Difference between CTE and Temp Table and Table Variable

Temp Table or Table variable or CTE are commonly used for storing data temporarily in SQL Server. In this blog, we will see the differences among these three.

CTE

CTE stands for Common Table expressions. It was introduced with SQL Server 2005. It is a temporary result set and typically it may be a result of complex sub-query. Unlike temporary table its life is limited to the current query. It is defined by using WITH statement. CTE improves readability and ease in maintenance of complex queries and sub-queries. Always begin CTE with semicolon.

A sub query without CTE is given below:

SELECT * FROM (

            SELECT Addr.Address, Emp.Name, Emp.Age From Address Addr

            Inner join Employee Emp on Emp.EID = Addr.EID) Temp

            WHERE Temp.Age > 50

            ORDER BY Temp.NAME

By using CTE above query can be re-written as follows

;With CTE1(Address, Name, Age)--Column names for CTE, which are optional

            AS

            (

                        SELECT Addr.Address, Emp.Name, Emp.Age from Address Addr

                        INNER JOIN EMP Emp ON Emp.EID = Addr.EID

            )

            SELECT * FROM CTE1 --Using CTE

            WHERE CTE1.Age > 50

            ORDER BY CTE1.NAME

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When to use CTE

1. This is used to store result of a complex sub query for further use.
2. This is also used to create a recursive query.

Temporary Tables

In SQL Server, temporary tables are created at run-time and you can do all the operations which you can do on a normal table. These tables are created inside Tempdb database. Based on the scope and behavior temporary tables are of two types as given below-

Local Temp Table

Local temp tables are only available to the SQL Server session or connection (means single user) that created the tables. These are automatically deleted when the session that created the tables has been closed. Local temporary table name is stared with single hash ("#") sign.

 CREATE TABLE #LocalTemp

                        (

                                    UserID int,

                                    Name varchar(50),

                                    Address varchar(150)

                        )

                        GO

                        insert into #LocalTemp values ( 1, 'Akash', 'Mumbai');

                        GO

                        Select * from #LocalTemp

Global Temp Table

Global temp tables are available to all SQL Server sessions or connections (means all the user). These can be created by any SQL Server connection user and these are  automatically deleted when all the SQL Server connections have been closed. Global temporary table name is stared with double hash ("##") sign.

CREATE TABLE ##GlobalTemp

                        (

                                    UserID int,

                                    Name varchar(50),

                                    Address varchar(150)

                        )

                        GO

                        insert into ##GlobalTemp values ( 1, 'Akash','Mumbai');

                        GO

                        Select * from ##GlobalTemp

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Table Variable

This acts like a variable and exists for a particular batch of query execution. It gets dropped once  it comes out of batch. This is also created in the Tempdb database but not the memory. This also allows you to create primary key, identity at the time of Table variable declaration but not non-clustered index.

GO

DECLARE @TProduct TABLE

(

SNo INT IDENTITY(1,1),

ProductID INT,

Qty INT

)

--Insert data to Table variable @Product

INSERT INTO @TProduct(ProductID,Qty)

SELECT DISTINCT ProductID, Qty FROM ProductsSales ORDER BYProductID ASC

--Select data

Select * from @TProduct

 

--Next batch

GO

Select * from @TProduct --gives error in next batch

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Note:

1. Temp Tables are physically created in the "Tempdb" database. These tables act as the normal table and also can have constraints, index like normal tables.
2. CTE is a named temporary result set which is used to manipulate the complex sub-queries data.This exists for the scope of statement. This is created in memory rather than "Tempdb" database.You cannot create any index on CTE.
3. Table Variable acts like a variable and exists for a particular batch of query execution. It gets dropped once it comes out of batch. This is also created in the Tempdb database but not the memory.

About Author:
Akash Verma is budding technology geek, who actively contributes to Systems Plus with his creativity and research on technology. He can be contacted at akash.v@spluspl.com