In database management systems, UNION and UNION ALL are used to combine the results of two or more SELECT statements into a single result set.
The main difference between UNION and UNION ALL is that UNION removes any duplicate rows from the result set, while UNION ALL preserves all rows, including duplicates.
Here are some more details on each operator:
- UNION: The UNION operator combines the results of two or more SELECT statements into a single result set, removing any duplicates that occur in the result set. The columns in each SELECT statement must have the same data type and be in the same order. Also, the number of columns in each SELECT statement must be the same. For example, if you have two tables, TableA and TableB, and you want to combine the results of two SELECT statements, you can use the following syntax:
SELECT column1, column2, column3 FROM TableA
UNION
SELECT column1, column2, column3 FROM TableB
- UNION ALL: The UNION ALL operator combines the results of two or more SELECT statements into a single result set, without removing any duplicates. This operator is generally faster than UNION because it does not need to compare the results for duplicates. For example, if you have two tables, TableA and TableB, and you want to combine the results of two SELECT statements, preserving duplicates, you can use the following syntax:
SELECT column1, column2, column3 FROM TableA
UNION ALL
SELECT column1, column2, column3 FROM TableB
In summary, UNION combines the results of two or more SELECT statements, removing duplicates, while UNION ALL combines the results of two or more SELECT statements, preserving duplicates.