Whether you are building a backend of a web application or working with huge datasets, SQL always remain one of the most vital tools for communicating with the databases. It allows you to access, update, and manage the data efficiently, that makes everyday development tasks much faster and well-organized. In this blog, we will walk through 10 SQL queries that every developer, whether you are just starting out or experienced already, must know confidently.
Who should read it?
SQL is language of database and having its knowledge is crucial for better database management. So, if you are using database in your daily work then you must read this blog till the end. This blog is good to read for all professionals from the industries like software development, web development, data science, machine learning, etc. Even if you are a student who is attending training under a data analytics course or web development course, this blog is for you.
So, dear data analysts, web developers, software developers, and individuals interested in database administration, let’s get ready to know SQL queries.
What is SQL?
SQL known as “Structured Query Language”. This programming language is capable of handling interactions with various types of relational databases.
With the rise of digitalization and massive data production, it is crucial to manage and manipulate data in a well-organized manner to obtain underlying information.
Essential SQL Queries Every Developer Must Know
Let’s start with this blog post on SQL Queries to understand what they are, including 10 essential questions.
So, now here we are getting started:
Query #1 | SELECT Statement
The SELECT Query is a powerful query that fetches data from multiple tables easily. There are various applications which use this query, one way is below:
Example of SELECT Statement:
SELECT empName, empAddress, empSalary FROM employee_details;
Explanation: This query fetches the employee’s name, employee’s address, and employee’s salary of all employees exists in the employee_details table.
When to use SELECT Statement:
When your task is to fetch any type of information that exist in one or more tables of a database.
Best Practices:
Always try to fetch specific records or the content you desire, rather than fetching all records with all columns for better performance.
Example:
SELECT name, age FROM students;
Explanation: It retrieves only name, age columns from student table.
Query #2 | WHERE Statement
The WHERE Statement is used to help SELECT Statement in filtering records, so that only desired results are being fetched using Operators, and different types of Clauses.
Example of WHERE Statement:
SELECT salary FROM employees_table WHERE deptName like 'S%';
Explanation: This query fetches the salaries of those employees whose department name’s first letter is ‘S’.
When to use WHERE Statement:
You should use WHERE Statement when you want to fetch specific records based on some condition. Also, you can use different type of relational operators like <, >, <=, <=; and clauses like BETWEEN, AND, OR, IN, etc.
Best Practices: It is recommended not to use WHERE Statement with aggregate functions such as AVG, MIN, MAX, SUM, COUNT, etc.
Query #3 | INSERT INTO Statement
The INSERT INTO statement is used to add new entries to a table. It allows you to populate your database with fresh data, such as new users, products, or any form of structured information.
Example of INSERT INTO Statement:
INSERT INTO employees_table (empName, deptName, empPosition) VALUES ('John Doe', 'Sales', 'Sales Manager');
Explanation: This query inserts a record in a table named employees_table specifically in three columns named “empName, deptName, and empPosition” with record values “John Doe”, “Sales”, and “Sales Manager”.
When to use INSERT INTO Statement:
You can use INSERT INTO when you’re storing new data into tables in a database like during user registration or when adding new product items to an inventory.
Best Practice:
- Always be careful of writing correct type of values in the VALUES parentheses (), which should match the type of their associated column.
- If inserting into all columns, you can skip specifying them.
Example:
INSERT INTO product_stocks VALUES (1, ‘Peanut Butter’, ‘Pure Farms’, 259, ’22-04-2025’);
Query #4 | UPDATE Statement
UPDATE Statement is an important query that modifies the existing records added in a table of a database. It let you to change values of attributes/columns that meet specific condition with the help of WHERE Statement.
Example of UPDATE Statement:
UPDATE customers_table SET cust_name=”Sachin Aggarwal” WHERE apmt_id = 2049;
This query modifies the customer’s name to “Sachin Aggarwal” whose appointment id matches “2049” in table “customers_table”.
When to use UPDATE Statement:
You can use this UPDATE statement when you want to make corrections to any of your records in a table of a database.
Best Practices:
- Always use WHERE clause with UPDATE statement to avoid making changes global.
Query #5 | DELETE Statement
The Delete Statement is another necessary query, that is used to delete specific records from a table.
Example of DELETE Statement:
DELETE from departments_table where depName = “Finance’;
Explanation: This query deletes all records from table “departments_table” whose department name matches to department “Finance”.
When to use DELETE Statement:
You can use it when you want one or more records from a table being deleted from a table.
Best Practices: It is recommended that if you want to empty your table, then simply use TRUNCATE statement instead of DELETE statement.
Query #6 | JOIN Clause
The JOIN clause helps in combining rows from different tables with similar column. It is essential to pull all connected data from relational databases.
Example of JOIN Clause:
SELECT e.emp_name, e.salary, d.dep_name FROM employees e JOIN departments WHERE e.dep_id = d.dep_id;
Explanation: This query will fetch all employee names, their salary, and their associated department name, by matching their relative department id that is common among both employees and departments table.
Types of Joins:
- INNER JOIN: This type of JOIN can fetch only matching records from both the tables.
- LEFT JOIN: This type of JOIN can fetch all the records from the leftward table, and all the matching records from the rightward table.
- RIGHT JOIN: This type of JOIN can fetch all the records from the rightward table, and the matching records from the leftward table.
- FULL OUTER JOIN: This type of JOIN can fetch all the matched records either of the tables.
When to use JOIN Clause:
You can use different types of JOINS if your tables are normalized and all tables has a common relationship attribute to other table, we using JOIN with.
Best Practices:
- Always use table aliasing for better readability of query.
- Always try to understand, what is your problem, and what type of join suits best to solve your problem.
QUERY #7 | GROUP BY Statement
Group By Statement is used to group the records that have similarities based on a specific column, and aggregate functions are used commonly to perform calculations on each group.
We use this query with aggregate functions like avg, max, min, sum, count, etc.
Example of GROUP BY Statement:
SELECT deptName, COUNT(*) AS ‘Total Number of Employees’ from employees_table GROUP BY deptName;
Explanation: This query will find out that how many employees work in each department.
When to use GROUP BY Statement:
You can use it when you want to group rows based on similar type of values in a column.
Best Practices: Use Having Clause to filter the records instead of WHERE, because WHERE is not likely to filter with aggregate functions.
Example:
SELECT deptName, COUNT(*) AS ‘Total Number of Employees’ FROM employees_table GROUP BY deptName HAVING COUNT(*) > 5;
Explanation: This query will fetch only those department names who have more than 5 employees.
Query #8 | Order By Statement
Order By statement helps us to bring our data in either ascending or descending order.
By default, this statement meant to arrange in ascending order but you can arrange in descending order using “DESC” clause.
Example of Order By Statement:
SELECT st_name, course_name, roll_no FROM students_table ORDER BY st_marks DESC;
Explanation: This query will fetch all the student’s name, course name, and rollno from table “students_table” and arrange it in descending order.
When to use Order By Statement:
You can use ORDER BY statement when you to arrange your records in specific order like if you want to show employee records in descending order based on their salary, etc.
Best Practices: Use it after WHERE Statement or SELECT statement if there is no WHERE statement to be used, and always arrange based on specific column.
Query #9 | LIMIT Clause
The LIMIT clause is used restrict or limit the number of records to be fetched by a query from a table.
Example of LIMIT Clause:
SELECT empId, empName, max(empSalary) AS highest_salary FROM employees_table GROUP BY empId, empName LIMIT 1;
Explanation: This query will display the empId, empName, and empSalary of the employee from the table named “employees_table” who has the highest salary.
When to use LIMIT Clause:
You can use LIMIT clause to limit the number or records being fetched.
Best practices: In MySQL and PostgreSQL, you can use LIMIT but while using SQL Server, you should use TOP clause instead.
Query #10 | Sub Query
The sub query or a nested query is written inside another query, it is used to use the result of another query to make data more filtered.
Example of Sub Query:
SELECT thrName, thrSalary FROM teachers_table WHERE thrSalary > (SELECT AVG(thrSalary) FROM teachers_table);
Explanation: This query will fetch the thrName, and thrSalary from table name “teachers_table” whose thrSalary is greater than the average salary in that table.
When to use Sub Query:
You can use it when you have a dynamic type of condition to solve your problem.
Best practices: Always test sub-queries independently before placing them inside other query, so it can give an expected result.
Our Conclusion
So, mastering SQL is not just about memorizing every command but it’s about understanding the core queries that let you work with data smoothly and effectively. Join our master level SQL course in Delhito practice all the 10 queries, we just covered in this blog. Remember, these are the building blocks of real-world database tasks and you should set your hands on them.
