In this Step by step SQL Tutorial I will show you a simple example using SQL Union.
Union is used to combine two queries together so they appear as one result set. There are a number of rules that you should follow that we will tell you below and we will also tell you the difference between Union and Union all from both behaviour and performance point of view.
Below is a simple example where I combine one row with another row using union.
Below are several basic SQL UNION rules (not all):
- Union should have two or more SELECT statement.
- Queries columns must be listed in the same order.
- Column data types must be the same.
IMPORTANT: Union shows DISTINCT rows in the final result. This is not always desired behavior so if you need to return exactly the same number of rows from both queries than use UNION ALL
PERFORMANCE: UNION applies DISTINCT; that means it will be slower comparing to UNION ALL that doesn't do that however always ensure you get the expected result.
You can also visit SQL UNION ALL Multiple queries issues with last UNION
I hope this very simple example of SQL UNION will help you