Tuesday, January 22, 2008

SQL Transaction Interview Questions

1. What is Transaction?
A transaction is a sequence of operations performed as a single logical unit of work. A logical unit of work must exhibit four properties, called the ACID (Atomicity, Consistency, Isolation, and Durability) properties, to qualify as a transaction:
o Atomicity - A transaction must be an atomic unit of work; either all of its data modifications are performed or none of them is performed.
o Consistency - When completed, a transaction must leave all data in a consistent state. In a relational database, all rules must be applied to the transaction's modifications to maintain all data integrity. All internal data structures, such as B-tree indexes or doubly-linked lists, must be correct at the end of the transaction.
o Isolation - Modifications made by concurrent transactions must be isolated from the modifications made by any other concurrent transactions. A transaction either sees data in the state it was in before another concurrent transaction modified it, or it sees the data after the second transaction has completed, but it does not see an intermediate state. This is referred to as serializability because it results in the ability to reload the starting data and replay a series of transactions to end up with the data in the same state it was in after the original transactions were performed.
o Durability - After a transaction has completed, its effects are permanently in place in the system. The modifications persist even in the event of a system failure.
2. After one Begin Transaction a truncate statement and a RollBack statements are there. Will it be rollbacked? Since the truncate statement does not perform logged operation how does it RollBack?
It will rollback.
3. Given a SQL like
Begin Tran
Select @@Rowcount
Begin Tran
Select @@Rowcount
Begin Tran
Select @@Rowcount
Commit Tran
Select @@Rowcount
Select @@Rowcount
Select @@Rowcount
What is the value of @@Rowcount at each stmt levels?
Ans : 0 – zero.
@@ROWCOUNT - Returns the number of rows affected by the last statement.
@@TRANCOUNT - Returns the number of active transactions for the current connection.
Each Begin Tran will add count, each commit will reduce count and ONE rollback will make it 0.

4. What are the constraints for Table Constraints define rules regarding the values allowed in columns and are the standard mechanism for enforcing integrity. SQL Server 2000 supports five classes of constraints.
5. There are 50 columns in a table. Write a query to get first 25 columns
Ans: Need to mention each column names.
6. How to list all the tables in a particular database?
USE pubs
7. What are cursors? Explain different types of cursors. What are the disadvantages of cursors? How can you avoid cursors?
Cursors allow row-by-row processing of the result sets.
Types of cursors: Static, Dynamic, Forward-only, Keyset-driven.
Disadvantages of cursors: Each time you fetch a row from the cursor, it results in a network roundtrip. Cursors are also costly because they require more resources and temporary storage (results in more IO operations). Further, there are restrictions on the SELECT statements that can be used with some types of cursors.
How to avoid cursor:
1. Most of the times, set based operations can be used instead of cursors. Here is an example: If you have to give a flat hike to your employees using the following criteria:
Salary between 30000 and 40000 -- 5000 hike
Salary between 40000 and 55000 -- 7000 hike
Salary between 55000 and 65000 -- 9000 hike
In this situation many developers tend to use a cursor, determine each employee's salary and update his salary according to the above formula. But the same can be achieved by multiple update statements or can be combined in a single UPDATE statement as shown below:
UPDATE tbl_emp SET salary =
CASE WHEN salary BETWEEN 30000 AND 40000 THEN salary + 5000
WHEN salary BETWEEN 40000 AND 55000 THEN salary + 7000
WHEN salary BETWEEN 55000 AND 65000 THEN salary + 10000
2. You need to call a stored procedure when a column in a particular row meets certain condition. You don't have to use cursors for this. This can be achieved using WHILE loop, as long as there is a unique key to identify each row. For examples of using WHILE loop for row by row processing, check out the 'My code library' section of my site or search for WHILE.
8. What is Dynamic Cursor? Suppose, I have a dynamic cursor attached to table in a database. I have another means by which I will modify the table. What do you think will the values in the cursor be?
Dynamic cursors reflect all changes made to the rows in their result set when scrolling through the cursor. The data values, order, and membership of the rows in the result set can change on each fetch. All UPDATE, INSERT, and DELETE statements made by all users are visible through the cursor. Updates are visible immediately if they are made through the cursor using either an API function such as SQLSetPos or the Transact-SQL WHERE CURRENT OF clause. Updates made outside the cursor are not visible until they are committed, unless the cursor transaction isolation level is set to read uncommitted.
9. What is DATEPART?
Returns an integer representing the specified datepart of the specified date.
10. Difference between Delete and Truncate?
TRUNCATE TABLE is functionally identical to DELETE statement with no WHERE clause: both remove all rows in the table.
(1) But TRUNCATE TABLE is faster and uses fewer system and transaction log resources than DELETE. 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's data, and only the page deallocations are recorded in the transaction log.
(2) Because TRUNCATE TABLE is not logged, it cannot activate a trigger.
(3) The counter used by an identity for new rows is reset to the seed for the column. If you want to retain the identity counter, use DELETE instead.
Of course, TRUNCATE TABLE can be rolled back.
11. Given a scenario where two operations, Delete Stmt and Truncate Stmt, where the Delete Statement was successful and the truncate stmt was failed. – Can u judge why?
12. What are global variables? Tell me some of them?
Transact-SQL global variables are a form of function and are now referred to as functions.
ABS - Returns the absolute, positive value of the given numeric expression.
13. What is DDL?
Data definition language (DDL) statements are SQL statements that support the definition or declaration of database objects (for example, CREATE TABLE, DROP TABLE, and ALTER TABLE).
You can use the ADO Command object to issue DDL statements. To differentiate DDL statements from a table or stored procedure name, set the CommandType property of the Command object to adCmdText. Because executing DDL queries with this method does not generate any recordsets, there is no need for a Recordset object.
14. What is DML?
Data Manipulation Language (DML), which is used to select, insert, update, and delete data in the objects defined using DDL
15. What are keys in RDBMS? What is a primary key/ foreign key?
There are two kinds of keys.
A primary key is a set of columns from a table that are guaranteed to have unique values for each row of that table.
Foreign keys are attributes of one table that have matching values in a primary key in another table, allowing for relationships between tables.
16. What is the difference between Primary Key and Unique Key?
Both primary key and unique key enforce uniqueness of the column on which they are defined. But by default primary key creates a clustered index on the column, where are unique creates a nonclustered index by default. Another major difference is that, primary key doesn't allow NULLs, but unique key allows one NULL only.
17. Define candidate key, alternate key, composite key?
A candidate key is one that can identify each row of a table uniquely. Generally a candidate key becomes the primary key of the table. If the table has more than one candidate key, one of them will become the primary key, and the rest are called alternate keys.
A key formed by combining at least two or more columns is called composite key.
18. What is the Referential Integrity?
Referential integrity refers to the consistency that must be maintained between primary and foreign keys, i.e. every foreign key value must have a corresponding primary key value.
19. What are defaults? Is there a column to which a default can't 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.
20. What is Query optimization? How is tuning a performance of query done?
21. What is the use of trace utility?
22. What is the use of shell commands? xp_cmdshell
Executes a given command string as an operating-system command shell and returns any output as rows of text. Grants nonadministrative users permissions to execute xp_cmdshell.
23. What is use of shrink database?
Microsoft® SQL Server 2000 allows each file within a database to be shrunk to remove unused pages. Both data and transaction log files can be shrunk.
24. If the performance of the query suddenly decreased where you will check?
25. What is a pass-through query?
Microsoft® SQL Server 2000 sends pass-through queries as un-interpreted query strings to an OLE DB data source. The query must be in a syntax the OLE DB data source will accept. A Transact-SQL statement uses the results from a pass-through query as though it is a regular table reference.
This example uses a pass-through query to retrieve a result set from a Microsoft Access version of the Northwind sample database.
FROM OpenRowset('Microsoft.Jet.OLEDB.4.0',
'c:\northwind.mdb';'admin'; '',
'SELECT CustomerID, CompanyName
FROM Customers
WHERE Region = ''WA'' ')
26. How do you differentiate Local and Global Temporary table?
You can create local and global temporary tables. Local temporary tables are visible only in the current session; global temporary tables are visible to all sessions. Prefix local temporary table names with single number sign (#table_name), and prefix global temporary table names with a double number sign (##table_name). SQL statements reference the temporary table using the value specified for table_name in the CREATE TABLE statement:
27. How the Exists keyword works in SQL Server?
USE pubs
SELECT au_lname, au_fname
FROM authors
WHERE exists
FROM publishers
WHERE authors.city = publishers.city)
When a subquery is introduced with the keyword EXISTS, it functions as an existence test. The WHERE clause of the outer query tests for the existence of rows returned by the subquery. The subquery does not actually produce any data; it returns a value of TRUE or FALSE.
28. ANY?
USE pubs
SELECT au_lname, au_fname
FROM authors
WHERE city = ANY
(SELECT city
FROM publishers)
29. to select date part only
SELECT CONVERT(char(10),GetDate(),101)
--to select time part only
SELECT right(GetDate(),7)
30. How can I send a message to user from the SQL Server?
You can use the xp_cmdshell extended stored procedure to run net send command. This is the example to send the 'Hello' message to JOHN:
EXEC master..xp_cmdshell "net send JOHN 'Hello'"
To get net send message on the Windows 9x machines, you should run the WinPopup utility. You can place WinPopup in the Startup group under Program Files.
31. What is normalization? Explain different levels of normalization? Explain Third normalization form with an example?
The process of refining tables, keys, columns, and relationships to create an efficient database is called normalization. This should eliminates unnecessary duplication and provides a rapid search path to all necessary information.
Some of the benefits of normalization are:
• Data integrity (because there is no redundant, neglected data)
• Optimized queries (because normalized tables produce rapid, efficient joins)
• Faster index creation and sorting (because the tables have fewer columns)
• Faster UPDATE performance (because there are fewer indexes per table)
• Improved concurrency resolution (because table locks will affect less data)
• Eliminate redundancy
There are a few rules for database normalization. Each rule is called a "normal form." If the first rule is observed, the database is said to be in "first normal form." If the first three rules are observed, the database is considered to be in "third normal form." Although other levels of normalization are possible, third normal form is considered the highest level necessary for most applications.
6. First Normal Form (1NF)
 Eliminate repeating groups in individual tables
 Create a separate table for each set of related data.
 Identify each set of related data with a primary key.
Do not use multiple fields in a single table to store similar data.
Subordinate1 Subordinate2 Subordinate3 Subordinate4
Bob Jim Mary Beth
Mary Mike Jason Carol Mark
Jim Alan
Eliminate duplicative columns from the same table. Clearly, the Subordinate1-Subordinate4 columns are duplicative. What happens when we need to add or remove a subordinate?
Bob Jim, Mary, Beth
Mary Mike, Jason, Carol, Mark
Jim Alan
This solution is closer, but it also falls short of the mark. The subordinates column is still duplicative and non-atomic. What happens when we need to add or remove a subordinate? We need to read and write the entire contents of the table. That’s not a big deal in this situation, but what if one manager had one hundred employees? Also, it complicates the process of selecting data from the database in future queries.

Bob Jim
Bob Mary
Bob Beth
Mary Mike
Mary Jason
Mary Carol
Mary Mark
Jim Alan
7. Second Normal Form (2NF)
 Create separate tables for sets of values that apply to multiple records.
 Relate these tables with a foreign key.
Records should not depend on anything other than a table's primary key (a compound key, if necessary).
For example, consider a customer's address in an accounting system. The address is needed by the Customers table, but also by the Orders, Shipping, Invoices, Accounts Receivable, and Collections tables. Instead of storing the customer's address as a separate entry in each of these tables, store it in one place, either in the Customers table or in a separate Addresses table.
8. Third Normal Form (3NF)
 Eliminate fields that do not depend on the key.
Values in a record that are not part of that record's key do not belong in the table. In general, any time the contents of a group of fields may apply to more than a single record in the table, consider placing those fields in a separate table.
For example, in an Employee Recruitment table, a candidate's university name and address may be included. But you need a complete list of universities for group mailings. If university information is stored in the Candidates table, there is no way to list universities with no current candidates. Create a separate Universities table and link it to the Candidates table with a university code key.
Another Example :
MemberId Name Company CompanyLoc
1 John Smith ABC Alabama
2 Dave Jones MCI Florida
The Member table satisfies first normal form - it contains no repeating groups. It satisfies second normal form - since it doesn't have a multivalued key. But the key is MemberID, and the company name and location describe only a company, not a member. To achieve third normal form, they must be moved into a separate table. Since they describe a company, CompanyCode becomes the key of the new "Company" table.
The motivation for this is the same for second normal form: we want to avoid update and delete anomalies. For example, suppose no members from the IBM were currently stored in the database. With the previous design, there would be no record of its existence, even though 20 past members were from IBM!
Member Table
MemberId Name CID
1 John Smith 1
2 Dave Jones 2

Company Table
CId Name Location
1 ABC Alabama
2 MCI Florida

9. Boyce-Codd Normal Form (BCNF)
A relation is in Boyce/Codd normal form if and only if the only determinants are candidate key. Its a different version of 3NF, indeed, was meant to replace it. [A determinant is any attribute on which some other attribute is (fully) functionally dependent.]
10. 4th Normal Form (4NF)
A table is in 4NF if it is in BCNF and if it has no multi-valued dependencies. This applies primarily to key-only associative tables, and appears as a ternary relationship, but has incorrectly merged 2 distinct, independent relationships.
Eg: This could be any 2 M:M relationships from a single entity. For instance, a member could know many software tools, and a software tool may be used by many members. Also, a member could have recommended many books, and a book could be recommended by many members.
11. The correct solution, to cause the model to be in 4th normal form, is to ensure that all M:M relationships are resolved independently if they are indeed independent.

12. 5th Normal Form (5NF)(PJNF)
A table is in 5NF, also called "Projection-Join Normal Form", if it is in 4NF and if every join dependency in the table is a consequence of the candidate keys of the table.
13. Domain/key normal form (DKNF). A key uniquely identifies each row in a table. A domain is the set of permissible values for an attribute. By enforcing key and domain restrictions, the database is assured of being freed from modification anomalies. DKNF is the normalization level that most designers aim to achieve.
Remember, these normalization guidelines are cumulative. For a database to be in 2NF, it must first fulfill all the criteria of a 1NF database.
32. If a database is normalized by 3 NF then how many number of tables it should contain in minimum? How many minimum if 2NF and 1 NF?
33. What is denormalization and when would you go for it?
As the name indicates, denormalization is the reverse process of normalization. It's the controlled introduction of redundancy in to the database design. It helps improve the query performance as the number of joins could be reduced.
34. How can I randomly sort query results?
To randomly order rows, or to return x number of randomly chosen rows, you can use the RAND function inside the SELECT statement. But the RAND function is resolved only once for the entire query, so every row will get same value. You can use an ORDER BY clause to sort the rows by the result from the NEWID function, as the following code shows:
FROM Northwind..Orders
35. sp_who
Provides information about current Microsoft® SQL Server™ users and processes. The information returned can be filtered to return only those processes that are not idle.
36. Have you worked on Dynamic SQL? How will You handled “ (Double Quotes) in Dynamic SQL?
37. How to find dependents of a table?
Verify dependencies with sp_depends before dropping an object
38. What is the difference between a CONSTRAINT AND RULE?
Rules are a backward-compatibility feature that perform some of the same functions as CHECK constraints. CHECK constraints are the preferred, standard way to restrict the values in a column. CHECK constraints are also more concise than rules; there can only be one rule applied to a column, but multiple CHECK constraints can be applied. CHECK constraints are specified as part of the CREATE TABLE statement, while rules are created as separate objects and then bound to the column.
39. How to call a COM dll from SQL Server 2000?
sp_OACreate - Creates an instance of the OLE object on an instance of Microsoft® SQL Server
sp_OACreate progid, | clsid,
objecttoken OUTPUT
[ , context ]
context - Specifies the execution context in which the newly created OLE object runs. If specified, this value must be one of the following:
1 = In-process (.dll) OLE server only
4 = Local (.exe) OLE server only
5 = Both in-process and local OLE server allowed
A. Use Prog ID - This example creates a SQL-DMO SQLServer object by using its ProgID.
DECLARE @object int
DECLARE @hr int
DECLARE @src varchar(255), @desc varchar(255)
EXEC @hr = sp_OACreate 'SQLDMO.SQLServer', @object OUT
IF @hr <> 0
EXEC sp_OAGetErrorInfo @object, @src OUT, @desc OUT
SELECT hr=convert(varbinary(4),@hr), Source=@src, Description=@desc
B. Use CLSID - This example creates a SQL-DMO SQLServer object by using its CLSID.
DECLARE @object int
DECLARE @hr int
DECLARE @src varchar(255), @desc varchar(255)
EXEC @hr = sp_OACreate '{00026BA1-0000-0000-C000-000000000046}',
@object OUT
IF @hr <> 0
EXEC sp_OAGetErrorInfo @object, @src OUT, @desc OUT
SELECT hr=convert(varbinary(4),@hr), Source=@src, Description=@desc
40. Difference between sysusers and syslogins?
sysusers - Contains one row for each Microsoft® Windows user, Windows group, Microsoft SQL Server™ user, or SQL Server role in the database.
syslogins - Contains one row for each login account.
41. What is the row size in SQL Server 2000?
8060 bytes.
42. How will you find structure of table, all tables/views in one db, all dbs?
//structure of table
sp_helpdb tbl_emp

//list of all databases
SELECT * FROM master.dbo.sysdatabases

//details about database pubs. .mdf, .ldf file locations, size of database
sp_helpdb pubs

//lists all tables under current database
SELECT * FROM information_schema.tables WHERE (table_type = 'base table')
SELECT * FROM sysobjects WHERE type = 'U' //faster
43. B-tree indexes or doubly-linked lists?
44. What is the system function to get the current user's user id?
USER_ID(). Also check out other system functions like USER_NAME(), SYSTEM_USER, SESSION_USER, CURRENT_USER, USER, SUSER_SID(), HOST_NAME().
45. What are the series of steps that happen on execution of a query in a Query Analyzer?
1) Syntax checking 2) Parsing 3) Execution plan
46. Which event (Check constraints, Foreign Key, Rule, trigger, Primary key check) will be performed last for integrity check?
Identity Insert Check
Nullability constraint
Data type check
Instead of trigger
Primary key
Check constraint
Foreign key
DML Execution (update statements)
After Trigger
47. How will you show many to many relation in sql?
Create 3rd table with 2 columns which having one to many relation to these tables.
48. When a query is sent to the database and an index is not being used, what type of execution is taking place?
A table scan.
49. What is #, ##, @, @@ means?
@@ - System variables
@ - user defined variables
50. What is the difference between a Local temporary table and a Global temporary table? How is each one denoted?
Local temporary table will be accessible to only current user session, its name will be preceded with a single hash (#mytable)
Global temporary table will be accessible to all users, & it will be dropped only after ending of all active connections, its name will be preceded with double hash (##mytable)
51. What is covered queries in SQL Server?

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