What is the difference between CTE and subquery?

A Common Table Expression

Common Table Expression

A hierarchical query is a type of SQL query that handles hierarchical model data. They are special cases of more general recursive fixpoint queries, which compute transitive closures. In standard SQL:1999 hierarchical queries are implemented by way of recursive common table expressions (CTEs).

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Hierarchical and recursive queries in SQL - Wikipedia

(aka CTE, aka WITH statement) is a temporary data set to be used as part of a query. It only exists during the execution of that query; it cannot be used in other queries even within the same session (from Wikipedia). A subquery is a nested query; it's a query within a query (more Wikipedia).

Is a CTE faster than a subquery?

The performance of CTEs and subqueries should, in theory, be the same since both provide the same information to the query optimizer. One difference is that a CTE used more than once could be easily identified and calculated once. The results could then be stored and read multiple times.

What is the difference between Common Table Expression and derived table and subquery?

CTEs were introduced back in SQL Server 2005. While there is some additional functionality, i.e. recursion, CTEs work similarly to derived tables. Derived tables are subqueries that are used in the FROM clause instead of named tables. I like using CTEs over derived tables because CTEs are so much easier to read.

What is the difference between temp table and CTE?

Temp Tables are physically created in the tempdb database. These tables act as the normal table and also can have constraints, an index like normal tables. CTE is a named temporary result set which is used to manipulate the complex sub-queries data. This exists for the scope of a statement.

Why CTE is faster than temp table?

Temp tables are always on disk - so as long as your CTE can be held in memory, it would most likely be faster (like a table variable, too). But then again, if the data load of your CTE (or temp table variable) gets too big, it'll be stored on disk, too, so there's no big benefit.

SQL: Subquery vs. Common Table Expression

Are CTE is stored in memory?

Answers. CTE results are not stored anywhere.... they don't produce results.... a CTE is just a definition, just like a VIEW is just a definition. Think of a CTE as being a View that only lasts for the duration of the query.

What is difference between CTE and view?

The key thing to remember about SQL views is that, in contrast to a CTE, a view is a physical object in a database and is stored on a disk. However, views store the query only, not the data returned by the query. The data is computed each time you reference the view in your query.

Why CTE is used in SQL?

Why to use a CTE. In SQL, we will use sub-queries to join the records or filter the records from a sub-query. Whenever we refer the same data or join the same set of records using a sub-query, the code maintainability will be difficult. A CTE makes improved readability and maintenance easier.

How do you convert CTE to subquery?

What is CTE with example in SQL?

CTE was introduced in SQL Server 2005, the common table expression (CTE) is a temporary named result set that you can reference within a SELECT, INSERT, UPDATE, or DELETE statement. You can also use a CTE in a CREATE a view, as part of the view's SELECT query.

What is the use of CTE in Oracle?

A common table expression (CTE) is a named temporary result set that exists within the scope of a single statement and that can be referred to later within that statement, possibly multiple times.

Can CTE be used in stored procedure?

According to the CTE documentation, Common Table Expression is a temporary result set or a table in which we can do CREATE, UPDATE, DELETE but only within that scope. That is, if we create the CTE in a Stored Procedure, we can't use it in another Stored Procedure.

Is CTE optimized?

CTEs have been originally implemented with an optimization intent. They are a way to tell the query planner “Materialize this intermediate table, because I will use it several times in the future. Please don't recompute it each time”. It's indeed positive in a few rare cases, like below.

Can we create index in CTE?

No. A CTE is a temporary, "inline" view - you cannot add an index to such a construct. If you need an index, create a regular view with the SELECT of your CTE, and make it an indexed view (by adding a clustered index to the view).

When should I use CTE in SQL Server?

Can we join two CTE?

But Can I Use One CTE Inside Another CTE? A straightforward question deserves a straightforward answer: yes, you can. Now that you know how to use multiple CTEs, writing a CTE that references another CTE is just a variation of what you've learned.

Can we use CTE multiple times?

Unlike a derived table, a CTE behaves more like an in-line view and can be referenced multiple times in the same query. Using a CTE makes complex queries easier to read and maintain. Because a CTE can be referred to multiple times in a query, syntax can be simpler.

Can we use CTE in joins?

CTEs can't be created within a SELECt query. As correctly suggested take the CTE definition outside the JOIN and use the CTE name in the JOIN. CTE definition that starts with WITH should not be preceded by any other statement, so as per the general practice, always use a semi colon before WITH.

What is the difference between temp table and normal table in SQL Server?

A Temp table is easy to create and back up data. Table variable involves the effort when you usually create the normal tables. Temp table result can be used by multiple users. Table variable can be used by the current user only.

Which is faster temp table or table variable?

So table variable is faster then temporary table. ⇒ Temporary tables are allowed CREATE INDEXes whereas, Table variables aren't allowed CREATE INDEX instead they can have index by using Primary Key or Unique Constraint.

What is difference between temp table and TEMP variable in SQL Server?

The Name of a temp variable can have a maximum of 128 characters and a Temp Table can have 116 characters. Temp Tables and Temp Variables both support unique key, primary key, check constraints, Not null and default constraints but a Temp Variable doesn't support Foreign Keys.

Can we update CTE in SQL Server?

If your CTE is based on a single table then you can update using CTE, which in turn updates the underlying table.

How can I improve my query performance?

What is difference between views temp table and staging table?

The main difference between temporary tables and views is that temporary tables are just the tables in tempdb, but views are just stored queries for existing data in existing tables. So, there is no need to populate the view, because the data is already here.