SQL Server Tutorial: Discover SQL's Common Table Expressions (CTE)

CTEs: What are they?

Within a SELECT statement, temporary result sets known as Common Table Expressions (CTEs) are defined. They offer a means of decomposing intricate queries into more manageable, simpler components. Consider them as makeshift tables that are only there while the primary query is running. CTEs are used within the query they are created in and are defined using the WITH clause; this method is more organized and cleaner than subqueries.

Why Use CTEs?

Improved readability: They make complex queries easier to understand by breaking them down into logical steps.
Enhanced maintainability: Changes to the CTE can be made independently without affecting the entire query.
Recursive queries: CTEs are essential for implementing recursive algorithms, such as calculating hierarchies or generating sequences.
Reusability: Once defined, a CTE can be referenced multiple times within the same query, reducing repetition.

Example of a Simple CTE

Let’s say you have a Sales table, and you want to find the total sales for each salesperson but only for those who achieved sales above a certain threshold. You can use a CTE to make this query more readable.

Recursive CTEs

Recursive CTEs are an extension of the basic CTE that allows a query to reference itself. This is particularly useful for dealing with hierarchical data.

Here’s an example of a recursive CTE to retrieve an employee hierarchy.

<span class="token keyword keyword-WITH">WITH</span> EmployeeHierarchy <span class="token keyword keyword-AS">AS</span> <span class="token punctuation">(</span>
    <span class="token keyword keyword-SELECT">SELECT</span> EmployeeID<span class="token punctuation">,</span> ManagerID<span class="token punctuation">,</span> Name<span class="token punctuation">,</span> <span class="token number">0</span> <span class="token keyword keyword-AS">AS</span> <span class="token keyword keyword-Level">Level</span>
    <span class="token keyword keyword-FROM">FROM</span> Employees
    <span class="token keyword keyword-WHERE">WHERE</span> ManagerID <span class="token operator">IS</span> <span class="token boolean">NULL</span>
    <span class="token keyword keyword-UNION">UNION</span> <span class="token keyword keyword-ALL">ALL</span>
    <span class="token keyword keyword-SELECT">SELECT</span> e<span class="token punctuation">.</span>EmployeeID<span class="token punctuation">,</span> e<span class="token punctuation">.</span>ManagerID<span class="token punctuation">,</span> e<span class="token punctuation">.</span>Name<span class="token punctuation">,</span> eh<span class="token punctuation">.</span><span class="token keyword keyword-Level">Level</span> <span class="token operator">+</span> <span class="token number">1</span>
    <span class="token keyword keyword-FROM">FROM</span> Employees e
    <span class="token keyword keyword-INNER">INNER</span> <span class="token keyword keyword-JOIN">JOIN</span> EmployeeHierarchy eh <span class="token keyword keyword-ON">ON</span> e<span class="token punctuation">.</span>ManagerID <span class="token operator">=</span> eh<span class="token punctuation">.</span>EmployeeID
<span class="token punctuation">)</span>
<span class="token keyword keyword-SELECT">SELECT</span> EmployeeID<span class="token punctuation">,</span> Name<span class="token punctuation">,</span> <span class="token keyword keyword-Level">Level</span>
<span class="token keyword keyword-FROM">FROM</span> EmployeeHierarchy<span class="token punctuation">;</span>

WITH EmployeeHierarchy AS (

SELECT EmployeeID, ManagerID, Name, 0 AS Level

FROM Employees

WHERE ManagerID IS NULL

UNION ALL

SELECT e.EmployeeID, e.ManagerID, e.Name, eh.Level + 1

FROM Employees e

INNER JOIN EmployeeHierarchy eh ON e.ManagerID = eh.EmployeeID

)

SELECT EmployeeID, Name, Level

FROM EmployeeHierarchy;

In this example, the recursive CTE starts by selecting the top-level managers (those with no ManagerID). It then recursively joins the Employees table to build out the hierarchy, adding a level of depth with each recursive step.

Conclusion

CTEs are essential for writing efficient, maintainable SQL queries. They simplify complex logic, support recursion, and enhance code clarity, making them a valuable tool for any SQL developer. Whether you’re dealing with large datasets or hierarchical data, CTEs offer a structured approach that can make your queries more efficient and easier to understand.

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