Mastering SQL Concepts

What is the purpose of SQL commands?

• The purpose of SQL commands is to manage and manipulate relational databases.

• To store data in non-relational formats.

• To create graphical user interfaces for applications.

• To perform calculations on numerical data.

Define a primary key constraint in SQL.

• A primary key constraint allows duplicate values in a table.

• A primary key constraint uniquely identifies each record in a table and ensures that no two records have the same value in the primary key column(s).

• A primary key constraint can be null in the primary key column(s).

• A primary key constraint is used to define foreign keys in other tables.

What is the difference between INNER JOIN and OUTER JOIN?

• INNER JOIN returns matching rows; OUTER JOIN returns all rows with NULLs for non-matches.

• INNER JOIN returns all rows; OUTER JOIN returns only matching rows.

• INNER JOIN includes NULLs for non-matches; OUTER JOIN does not.

• INNER JOIN and OUTER JOIN are the same and return all rows.

Explain the use of UNION in set operations.

• UNION is used to find the intersection of two sets.

• The UNION operation only combines sets with identical elements.

• The UNION operation removes all duplicate elements from a single set.

• The UNION operation combines multiple sets into one, including only unique elements.

What is a subquery and how is it used?

• A subquery is a type of database index.

• A subquery is a method for optimizing database performance.

• A subquery is a command to delete data from a table.

• A subquery is a nested query used to retrieve data for the main query.

How do you create a view in SQL?

• CREATE VIEW view_name AS SELECT column1, column2 FROM table_name WHERE condition;

• ALTER VIEW view_name SET column1 = value;

• CREATE TABLE view_name AS SELECT column1, column2 FROM table_name;

• DROP VIEW view_name;

What is PL/SQL and how does it differ from SQL?

• SQL is a programming language that supports procedural logic.

• PL/SQL is a database management system similar to SQL.

• PL/SQL is a procedural extension of SQL that allows for complex programming, while SQL is a declarative language for data manipulation.

• PL/SQL is a markup language used for web development.

Describe the function of triggers in a database.

• Triggers automatically execute predefined actions in response to specific events on a database table or view.

• Triggers store data permanently in the database.

• Triggers are used to create user interfaces in databases.

• Triggers are only applicable to read operations in a database.

What are cursors in PL/SQL?

• Cursors are only for storing static data in PL/SQL.

• Cursors are a type of variable in PL/SQL that cannot be used for data retrieval.

• Cursors are used to create tables in PL/SQL.

• Cursors are database objects in PL/SQL used to retrieve and manipulate data row by row.

How do you enforce a foreign key constraint?

• Define a foreign key in the child table that references the primary key of the parent table.

• Create a primary key in the child table that references the foreign key of the parent table.

• Remove the foreign key from the child table entirely.

• Use a unique constraint in the parent table instead of a foreign key.