SQL Interview Questions For Fresher
Q1. What is the difference between DELETE and TRUNCATE statements?
DELETE vs TRUNCATE
Delete command is used to delete a row in a table. Truncate is used to delete all the rows from a table.
You can rollback data after using delete statement. You cannot rollback data.
It is a DML command. It is a DDL command.
It is slower than truncate statement. It is faster.
Q2. What are the different subsets of SQL?
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.
Q3. What do you mean by DBMS? What are its different types?
Database – SQL Interview Questions – EdurekaA 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
Q4. What do you mean by table and field in SQL?
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:
Field: Stu Id, Stu Name, Stu Marks
Q5. What are joins in SQL?
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:
Q6. What is the difference between CHAR and VARCHAR2 datatype in SQL?
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.
Q7. What is a Primary key?
Table – SQL Interview Questions – EdurekaA 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.
Q8. What are Constraints?
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:
Q9. What is the difference between SQL and MySQL?
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.
Q10. What is a Unique key?
Uniquely identifies a single row in the table.
Multiple values allowed per table.
Null values allowed.
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