When it comes to working with databases, one term that often causes confusion is the difference between UNION and UNION ALL. Both of these commands are used to combine the results of two or more SELECT statements, but they have distinct differences that can greatly impact the outcome of your data.
So, what exactly are these distinctions and how do they affect your data? Let's dive in and explore the differences between UNION and UNION ALL.
To start, let's define what a UNION is. Essentially, a UNION combines the results of two or more SELECT statements into a single result set. This means that if you have two tables with similar columns, a UNION will merge the data from both tables into one result set, removing any duplicate rows.
On the other hand, a UNION ALL also combines the results of two or more SELECT statements, but it does not remove any duplicate rows. This means that if you have duplicate data in your tables, a UNION ALL will keep those duplicates in the final result set.
One key difference to note is that UNION ALL is typically faster than UNION because it does not have to scan the data for duplicates. However, this speed comes at a cost – the final result set may contain duplicate rows, which could potentially skew your data analysis.
Another important distinction is the number of columns in each SELECT statement. With a UNION, all SELECT statements must have the same number of columns, and the columns must be in the same order. This is because a UNION is essentially stacking the data on top of each other, so the columns need to align.
On the other hand, a UNION ALL does not have this requirement. Each SELECT statement can have a different number of columns and the columns can be in any order. This can be useful if you are combining data from multiple tables that have different column structures.
So, when should you use a UNION and when should you use a UNION ALL? The answer depends on your specific data and what you are trying to achieve.
If you want to combine data from multiple tables and remove any duplicate rows, a UNION is the way to go. This is especially useful when you are dealing with large amounts of data and want to streamline your results.
On the other hand, if you are working with data that may have duplicates and you want to keep those duplicates in the final result set, then a UNION ALL is the appropriate choice. This can be useful when you want to compare data from different tables or when you want to see all of the data in its entirety.
In conclusion, the main distinction between UNION and UNION ALL is the removal of duplicate rows. UNION removes duplicates while UNION ALL keeps them. Additionally, UNION requires all SELECT statements to have the same number of columns, while UNION ALL does not have this requirement.
Understanding these distinctions is crucial when working with databases, as it can greatly impact the accuracy of your data analysis. So, next time you are combining data from multiple tables, be sure to carefully consider whether you need a UNION or a UNION ALL to achieve your desired results.
