SQL interview Questions

What is the full form of SQL ?

Structured Query Language (SQL). It is pronounced “sequel”.SQl is a language that provides an interface to relational database systems. It was developed by IBM.

What are two methods of retrieving SQL?

What is a deadlock in SQL ?

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.

What is livelock in SQL ?

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.

Some important system function to get the current user details


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.

What cursor type do you use to retrieve multiple recordsets?

What is candidate key, alternate key, composite key in SQL ?

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.

Explain the architecture of SQL Server

What is the difference between a “where” clause and a “having” clause in SQL ?

“Where” Clause in SQL is a kind of restiriction statement. You use where clause to restrict all the data from DB.Where clause is using before result retrieving. But Having clause is using after retrieving the data.Having clause is a kind of filtering command from the selected data.

What is the basic form of a SQL statement to read data out of a table?

Basic form to read data out of table in SQL is “SELECT * FROM tablename”. Answer with “where” close wont be proper because it is an additional thing with basic select statement.

What’s the maximum size of a row in SQL table?
8060 bytes.

What are the tradeoffs with having indexes?

Faster selects
slower updates
Extra storage space to store indexes
Updates are slower because in addition to updating the table you have to update the index.

What’s the difference between DELETE TABLE and TRUNCATE TABLE commands in SQL?

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 de allocation of the data pages of the table, which makes it faster. TRUNCATE TABLE can be rolled back

What is a “join” statement in SQL?
‘join’ used to connect two or more tables logically with or without common field.

What is “normalization”? “Denormalization”? Why do you sometimes want to denormalize?

Normalizing data means eliminating redundant information from a table and organizing the data so that future changes to the table are easier. Denormalization means allowing redundancy in a table. The main benefit of denormalization is improved performance with simplified data retrieval and manipulation. This is done by reduction in the number of joins needed for data processing.

How to restart SQL Server?
from command line, using the SQLSERVR.EXE.

-m is used for starting SQL Server in single user mode
-f is used to start the SQL Server in minimal confuguration mode.

What is a “constraint” in SQL?

A constraint allows you to apply simple referential integrity checks to a table. There are four primary types of constraints that are currently supported by SQL Server

PRIMARY/UNIQUE – enforces uniqueness of a particular table column.
DEFAULT – specifies a default value for a column in case an insert operation does not provide one.
FOREIGN KEY – validates that every value in a column exists in a column of another table.
CHECK – checks that every value stored in a column is in some specified list.
NOT NULL is one more constraint which does not allow values in the specific column to be null. And also it the only constraint which is not a table level constraint.

Each type of constraint performs a specific type of action. Default is not a constraint.

Different Types of joins in SQL


What types of index data structures can you have?
An index helps to faster search values in tables. The three most commonly used index-types are:

B-Tree: builds a tree of possible values with a list of row IDs that have the leaf value. Needs a lot of space and is the default index type for most databases.
Bitmap: string of bits for each possible value of the column. Each bit string has one bit for each row. Needs only few space and is very fast.(however, domain of value cannot be large, e.g. SEX(m,f); degree(BS,MS,PHD)
Hash: A hashing algorithm is used to assign a set of characters to represent a text string such as a composite of keys or partial keys, and compresses the underlying data. Takes longer to build and is supported by relatively few databases.

Types of cursors in SQL ?


What is a “primary key”?

Primary Key is a type of a constraint enforcing uniqueness and data integrity for each row of a table. All columns participating in a primary key constraint must possess the NOT NULL property.For example “user Id” should be unique for users, so we can make that field a s primary key in some tables for making sure that value wont repeat.

What is a “trigger”?

Triggers are stored procedures created in order to enforce integrity rules in a database. A trigger is executed every time a data-modification operation occurs (i.e., insert, update or delete). Triggers are executed automatically on occurrence of one of the data-modification operations. A trigger is a database object directly associated with a particular table. It fires whenever a specific statement/type of statement is issued against that table. The types of statements are insert,update,delete and query statements. Basically, trigger is a set of SQL statements A trigger is a solution to the restrictions of a constraint.

What is “index covering” of a query?
Index covering means that “Data can be found only using indexes, without touching the tables”

What is a SQL view?

An output of a query can be stored as a view. View acts like small table which meets our criterion. View is a precomplied SQL query which is used to select data from one or more tables. A view is like a table but it doesn’t physically take any space. View is a good way to present data in a particular format if you use that query quite often. View can also be used to restrict users from accessing the tables directly.Its mainly used to view the data from various tables.

What is blocking and when it is happening?

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.

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.

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