What is the Difference Between Union and Union All in SQL Server?

The main difference between UNION and UNION ALL in SQL Server lies in how they handle duplicate records. Here are the key differences:

• UNION: This operator only keeps unique records, eliminating any duplicate rows from the result set.
• UNION ALL: This operator keeps all records, including duplicates, and does not remove any duplicate rows from the result set.

Both UNION and UNION ALL are used to concatenate two or more result sets, allowing you to write multiple SELECT statements, retrieve the desired results, and combine them into a final, unified set. However, they differ in their handling of duplicate records, with UNION removing duplicates and UNION ALL retaining them.

When using these operators, there are certain requirements for the data being combined:

• The same number of columns must be retrieved in each SELECT statement to be combined.
• The columns retrieved must be in the same order in each SELECT statement.
• The columns retrieved must be of similar data types.

From a performance perspective, UNION ALL is faster than UNION because it does not require the server to remove duplicate records. However, if you are certain that there will not be any duplicates or having duplicates is not a problem, using UNION ALL is recommended.

Comparative Table: Union vs Union All in SQL Server

The main difference between UNION and UNION ALL in SQL Server lies in the way they handle duplicate records. Here is a table summarizing the differences:

Both UNION and UNION ALL are SQL operators used to concatenate two or more result sets, allowing you to write multiple SELECT statements and retrieve the desired results before combining them into a final, unified set. However, UNION only returns unique records, while UNION ALL returns all records, including duplicates. The execution time of UNION ALL is typically lower than that of UNION, as UNION ALL does not remove duplicates, which is an additional step performed by UNION.