Month: August 2022

DECLARE @StartDate DATETIME
DECLARE @EndDate DATETIME
SET @StartDate = ‘2022/08/01’
SET @EndDate = ‘2022/08/25’

SELECT
(DATEDIFF(dd, @StartDate, @EndDate) + 1)
-(DATEDIFF(wk, @StartDate, @EndDate) * 2)
-(CASE WHEN DATENAME(dw, @StartDate) = ‘Sunday’ THEN 1 ELSE 0 END)
-(CASE WHEN DATENAME(dw, @EndDate) = ‘Saturday’ THEN 1 ELSE 0 END)

In the data world, duplicated data is a very common issue. It may duplicated for few fields or even worse you have more then two identical rows in the table and there is no way to distinguish those rows. So how to clean those duplicated data?

DELETE Duplicate Records Using TOP

Since SQL Server 2005 we can use the TOP command when we issue the delete, such as the following.

CREATE TABLE dbo.duplicateTest (
	[id] [INT] NULL,
	[name] [VARCHAR](100) 
)

INSERT INTO dbo.duplicateTest VALUES (1, 'a')
INSERT INTO dbo.duplicateTest VALUES (1, 'a')
INSERT INTO dbo.duplicateTest VALUES (1, 'a')
INSERT INTO dbo.duplicateTest VALUES (2, 'b')
INSERT INTO dbo.duplicateTest VALUES (3, 'c')

DECLARE @id int
WHILE EXISTS (SELECT COUNT(id) FROM dbo.duplicateTest  GROUP BY id HAVING COUNT(id) > 1 ) 
BEGIN
	SELECT TOP 1  @id = id
	FROM dbo.duplicateTest
	GROUP BY id 
	HAVING COUNT(id) > 1 

	DELETE TOP(1) FROM dbo.duplicateTest WHERE id =@id 
END

So as you can see using the top in while loop we can remove multiple duplicated records even you are not sure which id is duplicated.

DELETE Duplicate Records Using CTE

another way we can use to remove duplicated records is by using CTE.

TRUNCATE TABLE dbo.duplicateTest
INSERT INTO dbo.duplicateTest VALUES(1, 'Bob') 
INSERT INTO dbo.duplicateTest VALUES(2, 'Dave') 
INSERT INTO dbo.duplicateTest VALUES(3, 'Karen') 
INSERT INTO dbo.duplicateTest VALUES(1, 'Bob')
INSERT INTO dbo.duplicateTest VALUES(1, 'Bob')	


WITH cte1
AS (
   SELECT ROW_NUMBER() OVER ( PARTITION BY ID ORDER BY ID) rank1,*
   FROM dbo.duplicateTest 
)
DELETE FROM cte1 WHERE rank1 > 1

Index is used to speed-up query performance in SQL Server. It is similar to book or dictionary indexes. In the book, if you are looking for some particular chapter, you can looking the index and get the page number of the chapter then go directly to that page. Without index, it will take long time for you to find the desired chapter.

The similar logic for the database index. Well-designed indexes can reduce disk I/O operations and consume fewer system resources therefore improving query performance. In SQL Server table can have two types of indexes:

• Clustered Indexes
  Clustered index is as same as dictionary where the data is arranged by alphabetical order. The order clustered indexes stored the table or view data rows in the table is the order of key values order. Each table can have 1 clustered indexes only, because it can have one kind of order in the same. In SQL Server, the primary key constraint automatically creates a clustered index on that particular column.
  
  
  Example of clustered index

CREATE DATABASE schooldb
CREATE TABLE student
(
id INT PRIMARY KEY,
name VARCHAR(50) NOT NULL,
gender VARCHAR(50) NOT NULL,
DOB datetime NOT NULL,
total_score INT NOT NULL,
city VARCHAR(50) NOT NULL
)

The primary key of column ‘id’ will become clustered index automatically. You use system procedure “sp_helpindex” to see the indexes of the table.

USE schooldb
EXECUTE sp_helpindex student

Another way to view table indexes is by going to “Object Explorer-> Databases-> Database_Name-> Tables-> Table_Name -> Indexes”.

Non-clustered Indexes
Non-Clustered Index is similar to the index of textbook. The index of textbook listed the chapter name and related page number, you can directly go to the page by using the page number of the chapter if you are looking for some particular one.

The data and index are stored in different place. Therefore you can have multiple non-clustered index for each table, all the index will point to the same storage place.

To create a new clustered Index, execute the following script:

use schooldb
CREATE CLUSTERED INDEX IX_tblStudent_Gender_Score
ON student(gender ASC, total_score DESC)

Differences

differences between clustered and non-clustered indexes.

1. Can only have one clustered index per table. However, We can create multiple non-clustered indexes on the same table.
2. Clustered indexes only sort tables. Therefore, they do not consume extra storage. Non-clustered indexes are stored in a separate place from the actual table claiming more storage space.
3. Clustered indexes are faster than non-clustered indexes since they don’t involve any extra lookup step.
4. In clustered index, the clustered key defines order of data within table. In non-clustered index, the index key defines the order of data in the index only.