Using Common Table Expressions
A common table expression (CTE) can be thought of as a temporary result set that is defined within the execution scope of a single SELECT, INSERT, UPDATE, DELETE, or CREATE VIEW statement. A CTE is similar to a derived table in that it is not stored as an object and lasts only for the duration of the query. Unlike a derived table, a CTE can be self-referencing and can be referenced multiple times in the same query.
A CTE can be used to:
- Create a recursive query. For more information, see Recursive Queries Using Common Table Expressions.
- Substitute for a view when the general use of a view is not required; that is, you do not have to store the definition in metadata.
- Enable grouping by a column that is derived from a scalar subselect, or a function that is either not deterministic or has external access.
- Reference the resulting table multiple times in the same statement.
Using a CTE offers the advantages of improved readability and ease in maintenance of complex queries. The query can be divided into separate, simple, logical building blocks. These simple blocks can then be used to build more complex, interim CTEs until the final result set is generated.
CTEs can be defined in user-defined routines, such as functions, stored procedures, triggers, or views.
Structure of a CTE
A CTE is made up of an expression name representing the CTE, an optional column list, and a query defining the CTE. After a CTE is defined, it can be referenced like a table or view can in a SELECT, INSERT, UPDATE, or DELETE statement. A CTE can also be used in a CREATE VIEW statement as part of its defining SELECT statement.
The basic syntax structure for a CTE is:
WITH expression_name [ ( column_name [,...n] ) ]
( CTE_query_definition )
The list of column names is optional only if distinct names for all resulting columns are supplied in the query definition.
The statement to run the CTE is:
The following example shows the components of the CTE structure: expression name, column list, and query. The CTE expression
CTE has three columns (
MaxDate) and is defined as the total number of sales orders and the most recent sales order date in the
SalesOrderHeader table for each salesperson. When the statement is executed, the CTE is referenced two times: one time to return the selected columns for the salesperson, and again to retrieve similar details for the salesperson's manager. The data for both the salesperson and the manager are returned in a single row.
USE AdventureWorks; GO WITH Sales_CTE (SalesPersonID, NumberOfOrders, MaxDate) AS ( SELECT SalesPersonID, COUNT(*), MAX(OrderDate) FROM Sales.SalesOrderHeader GROUP BY SalesPersonID ) SELECT E.EmployeeID, OS.NumberOfOrders, OS.MaxDate, E.ManagerID, OM.NumberOfOrders, OM.MaxDate FROM HumanResources.Employee AS E JOIN Sales_CTE AS OS ON E.EmployeeID = OS.SalesPersonID LEFT OUTER JOIN Sales_CTE AS OM ON E.ManagerID = OM.SalesPersonID ORDER BY E.EmployeeID; GO
Here is a partial result set:
EmployeeID NumberOfOrders MaxDate ManagerID NumberOfOrders MaxDate ----------- -------------- ---------- --------- -------------- ---------- 268 48 2004-06-01 273 NULL NULL 275 450 2004-06-01 268 48 2004-06-01 276 418 2004-06-01 268 48 2004-06-01 277 473 2004-06-01 268 48 2004-06-01