Why use cte sql

The SalaryCTE has a subquery that will select the employeeID from the salary table for the employee's salary greater than or equal to CTE provides recursive programming. CTE makes code maintainability easier. Though it provides similar functionality as a view, it will not store the definition in metadata.

View All. Dhanasekar Updated date Nov 19, For this first we create an Item History table. Create Item History Table: In this history table, we add the same columns as item table along with MarketRate column as present or future Item price.

Here is the query to create an ItemHistory table. Output: When we run the query, we can see the below output as 30 records has been inserted into our Item History 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. A CTE query is defined in between the parentheses. It also calculates the average grade, using the column grade from the table exams. The tables students and exams are joined on the appropriate student ID column from each table. The records are grouped, since you need to obtain the result by student. It is possible to define and use more than one CTEs in a query.

Let me show you an example. With the same tables from the previous example, you have the following task: show the name of the subjects and their respective average and minimum grades, but only for those subjects in which everybody passed the exam, i. Then, it groups the data to get the result by subject. Now, you define the second CTE. It groups the data as was done in the first CTE.

Since you need the result only for the subjects in which everybody passed, you use a HAVING clause to select only the subjects in which the minimum grade is 5 or above. There are only two such subjects:. The task now is to calculate the average minimum grade and the average maximum grade by subject. Where would you start? If you think logically, you should first find the minimum and maximum grades per subject then find the average of the results by subject.

Logically speaking, this tries to calculate the minimum grade and the maximum grade by subject first, then the average of those values.

You now run the code, and get a message that looks like this:. Also, it makes it easier to be able to see what is actually being joined together.

Correlated subqueries can also be replaced with non recursive CTEs. Given this, and knowing that joins are easily moved into non recursive CTEs, you can see the possibilities.

The subquery is used to calculate the average line total for each sales order. To replicate this query using a CTE we first need to create query definition to calculate the average line total for each Sales Order.

Suppose you wanted to know how many departments have the same number of employees. What query could you use? Finding the number of employees by department name is pretty easy. We can use the following query to do so:. The number of employees in each department is calculated using a scalar sub select.

This is colored red in the above query for easy identification. Here is a sample result:. To do this we should group on NumberEmployees, but this statement is invalid:. Due to the grouping restriction. That is a SQL violation! As is. Now we query the CTE and group by the result of the scalar sub select red text. The results returned are:. Suppose we want to return the top sales within each territory.

To do this we can RANK sales within each territory as 1,2,…, and so on, and then select those with a rank of 1. Given this, then our job is to pick only those rows whose RANK is one. These are the top ranked sales within each territory. With a CTE we can define a query to return the sales data ranked by territory and then query that to only return those items ranked first in each territory:.

To help you see this, the original query used to generate rankings is highlighted in blue. The filter to only show the top ranked sales in each territory is in red. In doing so, the initial CTE is repeatedly executed, returning subsets of data, until the complete result is returned. This query is repeatedly run until no rows are returned. This is why the termination check is so important. Here is an example of the error you can expect if the termination check is missing or faulty:.

Of course, there may be an instance where the maximum recursion should be greater than The Adventure Works company manufactures bicycles, and as you can imagine, they contain a lot of parts! To keep it all straight the production department has put together a bill of materials or BOM for short. A BOM lists all the parts for each bike.