What Is Common Table Expressions (CTE) In SQL Server

The Common Table Expressions is a table expression that most of the database developers and DBAs work on. The CTE was first introduced in SQL Server 2005. The main intent of the introduction was to ease the developers with the result sets. Let’s take a look at the details of the CTE.

What is a Common Table Expressions?

The Common Table Expressions (CTEs) are a temporary result set. It is another standard form of table expression very similar to derived tables. Additionally, Common Table Expressions are represented in short form as CTE. While considering the CTEs, there comes a bundle of more advantages over the standard table expressions.

The inner query defined in the CTE must always follow all the requirements of SELECT from your table to be valid to define a table expression.

Syntax of a CTE:

Every CTE has a WITH statement and have the below general format.

WITH <Your CTE Name>[<target_Column_list>]
AS
(
<inner_query_defining_CTE>
)
<outer_query_against_CTE>;

Example:

with CTE1 as
(
select * from sys.databases
)
select name from CTE1;

Assigning Column Aliases in CTEs

There are two forms of column aliases, inline and external.

Inline Alias: Specify column alias <column_alias> right after the expression <expression>.

Example:

with CTE1 as
(
select name, database_id as InternalAlias from sys.databases
)
select name from CTE1;

External Alias: Specify column alias <column_alias> right after the CTE name in the parentheses.

Example:

with CTE1(External_Alias, ID) as
(
select name, database_id as ID from sys.databases
)
select External_Alias, ID from CTE1;

Using Arguments in CTE

Like most of the database developers and administrators are much familiar with using the parameters and arguments while writing the query, the similar applies to CTE too. You can use arguments inside CTE.

Example:

declare @DB_ID int=1;
with CTE1(External_Alias, ID) as
(
select name, database_id as ID from sys.databases where database_id=@DB_ID
)
select External_Alias, ID from CTE1;

Defining Multiple CTEs

The option of defining multiple CTEs come with a great advantage. And one of them is to avoid the query nesting that usually happens with the derived tables.

Example:

with CTE1(External_Alias, ID) as
(
select name, database_id as ID from sys.databases
),
CTE2 AS
(
Select name as database_name from sys.databases
)
select CTE1.External_Alias, CTE1.ID, CTE2.database_name from CTE1
inner join
CTE2
on
CTE1.External_Alias=CTE2.database_name;

Multiple References in CTEs

In most cases we tend to reference the CTE only once. But we can have multiple references to the same CTE and it comes with added advantages. Defining a single CTE and referencing the same CTE multiple times from the FROM clause of the outer query avoids multiple derivatives from the user tables.

This approach significantly increases the performance as it is considered as “modular approach” verse the “derived tables”.

Example:

with CTE1 as
(
select name, database_id as ID from sys.databases
)
select a.name, b.ID from CTE1 a
left outer join CTE1 b
on
a.ID=b.ID;

From our example, we have declared just one CTE by name CTE1 and referencing the CTE1 twice in the outer FROM clause. This is logically equivalent to deriving the data from two different derived tables. Additionally, this approach can give us a clearer picture and lessen the errors. 

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