fewer records and unique data would be ideal for the UNION ALL command.

Summary:

1. UNION ALL operation is performed faster than UNION command.

2. UNION ALL doesn’t perform sorting of data whereas UNION command gives the query result in a sorted manner.

3. UNION ALL includes the redundant records of the tables while UNION command eliminates the duplications in the table and its result does not contain redundant rows.

4. UNION is very effective with larger tables whereas UNION ALL is more effective in cases where tables are not very large and redundancy is not an issue.
Rating: 8. Good article. Minor editing carried out.