UNION in Visual C#.NET

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UNION
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UNION generates a result set combining the rows from both inputs. The following sections describe the differences between UNION (implicit DISTINCT) and UNION ALL.
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Joins and Set Operations
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UNION DISTINCT
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Specifying UNION without the ALL option combines the rows from both inputs and applies a DISTINCT on top (in other words, removes duplicate rows). For example, the following query returns all occurrences of country, region, city that appear in either the Employees table or the Customers table, with duplicate rows removed:
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USE InsideTSQL2008; SELECT country, region, city FROM HR.Employees UNION SELECT country, region, city FROM Sales.Customers;
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The query returns 71 unique rows.
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UNION ALL
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You can think of UNION ALL as UNION without duplicate removal. That is, you get one result set containing all rows from both inputs, including duplicates. For example, the following query returns all occurrences of country, region, city from both tables:
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SELECT country, region, city FROM HR.Employees UNION ALL SELECT country, region, city FROM Sales.Customers;
Because the Employees table has 9 rows and the Customers table has 91 rows, you get a result set with 100 rows.
EXCEPT
EXCEPT allows you to identify rows that appear in the rst input but not in the second.
EXCEPT DISTINCT
EXCEPT DISTINCT returns distinct rows that appear in the rst input but not in the second input. To achieve EXCEPT, programmers sometimes use the NOT EXISTS predicate, or an outer join ltering only outer rows, as I demonstrated earlier in the Semi Joins section. However, those solutions treat two NULLs as different from each other. For example, (UK, NULL, London) will not be considered equal to (UK, NULL, London). If both tables contain such a row, input1 EXCEPT input2 is not supposed to return it, yet the NOT EXISTS and outer join solutions will as typically written, unless you add logic that treats two NULLs as equal. As mentioned, the built-in set operations treat NULLs as equal. The following code uses the built-in EXCEPT set operation to return distinct cities that appear in Employees but not in Customers:
SELECT country, region, city FROM HR.Employees EXCEPT SELECT country, region, city FROM Sales.Customers;
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Note that of the three set operations, only EXCEPT is asymmetrical. That is, input1 EXCEPT input2 is not the same as input2 EXCEPT input1. For example, the query just shown returned the two cities that appear in Employees but not in Customers. The following query returns 66 cities that appear in Customers but not in Employees:
SELECT country, region, city FROM Sales.Customers EXCEPT SELECT country, region, city FROM HR.Employees;
EXCEPT ALL
EXCEPT ALL is trickier than EXCEPT DISTINCT and has not yet been implemented in SQL Server. Besides caring about the existence of a row, it also cares about the number of occurrences of each row. Say you request the result of input1 EXCEPT ALL input2. If a row appears n times in input1 and m times in input2 (both n and m will be >= 0), it will appear MAX(0, n m) times in the output. That is, if n is greater than m, the row will appear n m times in the result; otherwise, it won t appear in the result at all. Even though you don t have a native operator for EXCEPT ALL in SQL Server 2008, you can easily generate the logical equivalent using EXCEPT and the ROW_NUMBER function. Here s the solution:
WITH EXCEPT_ALL AS ( SELECT ROW_NUMBER() OVER(PARTITION BY country, region, city ORDER BY (SELECT 0) As rn, country, region, city FROM HR.Employees EXCEPT SELECT ROW_NUMBER() OVER(PARTITION BY country, region, city ORDER BY (SELECT 0) As rn, country, region, city FROM Sales.Customers ) SELECT country, region, city FROM EXCEPT_ALL;
To understand the solution, I suggest that you rst highlight sections (queries) within it and run them separately. This allows you to examine the intermediate result sets and get a better idea of what the following paragraph tries to explain. The code rst assigns row numbers to the rows of each of the inputs, partitioned by the whole attribute list. The row numbers will number the duplicate rows within the input. For example, a row that appears ve times in Employees and three times in Customers will get
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