Top Interview questions on SQL for 1 years of experience

Introduction:

Top Interview questions on SQL for 1 years of experience

1. What is the difference between the WHERE and HAVING clauses in SQL?

The WHERE and HAVING clauses are both used to filter data in SQL, but they operate on different parts of the query.

The WHERE clause is used to filter rows based on a condition that is applied to each individual row. It is used with the SELECT, UPDATE, and DELETE statements. The WHERE clause is used in the initial filtering of rows, and it is applied before any grouping or aggregation takes place. It operates on individual rows, evaluating each row to determine whether it should be included in the result set.

The HAVING clause, on the other hand, is used to filter the results of a GROUP BY clause based on a condition that is applied to groups of rows. It is used with the SELECT statement and is applied after the grouping and aggregation has taken place. The HAVING clause operates on groups of rows, evaluating each group to determine whether it should be included in the result set.

2. Explain the difference between UNION and UNION ALL.

UNION:

The UNION operator combines the result sets of two or more SELECT statements into a single result set.
It eliminates duplicate rows from the final result set.
The column names and data types of the corresponding columns in the SELECT statements must be the same.

UNION ALL:

The UNION ALL operator also combines the result sets of two or more SELECT statements into a single result set.
It includes all rows from each SELECT statement, including duplicates.
The column names and data types of the corresponding columns in the SELECT statements must be the same.

3. What is a subquery, and how can it be used in SQL?

A subquery, also known as a nested query, is a SQL statement that is used within another SQL statement. The subquery is enclosed in parentheses and is usually used within the WHERE or HAVING clauses of the main query.

Subqueries can be used in a variety of ways in SQL, including:

Filtering data: Subqueries can be used to filter data based on specific conditions. For example, a subquery can be used to retrieve all customers who have placed orders in the last 30 days.

Joining tables: Subqueries can be used to join tables that cannot be joined directly. For example, a subquery can be used to join two tables based on a common field.

4. What is a join, and what are the different types of joins available in SQL?

A join is a SQL operation that combines data from two or more tables based on a common column between them. A join operation allows you to retrieve data from multiple tables in a single query, and is one of the fundamental operations in SQL.

There are several types of joins available in SQL, including:

Inner join: An inner join returns only the rows that have matching values in both tables being joined. This is the most common type of join used in SQL.

Left join: A left join returns all the rows from the left table and the matching rows from the right table. If there is no match in the right table, the result will contain NULL values for those columns.

Right join: A right join returns all the rows from the right table and the matching rows from the left table. If there is no match in the left table, the result will contain NULL values for those columns.

Full outer join: A full outer join returns all the rows from both tables, including the rows that do not have matching values in the other table. If there is no match in one of the tables, the result will contain NULL values for those columns.

Cross join: A cross join returns the Cartesian product of the two tables being joined. This means that each row from the first table is paired with every row from the second table.

5. What is a view in SQL, and how can it be useful?

A view in SQL is a virtual table that is based on the result of a SELECT query. Views can be thought of as a saved SELECT statement that can be reused in other queries.

When a view is created, it stores the SELECT statement used to define the view as well as the resulting data. Any subsequent queries against the view will execute the underlying SELECT statement and return the data stored in the view.

Views can be useful in a number of ways, including:

Simplifying complex queries: Views can be used to simplify complex queries by breaking them down into smaller, more manageable pieces.
Enhancing security: Views can be used to limit access to sensitive data by only allowing users to view specific columns or rows of a table.
Improving performance: Views can improve query performance by pre-aggregating data, joining tables, and reducing the amount of data returned by a query.
Enforcing business rules: Views can be used to enforce business rules by limiting the types of data that can be returned by a query.

6. What is a stored procedure, and how can it be used in SQL?

A stored procedure in SQL is a set of pre-written SQL statements that are stored in a database and can be executed by calling the procedure. Stored procedures are typically used to perform complex database operations, such as querying multiple tables, updating data in multiple tables, or performing calculations on data.

Stored procedures can be used in a number of ways, including:

Improving performance: Stored procedures can improve database performance by reducing network traffic and optimizing database access.
Simplifying application development: By encapsulating complex database operations in stored procedures, developers can simplify application development and maintenance.

Enhancing security: Stored procedures can be used to control access to sensitive data by limiting the types of operations that can be performed on the data.
Promoting code reusability: Stored procedures can be reused across multiple applications, reducing the amount of code that needs to be written and maintained.

7. How do you use the GROUP BY clause in SQL, and what does it do?

The GROUP BY clause is a SQL statement that is used to group rows in a table based on the values of one or more columns. When used with an aggregate function, such as SUM, AVG, MAX, MIN, or COUNT, the GROUP BY clause calculates summary statistics for each group.

Here is an example of how to use the GROUP BY clause in SQL:

SELECT column1, column2, SUM(column3)
FROM table_name
GROUP BY column1, column2;

In this example, the columns column1 and column2 are used to group the rows in the table table_name, and the SUM() function is used to calculate the sum of column3 for each group.

8. What is normalization, and why is it important in database design?

Normalization is the process of organizing data in a database in such a way that it reduces data redundancy and improves data integrity. It involves breaking down large tables into smaller ones and establishing relationships between them to ensure data consistency and avoid data anomalies.

Normalization follows a set of rules called normal forms, which are progressively stricter as the level increases. The most commonly used normal forms are:

First Normal Form (1NF): Each table cell should contain only atomic values, meaning that it cannot be further divided into smaller pieces of data.
Second Normal Form (2NF): Every non-key attribute of the table must depend on the entire primary key and not just a part of it.
Third Normal Form (3NF): Every non-key attribute of the table must be independent of every other non-key attribute.

9. How do you handle NULL values in SQL queries?

In SQL, NULL values represent missing or unknown data. It is important to handle NULL values in SQL queries to avoid unexpected results and ensure accurate data analysis. Here are some common ways to handle NULL values in SQL queries:

IS NULL and IS NOT NULL: You can use the IS NULL and IS NOT NULL operators to check if a value is NULL or not. For example, to select all records where the “name” field is NULL, you can use the following query:

SELECT * FROM table WHERE name IS NULL;

COALESCE function: The COALESCE function returns the first non-NULL value from a list of expressions. For example, to select the “name” field and use “Unknown” as the default value if the field is NULL, you can use the following query:

SELECT COALESCE(name, ‘Unknown’) AS name FROM table;

NULLIF function: The NULLIF function returns NULL if two expressions are equal, or the first expression if they are not equal. This function can be useful for replacing a specific value with NULL. For example, to replace all occurrences of “N/A” with NULL in the “age” field, you can use the following query:

SELECT NULLIF(age, ‘N/A’) AS age FROM table;

10. What is the purpose of the ORDER BY clause in SQL?

The ORDER BY clause in SQL is used to sort the result set of a query based on one or more columns. It allows the user to specify the order in which the rows should be returned. The default order is ascending, but it can be changed to descending by using the DESC keyword.

For example, consider the following SQL query:

SELECT name, age
FROM students
ORDER BY age DESC;

This query will return the name and age of all students in the table “students,” sorted by age in descending order. The result set will show the oldest students first, followed by the younger ones