Calculating Columns in VS .NET

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Calculating Columns
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The DBMS can calculate values in a table using one of five built-in calculation functions You place the name of the column that you want calculated within the parentheses of the calculation function These are the functions: SUM() tallies values in a column that is passed to the built-in function
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SELECT SUM(Quantity) FROM Orders
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AVG() averages values in a column that is passed to the built-in function
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SELECT AVG(Quantity) FROM Orders
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MIN() determines the minimum value in a column that is passed to the built-in function
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SELECT MIN(Quantity) FROM Orders
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MAX() determines the maximum value in a column that is passed to the built-in function
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SELECT MAX (Quantity) FROM Orders
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COUNT() determines the number of rows in a column that is passed to the built-in function Rows without values in the column are excluded from the count
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SELECT COUNT(Quantity) FROM Orders
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The Count() function is also used to count the number of rows in a table You do this by using the wildcard character (asterisk) within the parentheses of the function as shown here:
SELECT COUNT ( * ) FROM Orders
You can perform multiple counts by extending the Select statement to reflect each count For example, here's how we determine the total number of rows in the table and the total value of the Quantity column:
SELECT COUNT ( * ) , COUNT (Quantity) FROM Orders
You can restrict the scope of a built-in calculation function by using a WHERE clause expression to specify the criteria for a row to be included in a calculation Any valid WHERE clause expression can be used to filter rows to be excluded from the calculation Let's say that you want to calculate the total number of orders, the average, and the total of the values in the Quantity column Here's how you do it:
SELECT COUNT (OrderNumber) AVG (Quantity), , FROM Orders o, customers c WHERE oCustomerNumber = cCustNumber SUM (Quantity)
There are two common problems that occur when using built-in functions Some rows won't contain any values, and others will contain duplicate values Both of these conditions can affect the calculation You can solve these problems by using the IS NULL operator and the Distinct modifier in your query Both of these were explained in detail previously in the chapter
Grouping and Ordering Data
Rows can be grouped or sorted by using the GROUP BY clause or the ORDER BY clause Grouping organizes rows according to similar values within the same column Let's say that you want to see a quantity for each product The selected columns can be grouped by product number Sorting organizes rows in natural order A DBMS is capable of simple and complex sorting A simple sort is when the values in a single column are used for the sort A complex sort is when multiple columns are used for the sort; for instance, rows may be sorted by customer last name and, within customer last name, by customer first name
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Here's how to group rows: The Select statement contains columns that will be returned to your application The Group By clause groups these by product number This means that rows that have the same product number appear one underneath the other
SELECT ProductNumber, SUM(Quantity) FROM Sales GROUP BY ProductNumber
You can also create a subgroup within a group This is illustrated in the next example, where the product number group is further grouped by customer number The results are shown in Table 11- 1
SELECT ProductNumber, CustomerNumber, SUM(Quantity) FROM Sales GROUP BY ProductNumber, CustomerNumber
The number of rows that are included in a group can be limited by including a conditional expression in the query A conditional expression is similar to the WHERE clause expression discussed previously in this chapter, except instead of the WHERE clause a Having clause is used In the next example, we use the Having clause to return rows whose Quantity column has a value that is greater than 1 Table 11-2 shows the results
SELECT ProductNumber, CustomerNumber, SUM(Quantity) FROM Sales GROUP BY ProductNumber, CustomerNumber HAVING Quantity > 1
ProductNumber 1052 3255 3255 5237 5237 7466 7466
CustomerNumber 54321 54321 54321 12345 87676 12345 67890
Quantity 2 1 4 1 1 1 3
Table 1-1 Rows Are Grouped by ProductNumber and Then by CustomerNumber
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I ProductNumber
CustomerNumber
Quantity
Table 1-2 Rows Are Grouped by ProductNumber and Then CustomerNumber if They Have a Quantity Greater than 1
Columns that are empty can create unexpected results when you execute a query This is because sometimes the empty column is included or excluded from the operation, depending on the nature of the query The DBMS may include or exclude a row in a group, depending on the conditional expression Here's how this works:
A row is included in a group if the empty column isn't used to group rows or used in the conditional expression in the HAVING clause A row is included in the group if the empty column is used to group rows Rows containing the empty column are placed in their own group
You can sort values returned to your application by using the Order By clause as shown here, where the rows are sorted by ProductNumber:
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