Correlation and dependence

Correlated subquery in sql Part 60

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• published: 10 Jan 2013
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C#, SQL Server, WCF, MVC and ASP .NET video tutorials for beginners http://www.youtube.com/user/kudvenkat/playlists In this video we will discuss about Corelated Subqueries Link for the text version of this video http://csharp-video-tutorials.blogspot.com/2013/01/correlated-subquery-in-sql-part-60.html In Part 59, we discussed about 2 examples that uses subqueries. Please watch Part 59, before proceeding with this video. We will be using the same tables and queries from Part 59. In the example below, sub query is executed first and only once. The sub query resutls are then used by the outer query. In a non-corelated subquery, the subquery can be executed independently of the outer query. Select [Id], [Name], [Description] from tblProducts where Id not in (Select Distinct ProductId from tblProductSales) If the subquery depends on the outer query for its values, then that sub query is called as a correlated subquery. In the where clause of the subquery below, "ProductId" column get it's value from tblProducts table that is present in the outer query. So, here the subquery is dependent on the outer query for it's value, hence this subquery is a correlated subquery. Correlated subqueries get executed, once for every row that is selected by the outer query. corelated subquery, cannot be executed independently of the outer query. Select [Name], (Select SUM(QuantitySold) from tblProductSales where ProductId = tblProducts.Id) as TotalQuantity from tblProducts order by Name