qr code generator c# Implement correlated subqueries. Implement noncorrelated subqueries. in C#

Printer QR Code in C# Implement correlated subqueries. Implement noncorrelated subqueries.

Implement correlated subqueries. Implement noncorrelated subqueries.
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Estimated lesson time: 20 minutes
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Noncorrelated Subqueries
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The main purpose of a noncorrelated subquery is to allow you to write code that is more dynamic and does not require that a user knows all the intermediate values that currently exist in the database. For example, if you wanted to return a list of customers that were assigned to a specific region, you first have to know the list of cities or states from which to retrieve the requested list. However, a table would exist within your database that specifies which cities or states are assigned to a given region and a noncorrelated subquery could be used to make your query resilient to changes in the way a region is organized. Here is an example of how it might work:
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SELECT a.CustomerID, a.FirstName, a.LastName, b.Address, b.City, b.StateProvince FROM Customer.Customer a INNER JOIN Customer.CustomerAddress b ON a.CustomerID = b.CustomerID WHERE b.City IN (SELECT c.City FROM Customer.CityRegion c INNER JOIN Customer.Region d ON c.RegionID = d.RegionID WHERE d.Region = 'RegionX')
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As another example, suppose that you wanted to return all the products with a list price greater than the average list price for all products. Instead of having to retrieve the average list price separately, store the value in a variable, and then use the variable in a second SELECT statement, you could use the following query:
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SELECT a.ProductID, a.Name, a.ListPrice FROM Production.Product a WHERE a.ListPrice > (SELECT AVG(b.ListPrice) FROM Production.Product b)
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Lesson 2: Implementing Subqueries
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Derived tables
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f you want to return a list of employees and the number of employees who have the same title, you might try to execute the following query:
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SELECT BusinessEntityID, JobTitle, count(*) FROM HumanResources.Employee GROUP BY BusinessEntityID, JobTitle
Now you have a problem. You need to calculate the number of employees with a given job title and then return a list of employees along with how many other employees have the same job title. But to meet the requirements of the GROUP BY, you have to include all the nonaggregate columns in the GROUP BY clause. You are faced with a dilemma because it appears that your query can t be satisfied. The issue with your query is the order of operations. You have to first calculate the number of people with a given job title. Then, based on that result, join it back to the Employee table to get a list of employees and how many other employees have the same title. Instead of resorting to temporary tables to store the intermediate result set, T-SQL can solve this dilemma by taking advantage of an interesting feature of a FROM clause; namely, it accepts a table source. A table is constructed of rows and columns. When you execute a SELECT statement, you get a result set that consists of rows and columns. Therefore, it seems possible that you could actually put an entire SELECT statement into the FROM clause because the only requirement is to have a source that has the structure of a table. When you embed a SELECT statement into a FROM clause, you are using a feature referred to as derived tables or virtual tables. A SELECT statement returns a result set, but no name exists for the result set to be referenced within a query. You get around the lack of a name by wrapping the entire SELECT statement in parentheses and specifying an alias. The solution to your original problem then becomes the following:
SELECT b.BusinessEntityID, b.JobTitle, a.numtitles FROM (SELECT JobTitle, count(*) numtitles FROM HumanResources.Employee GROUP BY JobTitle) a INNER JOIN HumanResources.Employee b ON a.JobTitle = b.JobTitle
SQL Server first executes the SELECT GROUP BY statement, loads the results into memory, and tags the results with the specified alias. You can then reference any column within the derived table in the remainder of the SELECT statement just as if you were working with a physical table. Keep in mind that any aggregate, concatenation, or computation within the derived table must have an alias specified because it is not possible to construct a table with a column that has no name.
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