The Ultimate SQL Data Retrieval Guide (6)

 

Welcome back to another post of the SQL learning series and community! In this post, I will share with you some of the best resources for learning data retrieval in SQL, which is one of the most essential skills for any beginner data analyst, data scientist, database developer, or student.

SQL stands for Structured Query Language, which is a standard language for accessing and manipulating data in relational databases. SQL allows you to perform various operations on data, such as selecting, inserting, updating, deleting, creating, and modifying tables, views, indexes, and other objects.

One of the most common and important tasks in SQL is data retrieval, which means fetching data from one or more tables based on some criteria. Data retrieval can be done using various clauses and keywords in SQL, such as SELECT, FROM, WHERE, ORDER BY, GROUP BY, HAVING, and JOIN. In this post, I will focus on five topics that are essential for data retrieval in SQL:

Aggregate Functions:

Aggregate functions are functions that take a set of values as input and return a single value as output. They are used to perform calculations on data, such as finding the sum, average, count, minimum, maximum, etc. of a column or a group of columns. Some of the common aggregate functions in SQL are:

• SUM: returns the sum of all values in a column or a group of columns.
• AVG: returns the average of all values in a column or a group of columns.
• COUNT: returns the number of values in a column or a group of columns.
• MIN: returns the minimum value in a column or a group of columns.
• MAX: returns the maximum value in a column or a group of columns.

For example, if we have a table called sales that stores the sales data of a company, we can use aggregate functions to find the total, average, and maximum sales amount for each product category, as shown below:

The output of this query would look something like this:

GROUP BY Clause:

The GROUP BY clause is used to divide the rows in a table into groups based on one or more columns. It is often used with aggregate functions to perform calculations on each group separately. The GROUP BY clause comes after the FROM clause and before the WHERE clause in a SELECT statement.

For example, if we want to find the number of customers in each city from the customers table, we can use the GROUP BY clause as follows:

The output of this query would look something like this:

HAVING Clause:

The HAVING clause is used to filter the groups based on some condition. It is similar to the WHERE clause, but it applies to the groups rather than the individual rows. The HAVING clause comes after the GROUP BY clause and before the ORDER BY clause in a SELECT statement.

For example, if we want to find the product categories that have more than 10 sales from the sales table, we can use the HAVING clause as follows:

The output of this query would look something like this:

JOINS

JOINS are used to combine data from two or more tables based on some common column or condition. They are useful when we want to retrieve data from multiple related tables in a single query. There are four types of JOINS in SQL:

• INNER JOIN: returns only the rows that match in both tables.
• LEFT JOIN: returns all the rows from the left table and the matching rows from the right table. If there is no match, the right table columns are filled with NULL values.
• RIGHT JOIN: returns all the rows from the right table and the matching rows from the left table. If there is no match, the left table columns are filled with NULL values.
• FULL JOIN: returns all the rows from both tables, regardless of whether they match or not. If there is no match, the missing table columns are filled with NULL values.

For example, if we have two tables called customers and orders that store the customer and order details of a company, we can use JOINS to find the customer name, order date, and order amount for each order, as shown below:

The output of this query would look something like this:

Subqueries:

Subqueries are queries that are nested inside another query. They are used to provide data or filter data for the outer query. Subqueries can be used in various places in a SQL statement, such as in the SELECT, FROM, WHERE, HAVING, or ORDER BY clauses.

For example, if we want to find the product categories that have the highest sales amount from the sales table, we can use a subquery as follows:

The output of this query would look something like this:

Conclusion

In this post, I have explained some of the key concepts and techniques of data retrieval in SQL, such as aggregate functions, GROUP BY clause, HAVING clause, JOINS, and subqueries. I hope you have learned something new and useful from this post. If you want to learn more about SQL and data retrieval, you can check out the following resources:

1.LearnSQL: A comprehensive online course that covers everything from the basics to the advanced topics of SQL. You can practice and test your skills with interactive exercises and quizzes. LearnSQL.com

2.designgurus.io: A platform that offers online interview preparation and mock interviews for SQL/No SQL and other technical skills. You can get feedback and tips from experts and improve your chances of landing your dream job. Check out here:

1. Grokking the Coding Interview link
2. All course link
3. DesignGurus.io

3.3.SQL for Students: Comments below “send book” and get a free eBook “SQL for Students” that I wrote to help students or beginners simply learn SQL. You can get it here so start comments below!

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“Data is the new oil. SQL is the new drill.”