Thursday, 20 April 2017

CTE and Temp Table and Table Variable - SQL SERVER

Temp tables

Behave just like normal tables, but are created in the TempDB database. They persist until dropped, or until the connection that created them disappears. They are visible in the procedure that created them and any procedures that that proc calls.

Just like normal tables, they can have primary keys, constraints and indexes, and column statistics are kept for the table. 

Temp tables, while they have space assigned to them in the tempDB database, will generally be accessed only from memory, unless the server is under memory pressure, or the amount of data in the table is large.



CREATE TABLE #LocalTemp
(
 UserID int,
 Name varchar(50), 
 Address varchar(150)
)
GO
insert into #LocalTemp values ( 1, 'Shailendra','Noida');
GO
Select * from #LocalTemp

Table Variables

These tables behave very much like other variables in their scoping rules. They are created when they are declared and are dropped when they go out of scope. They cannot be explicitly dropped.

Like with temp tables, table variables reside in TempDB. they have entries in the system tables in tempDB, just like temp tables, and they follow the same behaviour regarding whether they are in memory or on disk.

Table variables can have a primary key, but indexes cannot be created on them, neither are statistics maintained on the columns. This makes table variables less optimal for large numbers of rows, as the optimiser has no way of knowing the number of rows in the table variable.


 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 BY ProductID ASC 
 --Select data
 Select * from @TProduct
 
 --Next batch
 GO
 Select * from @TProduct --gives error in next batch

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.



;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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