Sql Basic Interview Questions And Answers PdfBy Saville D. In and pdf 25.05.2021 at 03:13 3 min read
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- 100+ Most Popular SQL Interview Questions And Answers
- Basic SQL Interview Questions And Answers Pdf
- SQL Interview Questions asked in Top Companies in 2021
- Top 65 SQL Interview Questions You Must Prepare In 2021
What are the properties of the transaction? Answer: Properties of the transaction are known as ACID properties, such as Atomicity: Ensures the completeness of all transactions performed. Checks whether every transaction is completed successfully if not then transaction is aborted at the failure point and the previous transaction is rolled back to its initial state as changes undone Consistency: Ensures that all changes made through successful transaction are reflected properly on database Isolation: Ensures that all transactions are performed independently and changes made by one transaction are not reflected on other Durability: Ensures that the changes made in the database with committed transactions persist as it is even after a system failure.
100+ Most Popular SQL Interview Questions And Answers
A database is an organized collection of data, stored and retrieved digitally from a remote or local computer system. Databases can be vast and complex, and such databases are developed using fixed design and modeling approaches.
DBMS is a system software responsible for the creation, retrieval, updation and management of the database. It ensures that our data is consistent, organized and is easily accessible by serving as an interface between the database and its end users or application softwares. The key difference here, compared to DBMS, is that RDBMS stores data in the form of a collection of tables and relations can be defined between the common fields of these tables. It is the standard language for relational database management systems.
It is especially useful in handling organized data comprised of entities variables and relations between different entities of the data. SQL is a standard language for retrieving and manipulating structured databases. What are Tables and Fields? A table is an organized collection of data stored in the form of rows and columns.
Columns can be categorized as vertical and rows as horizontal. The columns in a table are called fields while the rows can be referred to as records. Constraints are used to specify the rules concerning data in the table. A table in SQL is strictly restricted to have one and only one primary key, which is comprised of single or multiple fields columns. This provides uniqueness for the column s and helps identify each row uniquely. Unlike primary key, there can be multiple unique constraints defined per table.
Foreign key constraint ensures referential integrity in the relation between two tables. The table with the foreign key constraint is labelled as the child table, and the table containing the candidate key is labelled as the referenced or parent table.
The SQL Join clause is used to combine records rows from two or more tables in a SQL database based on a related column between the two.
This is the widely used join for queries. A self JOIN is a case of regular join where a table is joined to itself based on some relation between its own column s. Cross join can be defined as a cartesian product of the two tables included in the join.
The table after join contains the same number of rows as in the cross-product of number of rows in the two tables. A database index is a data structure that provides quick lookup of data in a column or columns of a table.
It enhances the speed of operations accessing data from a database table at the cost of additional writes and memory to maintain the index data structure. There are different types of indexes that can be created for different purposes: Unique and Non-Unique Index : Unique indexes are indexes that help maintain data integrity by ensuring that no two rows of data in a table have identical key values.
Once a unique index has been defined for a table, uniqueness is enforced whenever keys are added or changed within the index. Non-unique indexes, on the other hand, are not used to enforce constraints on the tables with which they are associated.
Instead, non-unique indexes are used solely to improve query performance by maintaining a sorted order of data values that are used frequently.
Clustered indexes are indexes whose order of the rows in the database correspond to the order of the rows in the index. This is why only one clustered index can exist in a given table, whereas, multiple non-clustered indexes can exist in the table.
The only difference between clustered and non-clustered indexes is that the database manager attempts to keep the data in the database in the same order as the corresponding keys appear in the clustered index.
Clustering index can improve the performance of most query operations because they provide a linear-access path to data stored in the database. As explained above, the differences can be broken down into three small factors - Clustered index modifies the way records are stored in a database based on the indexed column. Non-clustered index creates a separate entity within the table which references the original table.
Clustered index is used for easy and speedy retrieval of data from the database, whereas, fetching records from the non-clustered index is relatively slower. In SQL, a table can have a single clustered index whereas it can have multiple non-clustered indexes. Data Integrity is the assurance of accuracy and consistency of data over its entire life-cycle, and is a critical aspect to the design, implementation and usage of any system which stores, processes, or retrieves data.
It also defines integrity constraints to enforce business rules on the data when it is entered into an application or a database. A query is a request for data or information from a database table or combination of tables.
A database query can be either a select query or an action query. A subquery is a query within another query, also known as nested query or inner query. It is used to restrict or enhance the data to be queried by the main query, thus restricting or enhancing the output of the main query respectively.
For example, here we fetch the contact information for students who have enrolled for the maths subject:. There are two types of subquery - Correlated and Non-Correlated. A correlated subquery cannot be considered as an independent query, but it can refer the column in a table listed in the FROM of the main query. A non-correlated subquery can be considered as an independent query and the output of subquery is substituted in the main query.
The data returned is stored in a result table, called the result-set. A database cursor is a control structure that allows for traversal of records in a database. Cursors, in addition, facilitates processing after traversal, such as retrieval, addition and deletion of database records. They can be viewed as a pointer to one row in a set of rows. Entity : An entity can be a real-world object, either tangible or intangible, that can be easily identifiable. For example, in a college database, students, professors, workers, departments, and projects can be referred to as entities.
Each entity has some associated properties that provide it an identity. Relationships : Relations or links between entities that have something to do with each other. For example - The employees table in a company's database can be associated with the salary table in the same database.
One-to-One - This can be defined as the relationship between two tables where each record in one table is associated with the maximum of one record in the other table. Many-to-Many - This is used in cases when multiple instances on both sides are needed for defining a relationship.
Self Referencing Relationships - This is used when a table needs to define a relationship with itself. It is a temporary name assigned to the table or table column for the purpose of a particular SQL query. In addition, aliasing can be employed as an obfuscation technique to secure the real names of database fields.
A table alias is also called a correlation name. An alias is represented explicitly by the AS keyword but in some cases the same can be performed without it as well. Nevertheless, using the AS keyword is always a good practice. A view contains rows and columns, just like a real table.
The fields in a view are fields from one or more real tables in the database. Normalization represents the way of organizing structured data in the database efficiently. It includes creation of tables, establishing relationships between them, and defining rules for those relationships. Inconsistency and redundancy can be kept in check based on these rules, hence, adding flexibility to the database. Denormalization is the inverse process of normalization, where the normalized schema is converted into a schema which has redundant information.
The performance is improved by using redundancy and keeping the redundant data consistent. The reason for performing denormalization is the overheads produced in query processor by an over-normalized structure. Normal Forms are used to eliminate or reduce redundancy in database tables. The different forms are as follows: First Normal Form A relation is in first normal form if every attribute in that relation is a single-valued attribute.
If a relation contains composite or multi-valued attribute, it violates the first normal form. Let's consider the following students table. As we can observe, the Books Issued field has more than one values per record and to convert it into 1NF, this has to be resolved into separate individual records for each book issued.
Check the following table in 1NF form -. A relation is in second normal form if it satisfies the conditions for first normal form and does not contain any partial dependency. A relation in 2NF has no partial dependency , i. Often, specifying a single column Primary Key is the solution to the problem.
Examples - Example 1 - Consider the above example. As we can observe, Students Table in 1NF form has a candidate key in the form of [Student, Address] that can uniquely identify all records in the table. The field Books Issued non-prime attribute depends partially on the Student field. Hence, the table is not in 2NF. To convert it into 2nd Normal Form, we will partition the tables into two while specifying a new Primary Key attribute to identify the individual records in the Students table.
The Foreign Key constraint will be set on the other table to ensure referential integrity. A relation is said to be in the third normal form, if it satisfies the conditions for second normal form and there is no transitive dependency between the non-prime attributes, i. Example 1 - Consider the Students Table in the above example. The field Salutation non-prime attribute , however, depends on the Student Field rather than the candidate key. Hence, the table is not in 3NF.
To convert it into 3rd Normal Form, we will once again partition the tables into two while specifying a new Foreign Key constraint to identify the salutations for individual records in the Students table. The Primary Key constraint for the same will be set on the Salutations table to identify each record uniquely.
A relation is in Boyce-Codd Normal Form if satisfies the conditions for third normal form and for every functional dependency, Left-Hand-Side is super key.
If you drop a table, all the rows in the table is deleted and the table structure is removed from the database. If a table is dropped, all things associated with the tables are dropped as well. This includes - the relationships defined on the table with other tables, the integrity checks and constraints, access privileges and other grants that the table has. To create and use the table again in its original form, all these relations, checks, constraints, privileges and relationships need to be redefined.
However, if a table is truncated, none of the above problems exist and the table retains its original structure.
Basic SQL Interview Questions And Answers Pdf
A database is a collection of information in an organized form for faster and better access, storage, and manipulation. It can also be defined as a collection of tables, schema, views, and other database objects. Data warehouse refers to a central repository of data from multiple sources of information. Those data are consolidated, transformed and made available for the mining as well as online processing. A table is a database object used to store records in a field in the form of columns and rows that holds data.
SQL Interview Questions asked in Top Companies in 2021
SQL programming skills are highly desirable and required in the market, as there is a massive use of Database Management Systems DBMS in almost every software application. In order to get a job, candidates need to crack the interview in which they are asked various SQL interview questions. Following is a curated list of SQL interview questions and answers, which are likely to be asked during the SQL interview. Candidates are likely to be asked basic SQL interview questions to advance level SQL questions depending on their experience and various other factors. The below list covers all the SQL interview questions for freshers as well as SQL interview questions for experienced level candidates and some SQL query interview questions.
SQL is one of the most widely used languages. Almost all biggest names in the tech industry such as Uber, Netflix, Airbnb, etc. The reason for the popularity of SQL is that most of the professions require knowledge of database connectivity. Also, in all technical interviews, 30—40 percent of questions will be from SQL. So, this blog lists all the top SQL interview questions.
Top 65 SQL Interview Questions You Must Prepare In 2021
Dear readers, these SQL Interview Questions have been designed specially to get you acquainted with the nature of questions you may encounter during your interview for the subject of SQL. As per my experience good interviewers hardly plan to ask any particular question during your interview, normally questions start with some basic concept of the subject and later they continue based on further discussion and what you answer:. SQL or Structured Query Language is a language; language that communicates with a relational database thus providing ways of manipulating and creating databases. A NULL value is not same as zero or a blank space. Whereas, zero is a number and blank space is a character.
A database is an organized collection of data, stored and retrieved digitally from a remote or local computer system. Databases can be vast and complex, and such databases are developed using fixed design and modeling approaches. DBMS is a system software responsible for the creation, retrieval, updation and management of the database. It ensures that our data is consistent, organized and is easily accessible by serving as an interface between the database and its end users or application softwares. The key difference here, compared to DBMS, is that RDBMS stores data in the form of a collection of tables and relations can be defined between the common fields of these tables.
What is Failover clustering overview? Answer: Failover clustering is mainly used for data availability. Typically, in a failover cluster, there are two machines. One machine provides the basic services and the second is available to run the service when the primary system fails. The primary system is monitored periodically to check if it works. This monitoring may be performed by the failover computer or an independent system also called a cluster controller. In an event of failure of the primary computer, the failover system takes control.
SQL Essentials Training & Certification
I have listed all the series in this post so that it can be easily downloaded and used. All the questions are collected and listed in one PDF which is here to download. If you have any question or if you want to add to any of the questions please send me a email or write a comment. In this very first blog post — various aspect of the interview questions and answers are discussed. Some people like the subject for their helpful hints and thought provoking subject, and others dislike these posts because they feel it is nothing more than cheating.
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