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.net & SQL Samples, programming tips and tricks, performance tips, guidelines, and best practices

Friday, 30 December 2011

Happy New Year !

Wishing all my followers a very happy & prosperous new year !

Monday, 26 December 2011

Dynamic Sorting in SSRS

Introduction
Many times while developing any report we come across a situation where we need to SORT the TABLE columns or the columns in the GROUP dynamically. There are many well known ways to achieve this. However, here I will demonstrate a not-so-known way -
 Implementation
Assuming that the sorting is to be done on 3 columns – Col1,Col2 & Col3. But the ORDER is undecided until runtime. Now, what we can do is -
  1. Define 3 Report parameters. Say - @SortCol1Name,@SortCol2Name & @SortCol3Name of STRING type.
  2. Pass the name of the columns which need to sorted in the required order. Say -
    • @SortCol1Name = “ColumnX”,@SortCol2Name = “ColumnY” and @SortCol3Name = “ColumnZ”
    • OR @SortCol1Name = “ColumnY”,@SortCol2Name = “ColumnZ” and @SortCol3Name = “ColumnX”
    • OR any preferred order
  3. In the Sorting Option of the Properties dialog box of Table or Group, set the sort expression as under -
    • =Fields(Parameters!SortCol1Name.Value).Value
    • =Fields(Parameters!SortCol2Name.Value).Value
    • =Fields(Parameters!SortCol3Name.Value).Value
1
This finally gets converted to –Fields!ColumnX.Value,Fields!ColumnY.Value & Fields!ColumnZ.Value if we have the passed the following values for the parameters - @SortCol1Name = “ColumnX”,@SortCol2Name = “ColumnY” and @SortCol3Name = “ColumnZ”
Conclusion
With this trick, we can easily set the sort expression dynamically. However, there is a limitation that we can not set the Sort Direction dynamically. I could not find out a way for it. If you have any trick to get it done with this, please leave your suggestion as a comment to this post.

Wednesday, 7 December 2011

How many times the T-SQL inside the CTE is executed?

Introduction

Common Table Expression (CTE) has become very popular these days. However, many users of CTE still have a myth that the t-sql written inside the CTE will be executed only once irrespective of the number of times it is referred in the subsequent CTEs or the related query.
However, this is not correct. It is a pure misconception. And let’s try to prove this with a simple example.

Proof
The Query
;WITH CTE1 AS (
SELECT
* 
FROM
dbo.Employees
),CTE2 AS (
SELECT
*
FROM
CTE1
)
SELECT
C1.EmployeeID,
C1.Name,
C2.EmployeeID,
C2.Name,
C3.EmployeeID,
C3.Name
FROM
CTE1 C1
INNER JOIN CTE2 C2
ON C2.EmployeeID = C1.EmployeeID
INNER JOIN CTE1 C3
ON C3.EmployeeID = C1.EmployeeID

Execution Plan

CTE

Please have a look at the execution plan. It clearly shows that even though the t-sql to fetch the data from the table is written only once inside CTE1, the Scan is done thrice. This is because CTE1 is referred thrice in the final query -
  1. Two times CTE1 is directly used.
  2. Once CTE1 is used indirectly via CTE2.
Conclusion
The CTE should be thought of a view that is defined for the current query only. At the time of execution, the query optimizer will replace all the direct/indirect use of CTE with the actual query in the same way as it does for Views.