What is cte in sql

You can use them to simplify queries. For example, you could use one to eliminate a derived table from the main query body. It becomes a convenient way to manage complicated queries. You can define one or more common table expression in this fashion. The blue portion is the CTE. Do you see the contained SQL is a fully formed query? Notice that when we define the CTE we give the result a name as well its columns.

The result and columns are named differently. This allows you to encapsulate complicated query logic with the common table expression. These columns correspond to the columns returned from the inner query. It is only concerned with the name and columns, not the inner SQL. This can help you simplify some very complicated queries which are ultimately joined together. Some of them include:. Here you see picture of opposing mirrors. Due to the reflection, it becomes a picture in a picture. A recursive query repeatedly run on a subset of the data.

A recursive query is basically a query that calls itself. In a way when you look into the picture you can imagine each picture in a picture is the picture calling itself. At that point, the recursion starts to unwind, collect and calculate data as it reviews each successive result. CTEs are table expressions. The CTE has two parts. The first part defines the name of the CTE and it the columns contained within it. The second part is the query definition. You can define more than one CTE in a statement.

You see an example of this further along in this article. When writing the query definition keep in mind that the following cannot be used within it:. The first common table expression is colored green , the second blue. As queries get larger is can become really difficult to understand how they work. Instead, it means that it is understandable to you and others. I think CTEs help improve readability several ways. They help separate out query logic.

If you are joining two complex queries , you can use non recursive CTEs to separate out the complexity of the queries from the actual join.

This not only helps in debugging, as you can independently run the query definition to test it, but you can more easily identify the parts used to do the join. In this example, we will delete all records where the AddressLine2 is null. When we select from the table again, you can see there are no longer any records with NULL in the AddressLine2 column. Finally, you are not limited to just writing a single CTE in a query. You can write as many as you want and separate them with a comma:.

In this example, we create two CTEs with some joins, logic, and selecting only a few specific columns. In our final operation, we join the CTEs together and form an aggregate to show total sales for each product by calendar year. A word of caution, if you find yourself with many CTEs, the code can become more challenging to read. So, while they can be a great tool, try not to go overboard with them.

Though CTEs are a great way to write cleaner code, they should not be used in every scenario. For example, one major limitation is that a CTE is limited to the scope of a single execution. What that means in practice is that if you need to do more than one operation with this result set, you need to declare the CTE multiple times.

This is not performant because we are doing the work of the CTE multiple times. In this scenario it would probably be better to use a temporary table. Temporary tables can be used across multiple queries, and they can be indexed and store column statistics. Generally speaking, if you need to create a named result set that will be used by multiple subsequent queries and the data is very large, you are better off using a temporary table.

Output: When we run the query, we can see the below output from item history table. Now we see how to use the above CTE query can be used in a view. Here we create a view and we add the CTE result inside the view.

When we select the view as a result, we can see the CTE output will be displayed. Output: When we run the query, we can see the below output as result from the View. CTEs can be used to create a recursive query and can be used to reference itself multiple times.

CTEs can be used instead of views and finally a CTE is easy and simple for readability and code maintainability. Select [ Column1 , Column2 , Column3 …. FROM sys. Declare startDate datetime , endDate datetime ;. A sub-query is a query within a query. In SQL, we will use sub-queries to join the records or filter the records from a sub-query.

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