What Is Nolock?
Using the NOLOCK query optimizer hint is generally considered good practice in order to improve concurrency on a busy system. When the NOLOCK hint is included in a SELECT statement, no locks are taken when data is read. The result is a Dirty Read, which means that another process could be updating the data at the exact time you are reading it. There are no guarantees that your query will retrieve the most recent data. The advantage to performance is that your reading of data will not block updates from taking place, and updates will not block your reading of data. SELECT statements take Shared (Read) locks. This means that multiple SELECT statements are allowed simultaneous access, but other processes are blocked from modifying the data. The updates will queue until all the reads have completed, and reads requested after the update will wait for the updates to complete. The result to your system is delay (blocking).
How Would You Handle Error In Sql Server 2008?
SQL Server now supports the use of TRY…CATCH constructs for providing rich error handling. TRY…CATCH lets us build error handling at the level we need, in the way we need to, by setting a region where if any error occurs, it will break out of the region and head to an error handler. The basic structure is as follows
BEGIN TRY
<code>
END TRY
BEGIN CATCH
<code>
END CATCH
So if any error occurs in the TRY block, execution is diverted to the CATCH block, and the error can be dealt.
What Is Raiseerror?
RaiseError generates an error message and initiates error processing for the session. RAISERROR can either reference a user-defined message stored in thesys.messages catalog view or build a message dynamically. The message is returned as a server error message to the calling application or to an associated CATCH block of a TRY…CATCH construct.
How To Rebuild Master Databse?
Master database is system database and it contains information about running server’s configuration. When SQL Server 2005 is installed it usually creates master, model, msdb, tempdb resource and distribution system database by default. Only Master database is the one which is absolutely must have database. Without Master database SQL Server cannot be started. This is the reason it is extremely important to backup Master database.
To rebuild the Master database, Run Setup.exe, verify, and repair a SQL Server instance, and rebuild the system databases. This procedure is most often used to rebuild the master database for a corrupted installation of SQL Server.
How To Find Tables Without Indexes?
Run following query in Query Editor.
USE <database_name>;
GO
SELECT SCHEMA_NAME(schema_id) AS schema_name
,name AS table_name
FROM sys.tables
WHERE OBJECTPROPERTY(OBJECT_ID,'IsIndexed') = 0
ORDER BY schema_name, table_name;
GO
How To Copy The Tables, Schema And Views From One Sql Server To Another?
There are multiple ways to do this.
1. “Detach Database” from one server and “Attach Database” to another server.
2. Manually script all the objects using SSMS and run the script on new server.
3. Use Wizard of SSMS.
How To Copy Data From One Table To Another Table?
There are multiple ways to do this.
1) INSERT INTO SELECT
This method is used when table is already created in the database earlier and data is to be inserted into this table from another table. If columns listed in insert clause and select clause are same, they are not required to list them.
2) SELECT INTO
This method is used when table is not created earlier and needs to be created when data from one table is to be inserted into newly created table from another table. New table is created with same data types as selected columns.
What Are Sparse Columns?
A sparse column is another tool used to reduce the amount of physical storage used in a database. They are the ordinary columns that have an optimized storage for null values. Sparse columns reduce the space requirements for null values at the cost of more overhead to retrieve nonnull values.
What Does Top Operator Do?
The TOP operator is used to specify the number of rows to be returned by a query. The TOP operator has new addition in SQL SERVER 2008 that it accepts variables as well as literal values and can be used with INSERT, UPDATE, and DELETES statements.
What Is Cte?
CTE is an abbreviation Common Table Expression. A Common Table Expression (CTE) is an expression that can be thought of as a temporary result set which is defined within the execution of a single SQL statement. A CTE is similar to a derived table in that it is not stored as an object and lasts only for the duration of the query.
What Is Merge Statement?
MERGE is a new feature that provides an efficient way to perform multiple DML operations. In previous versions of SQL Server, we had to write separate statements to INSERT, UPDATE, or DELETE data based on certain conditions, but now, using MERGE statement we can include the logic of such data modifications in one statement that even checks when the data is matched then just update it and when unmatched then insert it. One of the most important advantages of MERGE statement is all the data is read and processed only once.
What is HEAP table ?
A table with NO CLUSTERED INDEXES is called as HEAP table. The data rows of a heap table are not stored in any particular order or linked to the adjacent pages in the table. This unorganized structure of the heap table usually increases the overhead of accessing a large heap table, when compared to accessing a large nonheap table (a table with clustered index). So, prefer not to go with HEAP tables ..
What is ROW LOCATOR ?
If you define a NON CLUSTERED index on a table then the index row of a nonclustered index contains a pointer to the corresponding data row of the table. This pointer is called a row locator. The value of the row locator depends on whether the data pages are stored in a heap or are clustered. For a nonclustered index, the row locator is a pointer to the data row. For a table with a clustered index, the row locator is the clustered index key value.
What is Covering Index ?
A covering index is a nonclustered index built upon all the columns required to satisfy a SQL query without going to the base table. If a query encounters an index and does not need to refer to the underlying data table at all, then the index can be considered a covering index. For Example
Select col1,col2 from table
where col3 = Value
group by col4
order by col5
Now if you create a clustered index for all the columns used in Select statement then the SQL doesn’t need to go to base tables as everything required are available in index pages.
What is Indexed View ?
A database view in SQL Server is like a virtual table that represents the output of a SELECT statement. A view is created using the CREATE VIEW statement, and it can be queried exactly like a table. In general, a view doesn’t store any data—only the SELECT statement associated with it. Every time a view is queried, it further queries the underlying tables by executing its associated SELECT statement.
A database view can be materialized on the disk by creating a unique clustered index on the view. Such a view is referred to as an indexed view. After a unique clustered index is created on the view, the view’s result set is materialized immediately and persisted in physical storage in the database, saving the overhead of performing costly operations during query execution. After the view is materialized, multiple nonclustered indexes can be created on the indexed view.
What is Bookmark Lookup ?
When a SQL query requests a small number of rows, the optimizer can use the nonclustered index, if available, on the column(s) in the WHERE clause to retrieve the data. If the query refers to columns that are not part of the nonclustered index used to retrieve the data, then navigation is required from the index row to the corresponding data row in the table to access these columns.This operation is called a bookmark lookup.
What is the difference between Primary Key and Unique Key ?
Both the Primary Key(PK) and Unique Key(UK) are meant to provide Uniqueness to the Column on which they are defined. PFB the major differences between these two.
By default PK defines Clustered Index in the column where as UK defines Non Clustered Index.
PK doesn’t allow NULL Value where as UK allow ONLY ONE NULL.
You can have only one PK per table where as UK can be more than one per table.
PK can be used in Foreign Key relationships where as UK cannot be used.
What is the difference between Delete and Truncate ?
Both Delete and Truncate commands are meant to remove rows from a table. There are many differences between these two and pfb the same.
Truncate is Faster where as Delete is Slow process.
Truncate doesn’t log where as Delete logs an entry for every record deleted in Transaction Log.
We can rollback the Deleted data where as Truncated data cannot be rolled back.
Truncate resets the Identity column where as Delete doesn’t.
We can have WHERE Clause for delete where as for Truncate we cannot have WHERE Clause.
Delete Activates TRIGGER where as TRUNCATE Cannot.
Truncate is a DDL statement where as Delete is DML statement.
What are Indexes or Indices ?
An Index in SQL is similar to the Index in a book. Index of a book makes the reader to go to the desired page or topic easily and Index in SQL helps in retrieving the data faster from database. An Index is a seperate physical data structure that enables queries to pull the data fast. Indexes or Indices are used to improve the performance of a query.
What are Constraints or Define Constraints ?
Generally we use Data Types to limit the kind of Data in a Column. For example, if we declare any column with data type INT then ONLY Integer data can be inserted into the column. Constraint will help us to limit the Values we are passing into a column or a table. In simple Constraints are nothing but Rules or Conditions applied on columns or tables to restrict the data.
Different types of Constraints ?
There are THREE Types of Constraints.
Domain
Entity
Referential
Domain has the following constraints types –
Not Null
Check
Entity has the following constraint types –
Primary Key
Unique Key
Referential has the following constraint types –
Foreign Key
Types of Indices in SQL ?
There are TWO types of Indices in SQL server.
Clustered
Non Clustered
How many Clustered and Non Clustered Indexes can be defined for a table ?
Clustered – 1
Non Clustered – 999
What is Transaction in SQL Server ?
Transaction groups a set of T-Sql Statements into a single execution unit. Each transaction begins with a specific task and ends when all the tasks in the group successfully complete. If any of the tasks fails, the transaction fails. Therefore, atransaction has only two results: success or failure. Incomplete steps result in the failure of the transaction.by programmers to group together read and write operations. In Simple Either FULL or NULL i.e either all the statements executes successfully or all the execution will be rolled back.
Types of Transactions ?
There are TWO forms of Transactions.
Implicit – Specifies any Single Insert,Update or Delete statement as Transaction Unit. No need to specify Explicitly.
Explicit – A group of T-Sql statements with the beginning and ending marked with Begin Transaction,Commit and RollBack. PFB an Example for Explicit transactions.
BEGIN TRANSACTION
Update Employee Set Emp_ID = 54321 where Emp_ID = 12345
If(@@Error <>0)
ROLLBACK
Update LEave_Details Set Emp_ID = 54321 where Emp_ID = 12345
If(@@Error <>0)
ROLLBACK
COMMIT
In the above example we are trying to update an EMPLOYEE ID from 12345 to 54321 in both the master table “Employee” and Transaction table “Leave_Details”. In this case either BOTH the tables will be updated with new EMPID or NONE.
What is the Max size and Max number of columns for a row in a table ?
Size – 8060 Bytes
Columns – 1024
What is Normalization and Explain different normal forms.
Database normalization is a process of data design and organization which applies to data structures based on rules that help building relational databases.
1. Organizing data to minimize redundancy.
2. Isolate data so that additions, deletions, and modifications of a field can be made in just one table and then propagated through the rest of the database via the defined relationships.
1NF: Eliminate Repeating Groups
Each set of related attributes should be in separate table, and give each table a primary key. Each field contains at most one value from its attribute domain.
2NF: Eliminate Redundant Data
1. Table must be in 1NF.
2. All fields are dependent on the whole of the primary key, or a relation is in 2NF if it is in 1NF and every non-key attribute is fully dependent on each candidate key of the relation. If an attribute depends on only part of a multi‐valued key, remove it to a separate table.
3NF: Eliminate Columns Not Dependent On Key
1. The table must be in 2NF.
2. Transitive dependencies must be eliminated. All attributes must rely only on the primary key. If attributes do not contribute to a description of the key, remove them to a separate table. All attributes must be directly dependent on the primary key.
BCNF: Boyce‐Codd Normal Form
for every one of its non-trivial functional dependencies X → Y, X is a superkey—that is, X is either a candidate key or a superset thereof. If there are non‐trivial dependencies between candidate key attributes, separate them out into distinct tables.
4NF: Isolate Independent Multiple Relationships
No table may contain two or more 1:n or n:m relationships that are not directly related.
For example, if you can have two phone numbers values and two email address values, then you should not have them in the same table.
5NF: Isolate Semantically Related Multiple Relationships
A 4NF table is said to be in the 5NF if and only if every join dependency in it is implied by the candidate keys. There may be practical constrains on information that justify separating logically related many‐to‐many relationships.
Using the NOLOCK query optimizer hint is generally considered good practice in order to improve concurrency on a busy system. When the NOLOCK hint is included in a SELECT statement, no locks are taken when data is read. The result is a Dirty Read, which means that another process could be updating the data at the exact time you are reading it. There are no guarantees that your query will retrieve the most recent data. The advantage to performance is that your reading of data will not block updates from taking place, and updates will not block your reading of data. SELECT statements take Shared (Read) locks. This means that multiple SELECT statements are allowed simultaneous access, but other processes are blocked from modifying the data. The updates will queue until all the reads have completed, and reads requested after the update will wait for the updates to complete. The result to your system is delay (blocking).
How Would You Handle Error In Sql Server 2008?
SQL Server now supports the use of TRY…CATCH constructs for providing rich error handling. TRY…CATCH lets us build error handling at the level we need, in the way we need to, by setting a region where if any error occurs, it will break out of the region and head to an error handler. The basic structure is as follows
BEGIN TRY
<code>
END TRY
BEGIN CATCH
<code>
END CATCH
So if any error occurs in the TRY block, execution is diverted to the CATCH block, and the error can be dealt.
What Is Raiseerror?
RaiseError generates an error message and initiates error processing for the session. RAISERROR can either reference a user-defined message stored in thesys.messages catalog view or build a message dynamically. The message is returned as a server error message to the calling application or to an associated CATCH block of a TRY…CATCH construct.
 |
| RELX Group Frequently Asked SQL Server Interview Questions Answers |
How To Rebuild Master Databse?
Master database is system database and it contains information about running server’s configuration. When SQL Server 2005 is installed it usually creates master, model, msdb, tempdb resource and distribution system database by default. Only Master database is the one which is absolutely must have database. Without Master database SQL Server cannot be started. This is the reason it is extremely important to backup Master database.
To rebuild the Master database, Run Setup.exe, verify, and repair a SQL Server instance, and rebuild the system databases. This procedure is most often used to rebuild the master database for a corrupted installation of SQL Server.
How To Find Tables Without Indexes?
Run following query in Query Editor.
USE <database_name>;
GO
SELECT SCHEMA_NAME(schema_id) AS schema_name
,name AS table_name
FROM sys.tables
WHERE OBJECTPROPERTY(OBJECT_ID,'IsIndexed') = 0
ORDER BY schema_name, table_name;
GO
How To Copy The Tables, Schema And Views From One Sql Server To Another?
There are multiple ways to do this.
1. “Detach Database” from one server and “Attach Database” to another server.
2. Manually script all the objects using SSMS and run the script on new server.
3. Use Wizard of SSMS.
How To Copy Data From One Table To Another Table?
There are multiple ways to do this.
1) INSERT INTO SELECT
This method is used when table is already created in the database earlier and data is to be inserted into this table from another table. If columns listed in insert clause and select clause are same, they are not required to list them.
2) SELECT INTO
This method is used when table is not created earlier and needs to be created when data from one table is to be inserted into newly created table from another table. New table is created with same data types as selected columns.
What Are Sparse Columns?
A sparse column is another tool used to reduce the amount of physical storage used in a database. They are the ordinary columns that have an optimized storage for null values. Sparse columns reduce the space requirements for null values at the cost of more overhead to retrieve nonnull values.
What Does Top Operator Do?
The TOP operator is used to specify the number of rows to be returned by a query. The TOP operator has new addition in SQL SERVER 2008 that it accepts variables as well as literal values and can be used with INSERT, UPDATE, and DELETES statements.
What Is Cte?
CTE is an abbreviation Common Table Expression. A Common Table Expression (CTE) is an expression that can be thought of as a temporary result set which is defined within the execution of a single SQL statement. A CTE is similar to a derived table in that it is not stored as an object and lasts only for the duration of the query.
What Is Merge Statement?
MERGE is a new feature that provides an efficient way to perform multiple DML operations. In previous versions of SQL Server, we had to write separate statements to INSERT, UPDATE, or DELETE data based on certain conditions, but now, using MERGE statement we can include the logic of such data modifications in one statement that even checks when the data is matched then just update it and when unmatched then insert it. One of the most important advantages of MERGE statement is all the data is read and processed only once.
What is HEAP table ?
A table with NO CLUSTERED INDEXES is called as HEAP table. The data rows of a heap table are not stored in any particular order or linked to the adjacent pages in the table. This unorganized structure of the heap table usually increases the overhead of accessing a large heap table, when compared to accessing a large nonheap table (a table with clustered index). So, prefer not to go with HEAP tables ..
What is ROW LOCATOR ?
If you define a NON CLUSTERED index on a table then the index row of a nonclustered index contains a pointer to the corresponding data row of the table. This pointer is called a row locator. The value of the row locator depends on whether the data pages are stored in a heap or are clustered. For a nonclustered index, the row locator is a pointer to the data row. For a table with a clustered index, the row locator is the clustered index key value.
What is Covering Index ?
A covering index is a nonclustered index built upon all the columns required to satisfy a SQL query without going to the base table. If a query encounters an index and does not need to refer to the underlying data table at all, then the index can be considered a covering index. For Example
Select col1,col2 from table
where col3 = Value
group by col4
order by col5
Now if you create a clustered index for all the columns used in Select statement then the SQL doesn’t need to go to base tables as everything required are available in index pages.
What is Indexed View ?
A database view in SQL Server is like a virtual table that represents the output of a SELECT statement. A view is created using the CREATE VIEW statement, and it can be queried exactly like a table. In general, a view doesn’t store any data—only the SELECT statement associated with it. Every time a view is queried, it further queries the underlying tables by executing its associated SELECT statement.
A database view can be materialized on the disk by creating a unique clustered index on the view. Such a view is referred to as an indexed view. After a unique clustered index is created on the view, the view’s result set is materialized immediately and persisted in physical storage in the database, saving the overhead of performing costly operations during query execution. After the view is materialized, multiple nonclustered indexes can be created on the indexed view.
What is Bookmark Lookup ?
When a SQL query requests a small number of rows, the optimizer can use the nonclustered index, if available, on the column(s) in the WHERE clause to retrieve the data. If the query refers to columns that are not part of the nonclustered index used to retrieve the data, then navigation is required from the index row to the corresponding data row in the table to access these columns.This operation is called a bookmark lookup.
What is the difference between Primary Key and Unique Key ?
Both the Primary Key(PK) and Unique Key(UK) are meant to provide Uniqueness to the Column on which they are defined. PFB the major differences between these two.
By default PK defines Clustered Index in the column where as UK defines Non Clustered Index.
PK doesn’t allow NULL Value where as UK allow ONLY ONE NULL.
You can have only one PK per table where as UK can be more than one per table.
PK can be used in Foreign Key relationships where as UK cannot be used.
What is the difference between Delete and Truncate ?
Both Delete and Truncate commands are meant to remove rows from a table. There are many differences between these two and pfb the same.
Truncate is Faster where as Delete is Slow process.
Truncate doesn’t log where as Delete logs an entry for every record deleted in Transaction Log.
We can rollback the Deleted data where as Truncated data cannot be rolled back.
Truncate resets the Identity column where as Delete doesn’t.
We can have WHERE Clause for delete where as for Truncate we cannot have WHERE Clause.
Delete Activates TRIGGER where as TRUNCATE Cannot.
Truncate is a DDL statement where as Delete is DML statement.
What are Indexes or Indices ?
An Index in SQL is similar to the Index in a book. Index of a book makes the reader to go to the desired page or topic easily and Index in SQL helps in retrieving the data faster from database. An Index is a seperate physical data structure that enables queries to pull the data fast. Indexes or Indices are used to improve the performance of a query.
What are Constraints or Define Constraints ?
Generally we use Data Types to limit the kind of Data in a Column. For example, if we declare any column with data type INT then ONLY Integer data can be inserted into the column. Constraint will help us to limit the Values we are passing into a column or a table. In simple Constraints are nothing but Rules or Conditions applied on columns or tables to restrict the data.
Different types of Constraints ?
There are THREE Types of Constraints.
Domain
Entity
Referential
Domain has the following constraints types –
Not Null
Check
Entity has the following constraint types –
Primary Key
Unique Key
Referential has the following constraint types –
Foreign Key
Types of Indices in SQL ?
There are TWO types of Indices in SQL server.
Clustered
Non Clustered
How many Clustered and Non Clustered Indexes can be defined for a table ?
Clustered – 1
Non Clustered – 999
What is Transaction in SQL Server ?
Transaction groups a set of T-Sql Statements into a single execution unit. Each transaction begins with a specific task and ends when all the tasks in the group successfully complete. If any of the tasks fails, the transaction fails. Therefore, atransaction has only two results: success or failure. Incomplete steps result in the failure of the transaction.by programmers to group together read and write operations. In Simple Either FULL or NULL i.e either all the statements executes successfully or all the execution will be rolled back.
Types of Transactions ?
There are TWO forms of Transactions.
Implicit – Specifies any Single Insert,Update or Delete statement as Transaction Unit. No need to specify Explicitly.
Explicit – A group of T-Sql statements with the beginning and ending marked with Begin Transaction,Commit and RollBack. PFB an Example for Explicit transactions.
BEGIN TRANSACTION
Update Employee Set Emp_ID = 54321 where Emp_ID = 12345
If(@@Error <>0)
ROLLBACK
Update LEave_Details Set Emp_ID = 54321 where Emp_ID = 12345
If(@@Error <>0)
ROLLBACK
COMMIT
In the above example we are trying to update an EMPLOYEE ID from 12345 to 54321 in both the master table “Employee” and Transaction table “Leave_Details”. In this case either BOTH the tables will be updated with new EMPID or NONE.
What is the Max size and Max number of columns for a row in a table ?
Size – 8060 Bytes
Columns – 1024
What is Normalization and Explain different normal forms.
Database normalization is a process of data design and organization which applies to data structures based on rules that help building relational databases.
1. Organizing data to minimize redundancy.
2. Isolate data so that additions, deletions, and modifications of a field can be made in just one table and then propagated through the rest of the database via the defined relationships.
1NF: Eliminate Repeating Groups
Each set of related attributes should be in separate table, and give each table a primary key. Each field contains at most one value from its attribute domain.
2NF: Eliminate Redundant Data
1. Table must be in 1NF.
2. All fields are dependent on the whole of the primary key, or a relation is in 2NF if it is in 1NF and every non-key attribute is fully dependent on each candidate key of the relation. If an attribute depends on only part of a multi‐valued key, remove it to a separate table.
3NF: Eliminate Columns Not Dependent On Key
1. The table must be in 2NF.
2. Transitive dependencies must be eliminated. All attributes must rely only on the primary key. If attributes do not contribute to a description of the key, remove them to a separate table. All attributes must be directly dependent on the primary key.
BCNF: Boyce‐Codd Normal Form
for every one of its non-trivial functional dependencies X → Y, X is a superkey—that is, X is either a candidate key or a superset thereof. If there are non‐trivial dependencies between candidate key attributes, separate them out into distinct tables.
4NF: Isolate Independent Multiple Relationships
No table may contain two or more 1:n or n:m relationships that are not directly related.
For example, if you can have two phone numbers values and two email address values, then you should not have them in the same table.
5NF: Isolate Semantically Related Multiple Relationships
A 4NF table is said to be in the 5NF if and only if every join dependency in it is implied by the candidate keys. There may be practical constrains on information that justify separating logically related many‐to‐many relationships.