data matrix code c# GROUP BY in Visual C#

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GROUP BY
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Using aggregations, as has just been demonstrated, works well when you just wish a single row of results for a specific filtered item. If you wish to find the average price of several shares, you may be thinking you need to provide a SELECT AVG() for each share. This section will demonstrate that this is not the case. By using GROUP BY, you instruct SQL Server to group the data to return and provide a summary value for each grouping of data. To clarify, as you will see in the
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CHAPTER 11 T-SQL ESSENTIALS
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upcoming examples, we could remove the WHERE ShareId=1 statement, which would then allow you to group the results by each different ShareId. The basic syntax for grouping is defined in the following code. It is possible to expand GROUP BY further to include rolling up or providing cubes of information; however, such a discussion falls outside the scope of this book. GROUP BY [ALL] (column1[,column2,...]) The option ALL is a bit like an OUTER JOIN. If you have a WHERE statement as part of your SELECT statement, any grouping filtered out will still return a row in the results, but instead of aggregating the column, a value of NULL will be returned. I tend to use this as a checking mechanism. I can see the rows with values and the rows without values, and visually this will tell me that my filtering is correct. When working with GROUP BY, the main point that you have to be aware of is that any column defined in the SELECT statement that does not form part of the aggregation MUST be contained within the GROUP BY clause and be in the same order as the SELECT statement. Failure to do this will mean that the query will give erroneous results, and in many cases use a lot of resources in giving these results.
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Try It Out: GROUP BY
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1. This first example will demonstrate how to find maximum and minimum values for every share that has a row in the ShareDetails.SharePrices table where the share ID < 9999. This means that the row we added earlier when looking at joins that has no Share record will be excluded. The code is as follows: SELECT ShareId, MIN(Price) MinPrice, Max(Price) MaxPrice FROM ShareDetails.SharePrices WHERE ShareId < 9999 GROUP BY ShareId 2. When the code is executed, you will see the two shares listed with their corresponding minimum and maximum values, as shown in Figure 11-16.
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Figure 11-16. Max and min of a group 3. If we wish to include any rows where there is a Price row, but the ShareId has a value of 9999 or greater, then we would use the ALL option with GROUP BY. In the following example, we are also linking into the ShareDetails.Shares table to retrieve the share description. SELECT FROM LEFT WHERE GROUP sp.ShareId, s.ShareDesc,MIN(Price) MinPrice, Max(Price) MaxPrice ShareDetails.SharePrices sp JOIN ShareDetails.Shares s ON s.ShareId = sp.ShareId sp.ShareId < 9999 BY ALL sp.ShareId, s.ShareDesc
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CHAPTER 11 T-SQL ESSENTIALS
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4. When you execute the code, the Price row that is outside of the filtering returns a NULL value. The other rows return details as shown in Figure 11-17.
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Figure 11-17. A JOIN with a max and min group
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HAVING
When using the GROUP BY clause, it is possible to supplement your query with a HAVING clause. The HAVING clause is like a filter, but it works on aggregations of the data rather than the rows of data prior to the aggregation. Hence, it has to be included with a GROUP BY clause. It will also include the aggregation you wish to check. The code would therefore look as follows: GROUP BY column1[,column2...] HAVING [aggregation_condition] The aggregation_condition would be where we place the aggregation and the test we wish to perform. For example, my bank charges me if I have more than 20 nonregular items pass through my account in a month. In this case, the query would group by customer ID, counting the number of nonregular transactions for each calendar month. If the count were less than or equal to 20 items, then you would like this list to not include the customer in question. To clarify this, the query code would look something like the following if we were running this in August 2005: SELECT CustomerId,COUNT(*) FROM CustomerBankTransactions WHERE TransactionDate BETWEEN '1 Aug 2005 ' AND '31 Aug 2005 ' GROUP BY CustomerId HAVING COUNT(*) > 20
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