Tag Archives: index

SQL INDEXING – Simplified

3 Apr SQL Indexing

Let us talk about indexing in SQL Server:

Depending on size of the data, a table can have more than a million rows. If you want to search for a specific record there are two ways:

  1. Go row by row and see if that is the row you want.
    Example: Let us say we want to search an employee table that has 1 million rows for an employee with last name “Smith”. Remember you can have more than one record with name “Smith”. One way to do this is to go record by record and check if last name is “Smith”. The problem with this approach is that this will be very slow as you have to go row by row. May be there is only one record with last name “Smith” and that could be the last record. In this case you will have to read one million times, only to find one record. This in SQL terms is called “TABLE SCAN”.
  2. The second approach is a better approach. What if I tell you that all names are sorted in “Alphabetical Order”. So, if you directly go to half a million record and if that name is say “Karsen” then you know “Smith” will be after “Karsen” (‘S’ comes after ‘K’). Now you can go directly to six hundred thousand record and if that name is “Tedd” then you know “Smith” is between half a million record and six hundred thousand record.  So now in two attempts you have narrowed it down from one million to hundred thousand. Isn’t that great? This is what is called as indexing.

Let us now get deep into details. Indexes are created on columns in tables. The index provides a fast way to look up data based on the values within those columns. An index is made up of a set of pages (index nodes) that are organized in a B-tree structure. Figure below shows how a typical B-tree looks like.

 

SQL Indexing Example
Fig.1 – B-tree.

 

Data is stored in the form of pages. Now let us try to find records with last name “Dull”.
Go to Root Page (level 1). With indexing now we know that any name starts with ‘Bennet’ and is before ‘Karsen’ we should go to page 1007. What this means is if we want to find out employees with name “Charlie” we should go to page 1007 and if we want to find out employees with name “Kyle’ we should go to page 1009 (“Ky” comes between “Ka” and “Sm”).

So, looking at page 1007 (level 0) we see that if name is between ‘Bennet’ and ‘Greane’ (‘D’ comes between ‘B’ and ‘G’) we need to go to page 1132. So, let us go there and find out.

Looking at page 1132 we can see record with name “Dull”. Hurray…. our search is over. We found our record. This is how indexing will work typically.

There are two types of indexes, Clustered index and Non Clustered Index. Clustered index is on primary key and data will be physically sorted. So, leaf level will have the actual data record. In a non-clustered index level will have a pointer to actual data page. This means you have to go one extra step to actually get your data. Remember indexes can make your search faster but insert, updates and delete will be slower.

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Written by Amit Uppal

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SQL Interview Questions

1 Apr

These are the most common yet tricky questions that you can expect in a .NET/SQL interview.

1)      What is the difference between truncate and Delete?

  1.  Delete will fire delete trigger, truncate will not.
  2. The DELETE statement removes rows one at a time and records an entry in the transaction log for each deleted row.TRUNCATE TABLE removes the data by deallocating the data pages used to store the table data and records only the page deallocations in the transaction log.Once you truncate you can’t rollback.
  3. If there is an identity column in the table truncate will reset identity to 1, delete will not.

2)      Name the different type of joins

  1. Inner Join
  2. Outer Join  (Left Outer Join and Right Outer Join)
  3. Cross join
  4. Self-Join

3)      What is an Index?

There are two types of indexes. Clustered index and Non Clustered Index. A table can have only 1 Clustered Index while it can have up to 249 & 999 nonclustered indexes on SQL Server 2005 & 2008 respectively. A clustered index stores the actual data rows at the leaf level of the index. Indexes can make Select statement fast but delete, update and insert statements will get slow.

4)      What are different types of Isolation levels in SQL Server

a)     READ UNCOMMITTED

b)      READ COMMITTED

c)       REPEATABLE READ

d)      SERIALIZABLE

e)       SNAPSHOT

 5)      What are different types of constraints in SQL Server?

  1.  Primary key Constraint
  2. Foreign key Constraint
  3. Check Constraint
  4. Unique Key Constraint

6)      What is the difference between where and having clause?

A having clause is typically used when you use group by clause.

Example:

SELECT titles.pub_id, AVG(titles.price)
FROM titles INNER JOIN publishers
ON titles.pub_id = publishers.pub_id
WHERE publishers.state = ‘CA’
GROUP BY titles.pub_id
HAVING AVG(titles.price) > 10

WHERE clause is applied first to the individual rows in the tables . Only the rows that meet the conditions in the WHERE clause are grouped. The HAVING clause is then applied to the rows in the result set

7)      How would you do “Error Handling” in SSIS?

SSIS package could mainly have two types of errors

a) Procedure Error: Can be handled in Control flow through the precedence control and redirecting the execution flow.

b) Data Error: is handled in DATA FLOW TASK buy redirecting the data flow using Error Output of a component.

8)      What is the difference between a primary key and a unique key?

Both primary key and unique enforce uniqueness of the column on which they are defined. But by default primary key creates a clustered index on the column. Major difference is that, primary key does not allow NULLs, but unique key allows NULL.

9)      What are defaults? Is there a column to which a default cannot be bound?

A default is a value that will be used by a column, if no value is supplied to that column while inserting data. IDENTITY columns and timestamp columns can’t have defaults bound to them.

You can create default and then bind a column to them.

Example:

//This will create default ” ZipCode ” in database

Create default ZipCode as “78746″

//This will bind the default we created to a column “PostalCode” in table “EmployeeData”
sp_bindefault ZipCode, “EmployeeData.PostalCode”
10)  What is SQL Blocking?
Blocking happens when one connection from an application holds a lock and a second connection requires a conflicting lock type. This forces the second connection to wait, blocked on the first.
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