Monday, 9 September 2013

CTE in SQL

                    CTE(Common Table Expressions) in SQL


CTE( Common Table Expressions) was first introduced with SQL 2005. It is the best replacement for Views.

There are times when we might require to work on small data sets that is a part of large database. For example, we might have a huge sized table that contains data for all transactions that a company has made, but we require current month's data only and apply some grouping or filters on that particular data of current month. Typically views are used to do the task. But, views sometimes over occupy our resources as views are stored at system level. CTE is the best substitute for Views. It is also helpful when we don't have access to create object in a database.


Common Table Expressions or CTE are more readable form of derived table. CTE can be declared once and can be used in multiple queries. CTE can be recursive and non-recursive (we will cover this topic in next post)

Common Table Expression Syntax
A Common Table Expression contains three core parts:
  • The CTE name (this is what follows the WITH keyword)
  • The column list (optional)
  • The query (appears within parentheses after the AS keyword)

EXAMPLE:

WITH ProductAndCategoryNamesOverTenDollars (ProductName, CategoryName, UnitPrice) AS
(
   SELECT
      p.ProductName,
      c.CategoryName,
      p.UnitPrice
   FROM Products p
      INNER JOIN Categories c ON
         c.CategoryID = p.CategoryID
   WHERE p.UnitPrice > 10.0
)


SELECT *
FROM ProductAndCategoryNamesOverTenDollars
ORDER BY CategoryName ASC, UnitPrice ASC, ProductName ASC


                            The CTE is part of the subsequent statement only.The subsequent statement can be a single SELECT/INSERT/UPDATE/DELETE, or a compound (with UNION, INTERSECT etc). So if we modify our above example and include a query between CTE creation query(WITH) and the query that refer the CTE.(like below)



WITH ProductAndCategoryNamesOverTenDollars (ProductName, CategoryName, UnitPrice) AS
(
   SELECT
      p.ProductName,
      c.CategoryName,
      p.UnitPrice
   FROM Products p
      INNER JOIN Categories c ON
         c.CategoryID = p.CategoryID
   WHERE p.UnitPrice > 10.0
)

SELECT * FROM Products 

SELECT *
FROM ProductAndCategoryNamesOverTenDollars
ORDER BY CategoryName ASC, UnitPrice ASC, ProductName ASC


Here, we will see that when the query that refers to CTE(ProductAndCategoryNamesOverTenDollars) is executed we will get an error "Invalid object name 'ProductAndCategoryNamesOverTenDollars'" 

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