CTE in SQL

A Common Table Expression (CTE) in SQL Server is a powerful, temporary result set that you can reference within a SELECT, INSERT, UPDATE, or DELETE statement. Acting like a temporary view, CTE in sql are used to simplify complex queries by breaking them down into simpler, more readable sections. Unlike subqueries, CTEs can be self-referencing and recursive, making them particularly useful for executing hierarchical or recursive queries, such as navigating tree structures or computing running totals.

Why do We Need CTE in the SQL server?

Using sub-queries in SQL, we can join the records or filter the records from a sub-query. We face maintainability issues whenever we refer to the same data or use the same sub-query to join the same records.

Additionally, there are Derived Tables, Temporary Tables, and Temporary Variables approaches in SQL that are somewhat similar to CTE, although each has its own set of cons compared to CTE. Common Table Expression proves its worth with improved readability and easier maintenance.

Using the CTE

Utilizing a Common Table Expression (CTE) in SQL Server provides a method for crafting temporary result sets that can be referred to within various types of SQL statements. This is achieved by preceding the main query with a WITH clause, which can encapsulate one or more CTEs, enhancing the structure and readability of complex SQL queries.

Syntax

The basic syntax framework for a CTE is outlined as follows:

Argument

• CTE Name: This serves as the unique identifier for the CTE within the scope of a query. It must be distinct from other CTE names defined in the same WITH clause but can share its name with base tables or views within the database. This name is used to reference the CTE within the main query.

• Column Names: This optional argument allows for the specification of column names in the CTE's result set. It ensures clarity and specificity in the structure of the CTE, especially when the result set columns need to be explicitly named for further operations. The count and order of column names must exactly match those in the CTE's result set. Duplicate column names within a single CTE are prohibited.

• CTE Query Definition: This is the core of the CTE, usually a SELECT statement that generates the result set the CTE represents. This query must produce a valid result set that conforms to the same standards as a view, with the limitation that it cannot define another CTE within it. If the CTE is part of a larger query involving multiple CTEs, their results can be combined using set operators like UNION, UNION ALL, EXCEPT, or INTERCEPT.

Types Of CTE In SQL Server

Recursive

A recursive common table expression (CTE) references itself. As a result, the CTE executes repeatedly, and returns subsets of data, until it obtains the complete result set. This makes it an excellent choice for dealing with hierarchical and tree-structured data.

Recursive CTEs use repeated procedural loops. The recursive query calls itself repeatedly until the condition is satisfied. In a recursive CTE, we should provide a where condition to stop the recursion.

Syntax :

Non-recursive

The non-recursive CTE uses no recursion or repeated processing. Previously, we discussed non-recursive CTEs and their examples.

Example Of CTE In SQL Server

After the discussion of theory and syntax, let's see some examples of CTE in SQL. First of all, we need to have some data for these examples, so let's define them.

CREATE DATABASE

SWITCH DATABASE

CREATE TABLES

Output :

INSERT DATA

After inserting some data our table will look something like this,

Now our data is ready so we can perform our query.

Scenario : 1

University provides an honors degree to those who secure more than $7.5$ CGPA, let's say academic cell wants to get records according to condition whether the student has secured more than $7.5$ CGPA or not.

Details Required :

• Unique ID of Student
• Full Name of Student
• CGPA of Student

Output :

Explanation :

• This is the simple CTE where we have just filtered the data of two tables with the help of join and based on a condition of having cgpa more than 7.5.

Scenario : 2

Let's say there is an urgent need for blood for a student having blood group B+, so how we can find all students having blood groups from B+, B-, O+, O- category.

Details Required :

• Unique ID of Student
• Full Name of Student
• Blood Group
• Contact Number
• Address

Output :

Explanation :

• We have created a CTE using the WITH keyword, which is having the name CTE_TABLE1.
• In the column list we are defining some required columns, also these columns must be the same as the ones written inside the AS() block.
• Later we are selecting student_id from the student, Concat function is used to concatenate the first name and last name.
• The phone, address, and blood group are being selected from the student_details table according to inner join.
• At the end, the where statement is filtering the record according to the required blood group.

When to Use CTE in SQL Server And When Not to?

• CTEs are highly beneficial for structuring complex and lengthy SQL queries.
• Functionally and performance-wise, there is no difference between a subquery and a CTE.
• CTEs enhance code readability by segmenting the query into distinct steps, making errors more identifiable and modifications simpler.
• They are particularly useful for recursive data access needs.
• CTEs have a limited scope of execution, making them less ideal for scenarios requiring data to be fetched multiple times.
• For repeated data access, temporary tables are a more efficient alternative due to the need to redefine CTEs for each use.

Advantages Of CTE In SQL Server

• The major advantage of CTE is it enhances code readability as compared to the subquery approach.
• Code maintenance becomes easier with CTEs.
• Because of recursive programming in CTE, these are especially effective at querying data for tree structures.
• The functionality of CTE is similar to view in the database but it doesn't store the definition in metadata.

Disadvantages Of CTE In SQL Server

• It is not possible to reuse CTE in another query like view or function.
• Although it has an advantage over view, it cannot be nested as a view.
• Performance issue will be there if the table is being referenced frequently because it will require equivalent cost to construct that table again.