Structured Query Language (SQL) is a programming language used to communicate with and manipulate data in a relational database management system (RDBMS). RDBMSs are the foundation of many modern database applications, and SQL is the lingua franca of data access and manipulation.

What is SQL used for?

SQL is used for a wide variety of tasks, including:

  • Data definition: Creating and modifying database tables, columns, and constraints.
  • Data manipulation: Inserting, updating, deleting, and querying data.
  • Data control: Granting and revoking permissions to access and modify data.
  • Data analysis: Aggregating and summarizing data to gain insights.

Application development: Building database-driven applications that allow users to interact with and manage data.

SQL basics

SQL statements are typically composed of three clauses:

  • SELECT: This clause specifies which columns of data to return.
  • FROM: This clause specifies the table(s) from which to retrieve the data.
  • WHERE: This clause specifies the conditions that the data must meet to be returned.

These are the basic clauses, but there are many other clauses that can be used to perform more complex operations, such as joining tables, grouping data, and aggregating data.

Here is an example of a more complex SQL statement:

SELECT customer_name, SUM(order_total) AS total_spent

FROM customers

INNER JOIN orders ON customers.id = orders.customer_id

GROUP BY customer_name

HAVING total_spent > 1000;

This statement will return the names of all customers who have spent more than $1000, along with the total amount of money they have spent. It uses the following clauses:

  • SELECT: Specifies the columns to return, which are the customer name and total amount spent.
  • FROM: Specifies the tables to join, which are the customers table and the orders table.
  • INNER JOIN: Joins the two tables on the customer_id column.
  • GROUP BY: Groups the data by customer name.
  • HAVING: Filters the results to only include customers who have spent more than $1000.

SQL operators

SQL operators are used to compare and manipulate data. The most common operators are:

  • Arithmetic operators: +, -, *, /, %
  • Comparison operators: =, !=, <, >, <=, >=
  • Logical operators: AND, OR, NOT

SQL operators can be used in expressions to create conditions for WHERE clauses, JOIN clauses, and other SQL statements.

SQL functions

SQL functions are built-in functions that can be used to perform various operations on data. Some common SQL functions include:

  • Aggregate functions: AVG(), COUNT(), MAX(), MIN(), SUM()
  • String functions: CONCAT(), LEFT(), LOWER(), RIGHT(), UPPER()
  • Date and time functions: CURRENT_DATE(), CURRENT_TIME(), YEAR(), MONTH(), DAY()

SQL functions can be used in expressions to perform complex operations on data, such as calculating the average age of customers or finding the date of the most recent order.

SQL subqueries

SQL subqueries are nested SQL statements that can be used to perform complex queries. Subqueries can be used in the SELECT, WHERE, and HAVING clauses of other SQL statements.

For example, the following SQL statement uses a subquery to find the most popular product among customers:

SELECT product_name

FROM products

WHERE product_id = (SELECT MAX(product_id) FROM orders);

This statement will return the name of the product that has been ordered the most times.

SQL joins

SQL joins are used to combine data from two or more tables. The most common joins are:

  • INNER JOIN: Returns all rows from both tables where the join condition is met.
  • LEFT JOIN: Returns all rows from the left table, even if there are no matching rows in the right table.
  • RIGHT JOIN: Returns all rows from the right table, even if there are no matching rows in the left table.
  • FULL JOIN: Returns all rows from both tables, even if there are no matching rows.

SQL joins can be used to perform complex queries, such as finding all customers who have not placed an order in the past 6 months or finding all products that have not been sold in the past year.

Conclusion

SQL is a powerful language for accessing and manipulating data in relational databases. It is used by millions of people around the world to build and maintain data-driven applications. By learning SQL, you can unlock a world of data and gain insights that can help you make better decisions.

Related Articles:

 

Author: Sana Ghani

Sana Ghani is currently working as a content writer. She is hardworking and looks forward to providing the best quality content for her clients. Click here for LinkedIn Profile.