The important thing to remember about SQL (and UNION vs UNION ALL) is that the structures you design aren’t solely monolithic entities. Databases are dynamic, as the data within them is constantly changing and being added.
One crucial element of manipulating your databases is in the form of a UNION operator. But how does the UNION clause differ from its counterpart, the UNION ALL?
Let’s take a look at use cases for both, what they are intended for, and how you would implement them in your queries and databases.
UNION vs UNION ALL: Side-by-Side Comparison
| UNION | UNION ALL | |
|---|---|---|
| Only unique records | Yes | No |
| Allows duplicates | No | Yes |
| Columns needed | The same number of columns are needed on both SELECT statements | The same number of columns are needed on both SELECT statements |
| Data types supported | Like data types between SELECT statements | Like data types between SELECT statements |
| Retrieval order | Columns are retrieved in the order of the SELECT statement | Also retrieved in the same order as the SELECT statement |
UNION vs UNION ALL: What’s the Difference?
Both the UNION and UNION ALL operators have similar use cases. Their constraints are identical, but how they return data is slightly different. Where to use these clauses is more of a situational case as opposed to one being preferable over the other.
How Do You Use the UNION or UNION ALL Statement?
UNION and UNION ALL function by combining the results of SELECT statements. SELECT statements themselves are nothing more than queries, as even beginners in SQL should know.
How UNION and UNION ALL have some utility is by allowing for the combination of more than one SELECT statement. This allows for the return of similar data types which is then rendered as a single result.
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UNION and UNION ALL only work on identical data types, and the columns queried have to be identical between however many SELECT statements you’re combining.
UNION and UNION ALL are not like a JOIN, however. A JOIN is the combination of two tables and, as such, the syntax is different. UNION and UNION ALL are meant to return results from queries and JOIN, and their variants are meant to combine datatypes in a singular table.
UNION and UNION ALL also differ from a VIEW because that statement is merely displaying specified tables. While you could potentially construct a VIEW query utilizing UNION or UNION ALL, they have vastly different syntax and purposes when used.
Where Do UNION and UNION ALL Differ?
UNION as a statement only returns unique results between two or more SELECT statements. Repeated values aren’t present whenever you select to use UNION. This is handy if you’re trying to go for unique results between two separate SELECT statements, for instance analyzing data from sales and salesperson tables.
UNION ALL, on the other hand, returns duplicate results, meaning you’ll get repeated rows when using it. This has its own use, of course, and, as an example, can be thought of when returning data from a SELECT statement querying a pair of tables with customer and shipping information.
One would expect repeated results considering the data, and both separate queries will likely have some identical values, given that you’ll be shipping to your customers.
Both have their usefulness, of course, and, as stated before, are purely situational. If you’re structuring queries where you need individual results, then UNION is going to be the best candidate. UNION ALL is ideal if the data returned isn’t hindered by duplicated results.
What Are the Requirements for Using UNION and UNION ALL?
The chief thing to keep in mind when deciding to use UNION or UNION ALL is that there are specific requirements needed before using them in a query. However, many SELECT statements will need an identical amount of columns and identical datatypes.
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If you’re looking to combine multiple results between disparate queries, it might be worth looking into using a JOIN statement where the joining constraint is directed towards a shared column despite differing datatypes within.
What this means is that you’re not going to use a UNION or UNION ALL to return dates from one table, and transactions from another. Instead, you’ll be needing to return dates from both tables.
Ignoring this requirement will likely lead to your SQL returning an error, as UNION and UNION ALL will not function when pointed at wildly different tables and datatypes.
UNION vs UNION ALL: 5 Must-Know Facts
- UNION is one of the four basic Set Operators utilized in SQL.
- UNION interacts solely with SELECT statements.
- UNION cannot interact with VIEW or JOIN unless part of a SELECT query.
- UNION ALL is faster than UNION.
- UNION ALL is identical in function to UNION, minus duplicated results.
UNION vs UNION ALL: Which One Is Better? Which One Should You Use?
There is no explicit winner when using either of these operators. Their use cases differ, but their actual implementation is identical when constructing your queries. UNION is less performant overall, as it does some work behind the scenes to remove duplicate entries from the query’s result. UNION ALL is faster on the whole, but has limited utility if you’re after only distinctive results.
SQL does have best practices, but when using Set Operators you’re likely to use all of them for different purposes. At the end of the day, both of these operators are merely tools that are used to accomplish the various tasks you’ll encounter when running databases.
As such, use either one as needed; there is no clear winner. The only distinction to be made when using either of these SQL operators is whether you need unique results or could stand to have duplicated results returned in your queries.
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