Difference Between Union and Union All in SQL Server

UNION ALL command is equal to UNION command, except that UNION ALL selects all the values. The difference between Union and Union all is that Union all will not eliminate duplicate rows, instead it just pulls all the rows from all the tables fitting your query specifics and combines them into a table.

1. What is the difference between union and union all in SQL?
2. Which is faster union or union all in SQL?
3. What is the use of union all in SQL?
4. Does Union all remove duplicates?
5. When would you use Union vs Union all what if there were no duplicates?
6. When should you use union as opposed to union all?
7. Is Union faster than join?
8. Are unions faster than two queries?
9. How do you order by in Union all?
10. Can we use order by in Union all?
11. What is difference between union and join?
12. How does SQL union work?

What is the difference between union and union all in SQL?

UNION ALL Difference

UNION ALL keeps all of the records from each of the original data sets, UNION removes any duplicate records. UNION first performs a sorting operation and eliminates of the records that are duplicated across all columns before finally returning the combined data set.

Which is faster union or union all in SQL?

UNION retrieves only distinct records from all queries or tables, whereas UNION ALL returns all the records retrieved by queries. Performance of UNION ALL is higher than UNION.

What is the use of union all in SQL?

The SQL UNION ALL operator is used to combine the result sets of 2 or more SELECT statements. It does not remove duplicate rows between the various SELECT statements (all rows are returned). Each SELECT statement within the UNION ALL must have the same number of fields in the result sets with similar data types.

Does Union all remove duplicates?

Both UNION and UNION ALL operators combine rows from result sets into a single result set. The UNION operator removes eliminate duplicate rows, whereas the UNION ALL operator does not. ... The ORDER BY clause is applied to the combined result set, not within the individual result set.

When would you use Union vs Union all what if there were no duplicates?

UNION ALL command is equal to UNION command, except that UNION ALL selects all the values. The difference between Union and Union all is that Union all will not eliminate duplicate rows, instead it just pulls all the rows from all the tables fitting your query specifics and combines them into a table.

When should you use union as opposed to union all?

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. When dealing with sets, you would not want numbers 2 and 4 appearing twice, as an element either is or is not in a set.

Is Union faster than join?

4 Answers. Union will be faster, as it simply passes the first SELECT statement, and then parses the second SELECT statement and adds the results to the end of the output table.

Are unions faster than two queries?

If you use UNION ALL and don't sort the output, then the performance of UNION should be more-or-less the same as the performance of multiple separate queries assuming the queries are identical to the ones you're UNIONing together.

How do you order by in Union all?

SQL SERVER – UNION ALL and ORDER BY – How to Order Table Separately While Using UNION ALL

1. USE tempdb. GO. -- Create table. ...
2. -- SELECT without ORDER BY. SELECT ID, Col1. FROM t1. ...
3. -- SELECT with ORDER BY. SELECT ID, Col1. FROM t1. ...
4. -- SELECT with ORDER BY - with ORDER KEY. SELECT ID, Col1, 'id1' OrderKey. FROM t1.

Can we use order by in Union all?

Without a transformation, a statement that contains both ORDER BY and UNION would require two separate sorting steps-one to satisfy ORDER BY and one to satisfy UNION (Currently Derby uses sorting to eliminate duplicates from a UNION. You can use UNION ALL to avoid sorting, but UNION ALL will return duplicates.

What is difference between union and join?

UNION in SQL is used to combine the result-set of two or more SELECT statements. The data combined using UNION statement is into results into new distinct rows. JOIN combines data from many tables based on a matched condition between them. ... It combines data into new columns.

How does SQL union work?

The Union operator combines the results of two or more queries into a distinct single result set that includes all the rows that belong to all queries in the Union. In this operation, it combines two more queries and removes the duplicates. For example, the table 'A' has 1,2, and 3 and the table 'B' has 3,4,5.