We should also mention that we can use set operators such as minus, union and intersection in SQL in all types of queries. But keep in mind that this will sort the whole query! We can't use ORDER BY before a SQL set operator or try to sort every SELECT separately. If we want a certain sort order or type, we can always use an ORDER BY at the end of the query. The MINUS, UNION and INTERSECT operators will always sort the returned results UNION ALL will not. The results table will always have more higher-precision columns.The columns being compared must be the same type and of equal number.You can use either query results or tables with set operators.If the column names or aliases being compared are different, the result column will be called after the column in the first SELECT query.There are a few things to remember about minus, union and intersection in SQL: Minus, Union, Intersection in SQL: Practical Tips So it is the only element in the results table. You know by now that "Harry Potter" (and only "Harry Potter") is present in both tables. This is the main role of the INTERSECT operator. But what should we do if we need to know what two queries have in common? OK, so we know how to add and subtract some elements using the UNION and MINUS operators. Don't worry – the function and results are exactly the same. Thanks to the MINUS set operator we are able to see only those titles that occur in the first table and are not present in the second.īy the way, some databases use the keyword EXCEPT instead of MINUS. Now "Harry Potter" doesn't appear in the results table it's the title of a book and a movie. The MINUS set operator is designed for this type of task. We need to "minus" everything from the BOOKS table that is also in the MOVIES table. Let's say we want to see only book titles that are not also movie titles. ![]() But if we know that having unique elements is our main goal, then UNION will be much more helpful. If we don't care about duplicates and we want to work fast, UNION ALL will be the perfect solution. In sum, UNION can be four times slower than UNION ALL, which doesn't eliminate duplicates and doesn't sort the data. This sorting process takes a lot of time and works with a large number of elements. UNION will eliminate any duplicates and sort all the elements in the results table. Let's say we need to merge two query results that each contain 10,000 elements. But when should we use UNION and when UNION ALL? Firstly, know that there is a huge difference in efficiency between them. Now we know the main difference between these two operators. This time, the little wizard appears twice in the results table. Let's do the same operation as in the UNION example and see what we get: You've probably guessed that UNION ALL is very similar to UNION, but with one exception: UNION ALL returns all data from all tables, no matter if it is a duplicate or not. Like we mentioned earlier, the UNION set operator does not return duplicate values. Note that "Harry Potter" – an item which appears in both tables – is shown only once in the results. Below, you'll see a Venn diagram representing this operation and the code that will make it happen:Īll the book and movie titles are now in one table. Important: UNION statements only return UNIQUE values. UNION merges the results of two SELECT statements. What if we wanted to make one table from all the content in the BOOKS and MOVIES tables? This is a perfect time to use UNION set operator. As you look at these tables, notice that one title appears in both: Table: BOOKS It has only two tables, BOOKS and MOVIES, which contain book and movie titles (respectively) and an ID number. Suppose we have a very simple database that stores information about books and movies. Operators like MINUS, UNION or INTERSECT are widely used in SQL queries. ![]() These records may be found in many different tables, so we need set operators such as union and intersection in SQL to merge them into one table or to find common elements.ĭuring such operations, we take two or more results from SELECT statements and create a new table with the collected data. ![]() making a report with the names of every customer served by every department of a large company). Let's say we want to present every aspect of some key data group in one results table (e.g. Of course, we can't expect that all necessary data will be stored in one table. SQL queries let us choose the most important bits from large amounts of information. ![]() In this article, we'll take a closer look at them. Ever heard terms such as union and intersection in SQL? They're examples of set operators, and they come in handy when you need to combine information from multiple tables or queries.
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