The following query finds the names of all the wheel products that Adventure Works Cycles makes. This statement is evaluated in two steps. First, the inner query returns the subcategory identification number that matches the name 'Wheel' Second, this value is substituted into the outer query, which finds the product names that go with the subcategory identification numbers in Production.
One difference in using a join rather than a subquery for this and similar problems is that the join lets you show columns from more than one table in the result. For example, if you want to include the name of the product subcategory in the result, you must use a join version. The following query finds the name of all vendors whose credit rating is good, from whom Adventure Works Cycles orders at least 20 items, and whose average lead time to deliver is less than 16 days.
The inner query is evaluated, producing the ID numbers of the vendors who meet the subquery qualifications. The outer query is then evaluated. Notice that you can include more than one condition in the WHERE clause of both the inner and the outer query.
A join can always be expressed as a subquery. A subquery can often, but not always, be expressed as a join. This is because joins are symmetric: you can join table A to B in either order and get the same answer. The same is not true if a subquery is involved. The following query finds the names of the products that are not finished bicycles. This statement cannot be converted to a join. The analogous not-equal join has a different meaning: It finds the names of products that are in some subcategory that is not a finished bicycle.
The following example doubles the value in the ListPrice column in the Production. Product table. ProductVendor table to restrict the rows updated in the Product table to just those supplied by BusinessEntity For clarity in case the same table is itself referenced in other subqueries, use the target table's alias:. A subquery introduced with an unmodified comparison operator a comparison operator not followed by ANY or ALL must return a single value rather than a list of values, like subqueries introduced with IN.
If such a subquery returns more than one value, SQL Server displays an error message. To use a subquery introduced with an unmodified comparison operator, you must be familiar enough with your data and with the nature of the problem to know that the subquery will return exactly one value. If, however, Linda Mitchell covered more than one sales territory, then an error message would result. Subqueries introduced with unmodified comparison operators often include aggregate functions, because these return a single value.
Writing a subquery as a column does not mean that the subquery is executed for each row retrieved from the Users table. If we display the estimated execution plan, it will show that the badges data is retrieved then left joined with the Users table.
Instead of using a table or view name in the FROM clause, we can use a SQL subquery as a data source, noting that assigning an alias is required. Let us try to write the previous query in another way:.
In the screenshot below, you can note how the TOP operator is executed directly after scanning the Users clustered index, while in the previous section, it was performed as the last step.
Besides, we can add joins within the FROM clause while using subqueries. In the example above, we used a subquery to retrieve the latest ten posts and their creation date. About us: Career Karma is a platform designed to help job seekers find, research, and connect with job training programs to advance their careers. Learn about the CK publication. James Gallagher is a self-taught programmer and the technical content manager at Career Karma. James has written hundreds of programming tutorials, and he frequently contributes to publications like Codecademy, Treehouse, Repl.
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Kris Wenzel. Thanks for clarification.. This is amazing. My entire life has been a lie. I blame being a regular programmer vs sql programmer for thinking that subqueries do the query for each row just like they would in a program with nested FOR loops This was very eye opening.
Is it possible to join a table to a subquery in the from clause?
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