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-- show the contents of the table SELECT * FROM dbo.Matrix;
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Aggregating and Pivoting Data
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This generates the following output:
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orderyear ----------2006 2007 2008 y2006 ----------1 NULL NULL y2007 ----------NULL 1 NULL y2008 ----------NULL NULL 1
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You join the base table (or table expression) with the Matrix table based on a match in orderyear. This means that each row from the base table will be matched with one row from Matrix the one with the same orderyear. In that row, only the corresponding orderyear s column value will contain a 1. So you can substitute the expression
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SUM(CASE WHEN orderyear = <some_year> THEN qty END) AS [<some_year>]
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with the logically equivalent expression
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SUM(qty*y<some_year>) AS [<some_year>]
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Here s what the full query looks like:
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SELECT custid, SUM(qty*y2006) AS [2006], SUM(qty*y2007) AS [2007], SUM(qty*y2008) AS [2008] FROM (SELECT custid, YEAR(orderdate) AS orderyear, qty FROM dbo.Orders) AS D JOIN dbo.Matrix AS M ON D.orderyear = M.orderyear GROUP BY custid;
If you need the number of orders instead of the sum of qty, in the original solution you produce a 1 instead of the qty column for each order and use the COUNT aggregate function, like so:
SELECT custid, COUNT(CASE WHEN orderyear = 2006 THEN COUNT(CASE WHEN orderyear = 2007 THEN COUNT(CASE WHEN orderyear = 2008 THEN FROM (SELECT custid, YEAR(orderdate) AS FROM dbo.Orders) AS D GROUP BY custid;
1 END) AS [2006], 1 END) AS [2007], 1 END) AS [2008] orderyear
This code generates the following output:
custid -----A B C D 2006 ----------2 1 1 1 2007 ----------1 1 1 0 2008 ----------1 1 1 0
Inside Microsoft SQL Server 2008: T-SQL Querying
With the Matrix table, simply specify the column corresponding to the target year:
SELECT custid, COUNT(y2006) AS [2006], COUNT(y2007) AS [2007], COUNT(y2008) AS [2008] FROM (SELECT custid, YEAR(orderdate) AS orderyear FROM dbo.Orders) AS D JOIN dbo.Matrix AS M ON D.orderyear = M.orderyear GROUP BY custid;
Of course, using the PIVOT operator, the query strings are pretty much as short as they can get. You don t explicitly specify the CASE expressions: those are constructed behind the scenes for you (you can actually see them by looking at the properties of the aggregate operator in the plan). In short, you don t need to use the Matrix table approach with the PIVOT operator. Here s the query using the PIVOT operator to calculate total yearly quantities per customer:
SELECT * FROM (SELECT custid, YEAR(orderdate) AS orderyear, qty FROM dbo.Orders) AS D PIVOT(SUM(qty) FOR orderyear IN([2006],[2007],[2008])) AS P;
And here s a query that counts the orders:
SELECT * FROM (SELECT custid, YEAR(orderdate) AS orderyear FROM dbo.Orders) AS D PIVOT(COUNT(orderyear) FOR orderyear IN([2006],[2007],[2008])) AS P;
Remember that static queries performing pivoting require you to know ahead of time the list of attributes you re going to rotate. For dynamic pivoting, you need to construct the query string dynamically.
Unpivoting
Unpivoting is the opposite of pivoting namely, rotating columns to rows. Unpivoting is usually used to normalize data, but it has other applications as well.
Note Unpivoting is not an exact inverse of pivoting it won t necessarily allow you to regenerate source rows that were pivoted. However, for the sake of simplicity, think of it as the opposite of pivoting. In my examples, I ll use the PvtCustOrders table, which you create and populate by running the following code:
USE tempdb; IF OBJECT_ID('dbo.PvtCustOrders') IS NOT NULL DROP TABLE dbo.PvtCustOrders; GO
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Aggregating and Pivoting Data
SELECT custid, COALESCE([2006], 0) AS [2006], COALESCE([2007], 0) AS [2007], COALESCE([2008], 0) AS [2008] INTO dbo.PvtCustOrders FROM (SELECT custid, YEAR(orderdate) AS orderyear, qty FROM dbo.Orders) AS D PIVOT(SUM(qty) FOR orderyear IN([2006],[2007],[2008])) AS P; UPDATE dbo.PvtCustOrders SET [2007] = NULL, [2008] = NULL WHERE custid = 'D'; -- Show the contents of the table SELECT * FROM dbo.PvtCustOrders;
This generates the following output:
custid -----A B C D 2006 ----------22 20 22 30 2007 ----------40 12 14 NULL 2008 ----------10 15 20 NULL
The goal in this case is to generate a result row for each customer and year, containing the customer ID (custid), order year (orderyear), and quantity (qty). I ll start with a solution that does not use the native UNPIVOT operator. Here as well, try to think in terms of logical query processing as described in 1. The rst step in the solution is to generate three copies of each base row one for each year. You can achieve this by performing a cross join between the base table and a virtual auxiliary table that has one row per year. The SELECT list can then return custid and orderyear and also calculate the target year s qty with the following CASE expression:
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