At some point there is an end condition, so it doesn’t call itself indefinitely. A recursive query is basically a query that calls itself. Due to the reflection, it becomes a picture in a picture.Ī recursive query repeatedly run on a subset of the data. Here you see picture of opposing mirrors. We’ll go deep into recursive CTE’s in a separate post, but for now let me introduce to you recursion using this diagram: Recursion can be a pretty difficult topic to grasp, I really didn’t get it until I took a LISP class way back in 1986, but hopefully I can explain it to you. Recursive CTE’s are common table expressions that reference themselves. Ranking – Whenever you want to use ranking function such as ROW_NUMBER(), RANK(), NTILE() etc.Ĭommon Table Expressions are categorized as: Recursive CTE’s and Non-Recursive CTE’s.Limitations – Overcome SELECT statement limitations, such as referencing itself (recursion), or performing GROUP BY using non-deterministic functions.This is handy when you need to work on hierarchical data such as organization charts. Recursion – Use CTE’s do create recursive queries, that is queries that can call themselves.This is handy if you don’t have permissions to create a view object or you don’t want to create one as it is only used in this one query. Substitute for a View – You can substitute a SQL CTE for a view.This lets you get the chunks of data you need and combine them in a final SELECT. Rather than lump all you query logic into one large query, create several CTE’s, which are the combined later in the statement. Readability – CTE’s promote readability.There are several reasons why you may want to use a SQL CTE over other methods. Each complicated piece can include in their own CTE which is then referred to and joined outside the WITH clause. This can help you simplify some very complicated queries which are ultimately joined together. I want to point out that you can define more than one SQL CTE within a WITH statement. Notice the column names, they’re based on the those defined in the CTE. It is only concerned with the name and columns, not the inner SQL. CTE Query Definition Column Mappingsįinally notice that our final query references the SQL CTE and columns defined.įrom our outer query’s perspective all it “sees” is this definition. These columns correspond to the columns returned from the inner query. There you’ll see the name and columns are defined. Now going back to the SQL CTE, notice that the WITH statement. This allows you to encapsulate complicated query logic with the common table expression. The result and columns are named differently. Notice that when we define the CTE we give the result a name as well its columns. When you run it you see results like: CTE Query Definition Results Naming Columns Do you see the contained SQL is a fully formed query? It is the SQL CTE query definition: SELECT NationalIDNumber, Here is a really simple example of one CTE: WITH Employee_CTE (EmployeeNumber, Title) You can define one or more common table expression in this fashion. You define Common Table Expressions using the WITH statement. It becomes a convenient way to manage complicated queries. What is a SQL CTE or Common Table Expression in SQL Server?Ī SQL CTE (Common Table Expression) defines a temporary result set which you can then use in a SELECT statement. #SQL STUDIO MANAGER TABLE MULTIPLE DELETE FREE#Start learning SQL Server using my free guide Getting Started Using SQL Server. Note: All the examples for this lesson are based on Microsoft SQL Server Management Studio and the AdventureWorks2012 database. For example, you could use one to eliminate a derived table from the main query body. They are SQL-compliant and part of the ANSI SQL 99 specification.Ī SQL CTE always returns a result set. They were introduced in SQL Server version 2005.
0 Comments
Leave a Reply. |
AuthorWrite something about yourself. No need to be fancy, just an overview. ArchivesCategories |