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CHAPTER 10 WRITING ADVANCED QUERIES
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employee may not be valid for a particular date. For example, the employee may have four changes and, therefore, four rows for an earlier date and only one row for the latest date.
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Figure 10-3. The results of calling one CTE from another CTE definition Code section 1 drops the JobHistory table if it already exists. Statement 2 creates the JobHistory table including a primary key composed of EmployeeID, EffDate, and EffSeq. Statement 3 inserts several rows into the table. Notice that the statement inserts one row for each change even if the changes happen on the same date. Statement 4 declares and initializes a variable, @Date, which will be used in the WHERE clause in Statement 5. You can change the value of this variable to validate the results for different dates. Statement 5 contains the SELECT statement. The first CTE, EffectiveDate, just determines the maximum EffDate from the JobHistory table for each employee that is valid for the @Date value. The second CTE, EffectiveSeq, joins the JobHistory table to the EffectiveDate CTE to find the maximum EffSeq for each employee for the date determined in the previous CTE, EffectiveDate. Finally, the outer query joins the JobHistory table on the EffectiveSeq CTE to display the valid data for each employee on the date stored in @Date.
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Using the Alternate CTE Syntax
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I prefer naming all the columns within the CTE definition, but you can also specify the column names outside the definition. There is no advantage to either syntax, but you should be familiar with both. Here is the syntax: WITH <cteName> (<col1>, <col2>) AS ( SELECT <col3>,<col4> FROM <table1>) SELECT <col1>,<col2> FROM <cteName> When using this technique, the column names defined outside the definition must be used in the outer query. If you have an expression within the definition, you do not have to give the expression an alias. Type in and execute the code in Listing 10-4 to practice this technique. Listing 10-4. Writing a Query with the Alternate CTE Syntax USE AdventureWorks; GO WITH Emp (EmployeeID, ManagerID, JobTitle,EmpName) AS (SELECT e.EmployeeID, e.ManagerID,e.Title, c.FirstName + ISNULL(' ' + c.MiddleName,'') + ' ' + FROM HumanResources.Employee AS e
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c.LastName
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CHAPTER 10 WRITING ADVANCED QUERIES
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INNER JOIN Person.Contact AS c ON e.ContactID = c.ContactID) SELECT Emp.EmployeeID, ManagerID, JobTitle, EmpName FROM Emp; Figure 10-4 shows the partial results. All the columns must be listed in parentheses between the CTE name and the definition. Only the columns listed are valid. Either syntax will work; it is just a matter of preference.
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Figure 10-4. The partial results of using the named column CTE syntax
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Writing a Recursive Query
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Recursive code, in any programming language, is code that calls itself. Programmers use this technique to follow paths in tree or directory structures. When following the paths in these structures, the code must start at the root, follow each path to the end, and back up again to the next path repeatedly. In T-SQL, you can use the same technique in a CTE. One common example is the HumanResources.Employee table in the AdventureWorks database. The self-join found in that table represents a hierarchical structure. To view the entire hierarchy, you must start at the root, the CEO of the company, and follow every possible manager-employee path down to the lowest person. Here is the syntax for writing a recursive CTE: WITH <cteName> (<col1>, <col2>, <col3>, level) AS ( --Anchor member SELECT <primaryKey>,<foreignKey>,<col3>, 0 AS level FROM <table1> WHERE <foreignKey> = <startingValue> UNION ALL --Recursive member SELECT a.<primaryKey>,a.<foreignKey>,a.<col3>, b.level + 1 FROM <table1> AS a INNER JOIN <cteName> AS b ON a.<foreignKey> = b.<primaryKey> )
CHAPTER 10 WRITING ADVANCED QUERIES
SELECT <col1>,<col2>,<col3>,level FROM <cteName> [OPTION (MAXRECURSION <number>)] To write the recursive CTE, you must have an anchor member, which is a statement that returns the top of your intended results. This is like the root of the directory. Following the anchor member, you will write the recursive member. The recursive member actually joins the CTE that contains it to the same table used in the anchor member. The results of the anchor member and the recursive member join in a UNION ALL query. Type in and execute the code in Listing 10-5 to see how this works. Listing 10-5. A Recursive CTE USE AdventureWorks; GO WITH OrgChart (EmployeeID, ManagerID, Title, Level,Node) AS (SELECT EmployeeID, ManagerID, Title, 0, CONVERT(VARCHAR(30),'/') AS Node FROM HumanResources.Employee WHERE ManagerID IS NULL UNION ALL SELECT a.EmployeeID, a.ManagerID,a.Title, b.Level + 1, CONVERT(VARCHAR(30),b.Node + CONVERT(VARCHAR,a.ManagerID) + '/') FROM HumanResources.Employee AS a INNER JOIN OrgChart AS b ON a.ManagerID = b.EmployeeID ) SELECT EmployeeID, ManagerID, SPACE(Level * 3) + Title AS Title, Level, Node FROM OrgChart ORDER BY Node; Figure 10-5 shows the results. The anchor member selects the EmployeeID, ManagerID, and Title from the HumanResources.Employee table for the CEO. The CEO is the only employee with a NULL ManagerID. The level is zero. The node column, added to help sorting, is just a slash. To get this to work, the query uses the CONVERT function to change the data type of the slash to a VARCHAR(30) because the data types in the columns of the anchor member and recursive member must match exactly. The recursive member joins HumanResources.Employee to the CTE, OrgChart. The query is recursive because the CTE is used inside its own definition. The regular columns in the recursive member come from the table, and the level is one plus the value of the level returned from the CTE. To sort in a meaningful way, the node shows the ManagerID values used to get to the current employee surrounded with slashes. This looks very similar to the node used in the HierarchyID example in 9.
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