SQL Interview Questions – Set 04

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What is a “TRIGGER” in SQL?

  • A trigger allows you to execute a batch of SQL code when an insert, update or delete command is run against a specific table as TRIGGER is said to be the set of actions that are performed whenever commands like insert, update or delete are given through queries.
  • The trigger is said to be activated when these commands are given to the system.
  • Triggers are the particular type of stored procedures that are defined to execute automatically in place or after data modifications.
  • Triggers are generated using CREATE TRIGGER statement.

Write an SQL query to get the third maximum salary of an employee from a table named employee_table.

SELECT TOP 1 salary
FROM (
SELECT TOP 3 salary
FROM employee_table
ORDER BY salary DESC ) AS emp
ORDER BY salary ASC;

What is SQL?

SQL stands for the Structured Query Language. SQL is a standard query language used for maintaining the relational database and perform many different operations of data manipulation on the data. SQL initially was invented in 1970. It is a database language used for database creation, deletion, fetching rows and modifying rows, etc. sometimes it is pronounced as ‘sequel.’

What is the primary use of Normalization?

Normalization is mainly used to add, delete or modify a field that can be made in a single table. The primary use of Normalization is to remove redundancy and to remove the insert, delete and update distractions. Normalization breaks the table into small partitions and then link them using different relationships so that it will avoid the chances of redundancy.

What are the different types of joins in SQL?

used to merge two tables or retrieve data from tables. It depends on the relationship between tables.

Following are the most commonly used joins in SQL:

Inner Join: inner joins are of three type:

  • Theta join
  • Natural join
  • Equijoin

Outer Join: outer joins are of three type:

  • right outer join
  • Left outer join
  • Full outer join

What is a Data Manipulation Language?

Data manipulation language makes the user able to retrieve and manipulate data. It is used to perform the following operations.

  • Insert data into database through INSERT command.
  • Retrieve data from the database through SELECT command.
  • Update data in the database through UPDATE command.
  • Delete data from the database through DELETE command.

What is view in SQL?

A view is a virtual table which contains a subset of data within a table. Views are not originally present, and it takes less space to store. A view can have data from one or more tables combined, and it depends on the relationship. Views are used to apply security mechanism in the SQL Server. The view of the database is the searchable object we can use a query to search the view as we use for the table.

What is the difference between primary key and unique key?

Primary key and unique key both are the essential constraints of the SQL, but there is a small difference between them

Primary key carries unique value but the field of the primary key cannot be Null on the other hand unique key also carry unique value but it can have a single Null value field.

What is the difference between SQL, MySQL and SQL Server?

SQL or Structured Query Language is a language which is used to communicate with a relational database. It provides a way to manipulate and create databases. On the other hand, MySQL and Microsoft’s SQL Server both are relational database management systems that use SQL as their standard relational database language.

MySQL is available for free as it is open source whereas SQL server is not an open source software.

Which are the different character-manipulation functions in SQL?

  • CONCAT: join two or more values together.
  • SUBSTR: used to extract the string of specific length.
  • LENGTH: return the length of the string in numerical value.
  • INSTR: find the exact numeric position of a specified character.
  • LPAD: padding of the left-side character value for right-justified value.
  • RPAD: padding of right-side character value for left-justified value.
  • TRIM: remove all the defined character from the beginning, end or both beginning and end.
  • REPLACE: replace a specific sequence of character with other sequences of character.