SQL Interview Questions for Freshers

A database is a structured collection of data. A Database Management System (DBMS) is a software application that interacts with the user, applications and the database itself to capture and analyze data. A DBMS allows a user to interact with the database. The data stored in the database can be modified, retrieved and deleted and can be of any type like strings, numbers, images etc.

There are two types of DBMS:

Relational Database Management System: The data is stored in relations (tables). Example – MySQL

. Non-Relational Database Management System: There is no concept of relations, tuples and attributes. Example – Mongo

A table refers to a collection of data in an organised manner in form of rows and columns. A field refers to the number of columns in a table. For example:

Table: StudentInformation

Field: Stu Id, Stu Name, Stu Marks

The different subsets of SQL are:

DDL (Data Definition Language): It allows you to perform various operations on the database such as CREATE, ALTER and DELETE objects.

DML ( Data Manipulation Language) – It allows you to access and manipulate data. It helps you to insert, update, delete and retrieve data from the database.

DCL ( Data Control Language) – It allows you to control access to the database. Example – Grant, Revoke access permissions.

A JOIN clause is used to combine rows from two or more tables, based on a related column between them. It is used to merge two tables or retrieve data from there. There are 4 joins in SQL namely:

• inner join
• Right join
• Left join
• full join

SQL is a standard language which stands for Structured Query Language based on the English language whereas MySQL is a database management system. SQL is the core of relational database which is used for accessing and managing database, MySQL is an RDMS (Relational Database Management System) such as SQL Server, Informix etc.

• Uniquely identifies a single row in the table.
• Multiple values allowed per table.
• Null values allowed.

Both Char and Varchar2 are used for characters datatype but varchar2 is used for character strings of variable length whereas Char is used for strings of fixed length. For example, char(10) can only store 10 characters and will not be able to store a string of any other length whereas varchar2(10) can store any length i.e 6,8,2 in this variable.

A Primary key is a column (or collection of columns) or a set of columns that uniquely identifies each row in the table.

Uniquely identifies a single row in the table

Null values not allowed

Example- In the Student table, Stu_ID is the primary key.

Constraints are used to specify the limit on the data type of the table. It can be specified while creating or altering the table statement. The sample of constraints are:

• Not NULL
• CHECK
• DEFAULT
• UNIQUE
• PRIMARY KEY
• FORIEGN KEY

Clustered index is used for easy retrieval of data from the database and its faster whereas reading from non clustered index is relatively slower.

Clustered index alters the way records are stored in a database as it sorts out rows by the column which is set to be clustered index whereas in a non clustered index, it does not alter the way it was stored but it creates a separate object within a table which points back to the original table rows after searching.

One table can only have one clustered index whereas it can have many non clustered index.

In SQL, there is a built-in function called GetDate() which helps to return the current timestamp/date.

Foreign key maintains referential integrity by enforcing a link between the data in two tables.

The foreign key in the child table references the primary key in the parent table.

The foreign key constraint prevents actions that would destroy links between the child and parent tables.

Data Integrity defines the accuracy as well as the consistency of the data stored in a database. It also defines integrity constraints to enforce business rules on the data when it is entered into an application or a database.

There are various types of joins which are used to retrieve data between the tables. There are four types of joins, namely:

Inner join: Inner Join in MySQL is the most common type of join. It is used to return all the rows from multiple tables where the join condition is satisfied.

Left Join: Left Join in MySQL is used to return all the rows from the left table but only the matching rows from the right table where the join condition is fulfilled.

Right Join: Right Join in MySQL is used to return all the rows from the right table but only the matching rows from the left table where the join condition is fulfilled.

Full Join: Full join returns all the records when there is a match in any of the tables. Therefore, it returns all the rows from the left-hand side table and all the rows from the right-hand side table.

Denormalization refers to a technique which is used to access data from higher to lower forms of a database. It helps the database managers to increase the performance of the entire infrastructure as it introduces redundancy into a table. It adds the redundant data into a table by incorporating database queries that combine data from various tables into a single table.

Entities: A person, place, or thing in the real world about which data can be stored in a database. Tables store data that represents one type of entity.

For example – A bank database has a customer table to store customer information. Customer table stores this information as a set of attributes (columns within the table) for each customer.

Relationships: Relation or links between entities that have something to do with each other. For example – The customer name is related to the customer account number and contact information, which might be in the same table. There can also be relationships between separate tables (for example, customer to accounts).

An index refers to a performance tuning method of allowing faster retrieval of records from the table. An index creates an entry for each value and hence it will be faster to retrieve data.

There are three types of index namely:

Unique Index:

This index does not allow the field to have duplicate values if the column is unique indexed. If a primary key is defined, a unique index can be applied automatically.

Clustered Index:

This index reorders the physical order of the table and searches based on the basis of key values. Each table can only have one clustered index.

Non-Clustered Index:

Non-Clustered Index does not alter the physical order of the table and maintains a logical order of the data. Each table can have many nonclustered indexes.

Normalization is the process of organizing data to avoid duplication and redundancy. Some of the advantages are:

• Better Database organization
• More Tables with smaller rows
• Efficient data access
• Greater Flexibility for Queries
• Quickly find the information
• Easier to implement Security
• Greater Flexibility for Queries
• Quickly find the information
• Easier to implement Security
• Allows easy modification
• Reduction of redundant and duplicate data
• More Compact Database
• Ensure Consistent data after modification

DROP command removes a table and it cannot be rolled back from the database whereas TRUNCATE command removes all the rows from the table.

There are many successive levels of normalization. These are called normal forms. Each consecutive normal form depends on the previous one.

The first three normal forms are usually adequate.

First Normal Form (1NF) – No repeating groups within rows

Second Normal Form (2NF) – Every non-key (supporting) column value is dependent on the whole primary key.

Third Normal Form (3NF) – Dependent solely on the primary key and no other non-key (supporting) column value.

ACID stands for Atomicity, Consistency, Isolation, Durability. It is used to ensure that the data transactions are processed reliably in a database system.

Atomicity: Atomicity refers to the transactions that are completely done or failed where transaction refers to a single logical operation of a data.

It means if one part of any transaction fails, the entire transaction fails and the database state is left unchanged.

Consistency: Consistency ensures that the data must meet all the validation rules. In simple words, you can say that your transaction never leaves the database without completing its state.

Isolation: The main goal of isolation is concurrency control.

Durability: Durability means that if a transaction has been committed, it will occur whatever may come in between such as power loss, crash or any sort of error.

Trigger in SQL is are a special type of stored procedures that are defined to execute automatically in place or after data modifications.

It allows you to execute a batch of code when an insert, update or any other query is executed against a specific table.

There are three operators available in SQL, namely:

Arithmetic Operators

Logical Operators

Comparison Operators

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A NULL value is not at all same as that of zero or a blank space. NULL value represents a value which is unavailable, unknown, assigned or not applicable whereas a zero is a number and blank space is a character.

The cross join produces the cross product or Cartesian product of two tables whereas the natural join is based on all the columns having the same name and data types in both the tables.

A subquery is a query inside another query where a query is defined to retrieve data or information back from the database. In a subquery, the outer query is called as the main query whereas the inner query is called subquery. Subqueries are always executed first and the result of the subquery is passed on to the main query. It can be nested inside a SELECT, UPDATE or any other query. A subquery can also use any comparison operators such as >,< or =.

There are two types of subquery namely, Correlated and Non-Correlated. Correlated subquery: These are queries which select the data from a table referenced in the outer query. It is not considered as an independent query as it refers to another table and refers the column in a table. Non-Correlated subquery: This query is an independent query where the output of subquery is substituted in the main query.

This statement allows conditional update or insertion of data into a table. It performs an UPDATE if a row exists, or an INSERT if the row does not exist.

SQL functions are used for the following purposes:

To perform some calculations on the data

To modify individual data items

To manipulate the output

To format dates and numbers

To convert the data types