SQL Server Interview Questions: Part 1

  1. 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.
  2. How do you implement one-to-one, one-to-many and many-to-many relationships while designing tables?
    One-to-One relationship can be implemented as a single table and rarely as two tables with primary and foreign key relationships. One-to-Many relationships are implemented by splitting the data into two tables with primary key and foreign key relationships. Many-to-Many relationships are implemented using a junction table with the keys from both the tables forming the composite primary key of the junction table.
  3. What's 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, 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.
  4. What are user defined datatypes and when you should go for them?
    User defined datatypes let you extend the base SQL Server datatypes by providing a descriptive name, and format to the database. Take for example, in your database, there is a column called ZIP_Code which appears in many tables. In all these tables it should be varchar(6). In this case you could create a user defined datatype called ZIP_Code_Type of varchar(6) and use it across all your tables.
  5. What is bit datatype and what's the information that can be stored inside a bit column?
    Bit datatype is used to store boolean information like 1 or 0 (true or false). Untill SQL Server 6.5 bit datatype could hold either a 1 or 0 and there was no support for NULL. But from SQL Server 7.0 onwards, bit datatype can represent a third state, which is NULL.
  6. 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.
  7. 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. See CREATE DEFUALT in books online.
  8. What is a transaction and what are ACID properties?
    A transaction is a logical unit of work in which, all the steps must be performed or none. ACID stands for Atomicity, Consistency, Isolation, Durability. These are the properties of a transaction. For more information and explanation of these properties, see SQL Server books online or any RDBMS fundamentals text book.
  9. Explain different isolation levels An isolation level determines the degree of isolation of data between concurrent transactions.
    The default SQL Server isolation level is Read Committed. Here are the other isolation levels (in the ascending order of isolation): Read Uncommitted, Read Committed, Repeatable Read, Serializable.
  10. How does .NET and SQL SERVER thread is work?
    There are two types of threading pre-emptive and Non-preemptive but Sql Server support Non-preemptive and .NET thread model is different. Because Sql have to handle thread in different way for SQLCLR this different thread are known as Tasking of Threads . In this thread there is a switch between SQLCLR and SQL SERVER threads .SQL SERVER uses blocking points for transition to happen between SQLCLR and SQL SERVER threads.
  11. What's the maximum size of a row?
    8060 bytes.
  12. What is lock escalation?
    Strong names are required to store shared assemblies in the global assembly cache (GAC). This is because the GAC allows multiple versions of the same assembly to reside on your system simultaneously, so that each application can find and use its own version of your assembly. This helps avoid DLL Hell, where applications that may be compiled to different versions of your assembly could potentially break because they are all forced to use the same version of your assembly. Another reason to use strong names is to make it difficult for hackers to spoof your assembly, in other words, replace or inject your assembly with a virus or malicious code.
  13. What's the difference between DELETE TABLE and TRUNCATE TABLE commands?
    DELETE TABLE is a logged operation, so the deletion of each row gets logged in the transaction log, which makes it slow. TRUNCATE TABLE also deletes all the rows in a table, but it won't log the deletion of each row, instead it logs the deallocation of the data pages of the table, which makes it faster. Of course, TRUNCATE TABLE can be rolled back.
  14. What are constraints? Explain different types of constraints.
    Constraints enable the RDBMS enforce the integrity of the database automatically, without needing you to create triggers, rule or defaults. Types of constraints: NOT NULL, CHECK, UNIQUE, PRIMARY KEY, FOREIGN KEY 
  15. What is an index?
    Indexes in SQL Server are similar to the indexes in books. They help SQL Server retrieve the data quicker.
  16. What are the types of indexes?
    Indexes are of two types. Clustered indexes and non-clustered indexes. When you craete a clustered index on a table, all the rows in the table are stored in the order of the clustered index key. So, there can be only one clustered index per table. Non-clustered indexes have their own storage separate from the table data storage. Non-clustered indexes are stored as B-tree structures (so do clustered indexes), with the leaf level nodes having the index key and it's row locater. The row located could be the RID or the Clustered index key, depending up on the absence or presence of clustered index on the table.
  17. I create a separate index on each column of a table. what are the advantages and disadvantages of this approach?
    If you create an index on each column of a table, it improves the query performance, as the query optimizer can choose from all the existing indexes to come up with an efficient execution plan. At the same t ime, data modification operations (such as INSERT, UPDATE, DELETE) will become slow, as every time data changes in the table, all the indexes need to be updated. Another disadvantage is that, indexes need disk space, the more indexes you have, more disk space is used.
  18. What is RAID and what are different types of RAID configurations?
    RAID stands for Redundant Array of Inexpensive Disks, used to provide fault tolerance to database servers. There are six RAID levels 0 through 5 offering different levels of performance, fault tolerance.
  19. What are the steps you will take, if you are tasked with securing an SQL Server?
    Preferring NT authentication, using server, databse and application roles to control access to the data, securing the physical database files using NTFS permissions, using an unguessable SA password, restricting physical access to the SQL Server, renaming the Administrator account on the SQL Server computer, disabling the Guest account, enabling auditing, using multiprotocol encryption, setting up SSL, setting up firewalls, isolating SQL Server from the web server etc.
  20. What is a deadlock and what is a live lock? How will you go about resolving deadlocks?
    Deadlock is a situation when two processes, each having a lock on one piece of data, attempt to acquire a lock on the other's piece. Each process would wait indefinitely for the other to release the lock, unless one of the user processes is terminated. SQL Server detects deadlocks and terminates one user's process. A livelock is one, where a request for an exclusive lock is repeatedly denied because a series of overlapping shared locks keeps interfering. SQL Server detects the situation after four denials and refuses further shared locks. A livelock also occurs when read transactions monopolize a table or page, forcing a write transaction to wait indefinitely.