Sql Question 2

2) Common Questions Asked

Which TCP/IP port does the SQL Server run on? How can it be Changed?

SQL Server runs on port 1433. It can be changed from the Network Utility TCP/IP properties –> Port number, both on client and the server.

What are the Difference between Clustered and a Non-clustered Index?

A clustered index is a special type of index that reorders the way records in the table are physically stored. Therefore, the table can have only one clustered index. The leaf nodes of a clustered index contain the data pages.

A non-clustered index is a special type of index in which the logical order of the index does not match the physical stored order of the rows on disk. The leaf node of a non-clustered index does not consist of the data pages. Instead, the leaf nodes contain index rows. (Read more here)

What are the Different Index Configurations a Table can have?

A table can have one of the following index configurations:

• No indexes
• A clustered index
• A clustered index and many non-clustered indexes
• A non-clustered index
• Many non-clustered indexes

What are Different Types of Collation Sensitivity?

Case sensitivity – A and a, B and b, etc.

Accent sensitivity – a and á, o and ó, etc.

Kana Sensitivity – When Japanese Kana characters Hiragana and Katakana are treated differently, it is called Kana sensitive.

Width sensitivity – When a single-byte character (half-width) and the same character represented as a double-byte character (full-width) are treated differently, it is width sensitive. (Read more here)

What is OLTP (Online Transaction Processing)?

In OLTP –(online transaction processing) systems, relational database design uses the discipline of data modeling and generally follows the Codd rules of data normalization in order to ensure absolute data integrity. Using these rules, complex information is broken down into its most simple structures (a table) where all of the individual atomic level elements relate to each other and satisfy the normalization rules.

What’s the Difference between a Primary Key and a Unique Key?

Both primary key and unique key enforce uniqueness of the column on which they are defined. But by default, the primary key creates a clustered index on the column, whereas unique key creates a non-clustered index by default. Another major difference is that primary key doesn’t allow NULLs, but unique key allows one NULL only. (Read more here)

What is Difference between DELETE and TRUNCATE Commands?

Delete command removes the rows from a table on the basis of the condition that we provide with a WHERE clause. Truncate will actually remove all the rows from a table, and there will be no data in the table after we run the truncate command.

TRUNCATE

• TRUNCATE is faster and uses fewer system and transaction log resources than DELETE. (Read all the points below)
• TRUNCATE removes the data by deallocating the data pages used to store the table’s data, and only the page deallocations are recorded in the transaction log.
• TRUNCATE removes all the rows from a table, but the table structure, its columns, constraints, indexes and so on remains. The counter used by an identity for new rows is reset to the seed for the column.
• You cannot use TRUNCATE TABLE on a table referenced by a FOREIGN KEY constraint.
• Using T-SQL – TRUNCATE cannot be rolled back unless it is used in TRANSACTION. OR TRUNCATE can be rolled back when used with BEGIN … END TRANSACTION using T-SQL.
• TRUNCATE is a DDL Command.
• TRUNCATE resets the identity of the table.

DELETE

• DELETE removes rows one at a time and records an entry in the transaction log for each deleted row.
• DELETE does not reset Identity property of the table.
• DELETE can be used with or without a WHERE clause
• DELETE activates Triggers if defined on table.
• DELETE can be rolled back.
• DELETE is DML Command.
• DELETE does not reset the identity of the table.

What are Different Types of Locks?

• Shared Locks: Used for operations that do not change or update data (read-only operations), such as a SELECT statement.
• Update Locks: Used on resources that can be updated. It prevents a common form of deadlock that occurs when multiple sessions are reading, locking, and potentially updating resources later.
• Exclusive Locks: Used for data-modification operations, such as INSERT, UPDATE, or DELETE. It ensures that multiple updates cannot be made to the same resource at the same time.
• Intent Locks: Used to establish a lock hierarchy. The types of intent locks are as follows: intent shared (IS), intent exclusive (IX), and shared with intent exclusive (SIX).
• Schema Locks: Used when an operation dependent on the schema of a table is executing. The types of schema locks are schema modification (Sch-M) and schema stability (Sch-S).
• Bulk Update Locks: Used when bulk-copying data into a table and the TABLOCK hint is specified.

What are Pessimistic Lock and Optimistic Lock?

Optimistic Locking is a strategy where you read a record, take note of a version number and check that the version hasn’t changed before you write the record back. If the record is dirty (i.e. different version to yours), then you abort the transaction and the user can re-start it.

Pessimistic Locking is when you lock the record for your exclusive use until you have finished with it. It has much better integrity than optimistic locking but requires you to be careful with your application design to avoid Deadlocks.

When is the use of UPDATE_STATISTICS command?

This command is basically used when a large amount of data is processed. If a large amount of deletions, modifications or Bulk Copy into the tables has occurred, it has to update the indexes to take these changes into account. UPDATE_STATISTICS updates the indexes on these tables accordingly.

What is the Difference between a HAVING clause and a WHERE clause?

They specify a search condition for a group or an aggregate. But the difference is that HAVING can be used only with the SELECT statement. HAVING is typically used in a GROUP BY clause. When GROUP BY is not used, HAVING behaves like a WHERE clause. Having Clause is basically used only with the GROUP BY function in a query, whereas WHERE Clause is applied to each row before they are part of the GROUP BY function in a query. (Read more here)

What is Connection Pooling and why it is Used?

To minimize the cost of opening and closing connections, ADO.NET uses an optimization technique called connection pooling.

The pooler maintains ownership of the physical connection. It manages connections by keeping alive a set of active connections for each given connection configuration. Whenever a user calls Open on a connection, the pooler looks for an available connection in the pool. If a pooled connection is available, it returns it to the caller instead of opening a new connection. When the application calls Close on the connection, the pooler returns it to the pooled set of active connections instead of closing it. Once the connection is returned to the pool, it is ready to be reused on the next Open call.

What are the Properties and Different Types of Sub-Queries?

Properties of a Sub-Query

• A sub-query must be enclosed in the parenthesis.
• A sub-query must be put on the right hand of the comparison operator, and
• A sub-query cannot contain an ORDER BY clause, however sub-query can use ORDER BY when used with TOP clause. Read Comment by David Bridge
  
• A query can contain more than one sub-query.

Types of Sub-query

• Single-row sub-query, where the sub-query returns only one row.
• Multiple-row sub-query, where the sub-query returns multiple rows, and
• Multiple column sub-query, where the sub-query returns multiple columns

What is an SQL Profiler?

SQL Profiler is a graphical tool that allows system administrators to monitor events in an instance of Microsoft SQL Server. You can capture and save data about each event to a file or SQL Server table to analyze later. For example, you can monitor a production environment to see which stored procedures are hampering performances by executing very slowly.

Use SQL Profiler to monitor only the events in which you are interested. If traces are becoming too large, you can filter them based on the information you want, so that only a subset of the event data is collected. Monitoring too many events adds overhead to the server and the monitoring process and can cause the trace file or trace table to grow very large, especially when the monitoring process takes place over a long period of time.

What are the Authentication Modes in SQL Server? How can it be Changed?

There are two authentication modes in SQL Server.

• Windows Mode
• Mixed Mode – SQL and Windows

To change authentication mode in SQL Server, go to Start -> Programs- > Microsoft SQL Server and click SQL Enterprise Manager to run SQL Enterprise Manager from the Microsoft SQL Server program group. Select the server; then from the Tools menu, select SQL Server Configuration Properties and choose the Security page.

What is an SQL Server Agent?

The SQL Server agent plays an important role in the day-to-day tasks of a database administrator (DBA). It is often overlooked as one of the main tools for SQL Server management. Its purpose is to ease the implementation of tasks for the DBA, with its full-function scheduling engine, which allows you to schedule your own jobs and scripts. (Read more here)

Can a Stored Procedure call itself or a Recursive Stored Procedure? How many levels of SP nesting is possible?

Yes. As T-SQL supports recursion, you can write stored procedures that call themselves. Recursion can be defined as a method of problem solving wherein the solution is arrived at by repetitively applying it to subsets of the problem. A common application of recursive logic is to perform numeric computations that lend themselves to repetitive evaluation by the same processing steps. Stored procedures are nested when one stored procedure calls another or executes managed code by referencing a CLR routine, type, or aggregate. You can nest stored procedures up to 32 levels. Any reference to managed code from a Transact-SQL stored procedure counts as one level against the 32-level nesting limit. Methods invoked from within managed code do not count against this limit. (Read more here) (Courtesy: Vinod Kumar)

What is Log Shipping?

Log shipping is the process of automating the backup of database and transaction log files on a production SQL server and then restoring them onto a standby server. All Editions (except Express Edition) supports log shipping. In log shipping, the transactional log file from one server is automatically updated into the backup database on the other server. If one server fails, the other server will have the same db and can be used this as the Disaster Recovery plan. The key feature of log shipping is that it will automatically backup transaction logs throughout the day and automatically restore them on the standby server at defined intervals. (Courtney: Rhys)

Name 3 ways to get an Accurate Count of the Number of Records in a Table?

SELECT * FROM table1

SELECT COUNT(*) FROM table1

SELECT rows FROM sysindexes WHERE id = OBJECT_ID(table1) AND indid < 2


What does it mean to have QUOTED_IDENTIFIER ON? What are the Implications of having it OFF?

When SET QUOTED_IDENTIFIER is ON, identifiers can be delimited by double quotation marks, and literals must be delimited by single quotation marks. When SET QUOTED_IDENTIFIER is OFF, identifiers cannot be quoted and must follow all T-SQL rules for identifiers. (Read more here)

What is the Difference between a Local and a Global Temporary Table?

A local temporary table exists only for the duration of a connection, or if defined inside a compound statement, for the duration of the compound statement.

A global temporary table remains in the database accessible across the connections. Once the connection where original global table is declared dropped this becomes unavailable.

What is the STUFF Function and How Does it Differ from the REPLACE Function?

STUFF function is used to overwrite existing characters using this syntax: STUFF (string_expression, start, length, replacement_characters), where string_expression is the string that will have characters substituted, start is the starting position, length is the number of characters in the string that are substituted, and replacement_characters are the new characters interjected into the string. REPLACE function is used to replace existing characters of all occurrences. Using the syntax REPLACE (string_expression, search_string, replacement_string), every incidence of search_string found in the string_expression will be replaced with replacement_string.

What is PRIMARY KEY?

A PRIMARY KEY constraint is a unique identifier for a row within a database table. Every table should have a primary key constraint to uniquely identify each row, and only one primary key constraint can be created for each table. The primary key constraints are used to enforce entity integrity.

What is UNIQUE KEY Constraint?

A UNIQUE constraint enforces the uniqueness of the values in a set of columns; so no duplicate values are entered. The unique key constraints are used to enforce entity integrity as the primary key constraints.

What is FOREIGN KEY?

A FOREIGN KEY constraint prevents any actions that would destroy links between tables with the corresponding data values. A foreign key in one table points to a primary key in another table. Foreign keys prevent actions that would leave rows with foreign key values when there are no primary keys with that value. The foreign key constraints are used to enforce referential integrity.

What is CHECK Constraint?

A CHECK constraint is used to limit the values that can be placed in a column. The check constraints are used to enforce domain integrity. (Read more here)

What is NOT NULL Constraint?

A NOT NULL constraint enforces that the column will not accept null values. The not null constraints are used to enforce domain integrity, as the check constraints.

(Read more here)

What is the difference between UNION and UNION ALL?

UNION
The UNION command is used to select related information from two tables, much like the JOIN command. However, when using the UNION command all selected columns need to be of the same data type. With UNION, only distinct values are selected.

UNION ALL
The UNION ALL command is equal to the UNION command, except that UNION ALL selects all values.

The difference between UNION and UNION ALL is that UNION ALL will not eliminate duplicate rows, instead it just pulls all rows from all the tables fitting your query specifics and combines them into a table. (Read more here)

What is B-Tree?

The database server uses a B-tree structure to organize index information. B-Tree generally has following types of index pages or nodes:

• Root node: A root node contains node pointers to only one branch node.
• Branch nodes: A branch node contains pointers to leaf nodes or other branch nodes, which can be two or more.
• Leaf nodes: A leaf node contains index items and horizontal pointers to other leaf nodes, which can be many.

How to get @@ERROR and @@ROWCOUNT at the Same Time?

If @@Rowcount is checked after Error checking statement, then it will have 0 as the value of @@Recordcount as it would have been reset. And if @@Recordcount is checked before the error-checking statement, then @@Error would get reset. To get @@error and @@rowcount at the same time, include both in same statement and store them in a local variable. SELECT @RC = @@ROWCOUNT, @ER = @@ERROR

What is a Scheduled Job or What is a Scheduled Task?

Scheduled tasks let user automate processes that run on regular or predictable cycles. User can schedule administrative tasks, such as cube processing, to run during times of slow business activity. User can also determine the order in which tasks run by creating job steps within a SQL Server Agent job, e.g. back up database and update statistics of the tables. Job steps give user control over flow of execution. If one job fails, then the user can configure SQL Server Agent to continue to run the remaining tasks or to stop execution.

What are the Advantages of Using Stored Procedures?

• Stored procedure can reduced network traffic and latency, boosting application performance.
• Stored procedure execution plans can be reused; they staying cached in SQL Server’s memory, reducing server overhead.
• Stored procedures help promote code reuse.
• Stored procedures can encapsulate logic. You can change stored procedure code without affecting clients.
• Stored procedures provide better security to your data.

What is a Table Called, if it has neither Cluster nor Non-cluster Index? What is it Used for?

Unindexed table or Heap. Microsoft Press Books and Book on Line (BOL) refers it as Heap. A heap is a table that does not have a clustered index and therefore, the pages are not linked by pointers. The IAM pages are the only structures that link the pages in a table together. Unindexed tables are good for fast storing of data. Many times, it is better to drop all the indexes from table and then do bulk of INSERTs and restore those indexes after that.

Can SQL Servers Linked to other Servers like Oracle?

SQL Server can be linked to any server provided it has OLE-DB provider from Microsoft to allow a link, e.g. Oracle has an OLE-DB provider that Microsoft provides to add it as a linked server to the SQL Server group

What is BCP? When is it Used?

BCP or BulkCopy is a tool used to copy huge amounts of data from tables and views. BCP does not copy the complete structures from source to destination. BULK INSERT command helps to import a data file into a database table or view in a user-specified format.