5.3 DQL — SELECT

Level: 5 | Version: v1.1 | Author: Meptrasoft

Overview

SELECT is used to retrieve data from a database.

It can return all columns, specific columns, or only rows that match a condition.

SELECT
  ↓
FROM
  ↓
WHERE
  ↓
RESULT

SELECT = Retrieve the data you need.

SQL SELECT — How a Query Retrieves Data: Source Table, SQL Query (SELECT, FROM, WHERE), and Result — Figure 1: A SELECT query chooses data from a table and can filter the rows returned.

Practice Table

This topic reuses the employees table from 5.2 DDL.

emp_id	emp_name	department	salary
1	Ravi Kumar	IT	45000.00
2	Anjali Mehta	HR	38000.00
3	Suresh Rao	Finance	42000.00
4	Priya Nair	IT	50000.00

Basic SELECT

To retrieve all columns:

SELECT *
FROM employees;

* means all columns.

SELECT Specific Columns

You can choose only the columns you need:

SELECT emp_name, department
FROM employees;

Result:

emp_name	department
Ravi Kumar	IT
Anjali Mehta	HR
Suresh Rao	Finance
Priya Nair	IT

This is usually clearer than selecting unnecessary columns.

SELECT with WHERE

WHERE is used to filter rows.

SELECT *
FROM employees
WHERE department = 'IT';

Result:

emp_id	emp_name	department	salary
1	Ravi Kumar	IT	45000.00
4	Priya Nair	IT	50000.00

Simple Idea

FROM   → Which table?
SELECT → Which columns?
WHERE  → Which rows?

Logical Query Processing Order

The SQL statement is written in one order, but the database conceptually processes its clauses in a different logical order.

A simplified order is:

FROM / JOIN
      ↓
WHERE
      ↓
GROUP BY
      ↓
HAVING
      ↓
SELECT
      ↓
ORDER BY
      ↓
LIMIT

For example, the database first determines the source rows, applies filtering, performs grouping when required, and then produces the selected output.

The order you write SQL and the logical order used to evaluate it are not always the same.

Note: The exact physical execution may differ because the optimizer can choose a different execution plan.

Common Beginner Mistakes

✗ Mistake	✓ Correct Understanding
`SELECT *` means select all tables	It means all columns from the selected source rows
`WHERE` chooses columns	`WHERE` filters rows
`FROM` filters the result	`FROM` identifies the source table or tables
`SELECT` always returns every row	`WHERE` or other clauses can reduce the returned rows
SQL always executes exactly in the written order	SQL has a logical processing order, and the optimizer chooses the physical execution plan
`SELECT` changes stored data	`SELECT` normally retrieves data without modifying it

Placement Quick Points

SELECT
→ CHOOSE COLUMNS

FROM
→ CHOOSE SOURCE

WHERE
→ FILTER ROWS

SELECT retrieves data.
FROM identifies the source table or tables.
WHERE filters rows.
* means all selected columns.
You can select only the columns needed for the result.
SQL has a logical query-processing order that differs from the written order.

Interview Questions

What is SELECT?

SELECT is used to retrieve data from a database.

What is the purpose of FROM?

FROM identifies the table or other data source from which the query retrieves data.

What is the purpose of WHERE?

WHERE filters rows based on a condition.

What is the difference between SELECT and WHERE?

SELECT → Which columns?
WHERE  → Which rows?

What is the logical order of processing a query?

A simplified order is:

FROM → WHERE → GROUP BY → HAVING → SELECT → ORDER BY → LIMIT

Does SELECT modify data?

Normally, no. SELECT retrieves data without changing the stored rows.

Practice & Hands-On Exercises

Using the employees table:

Display all columns:

SELECT * FROM employees;

Display only emp_name and salary:

SELECT emp_name, salary FROM employees;

Display employees from the IT department:

SELECT * FROM employees WHERE department = 'IT';

Display employees whose salary is greater than 40000:

SELECT * FROM employees WHERE salary > 40000;

Display only emp_name for employees in HR:

SELECT emp_name FROM employees WHERE department = 'HR';

Explain the difference between SELECT * and selecting specific columns.
Identify the roles of SELECT, FROM, and WHERE in a query.

💡 Tip: Test your queries using the in-browser interactive runner above.

Key Takeaway

SELECT → COLUMNS
FROM   → SOURCE
WHERE  → ROWS TO KEEP

SELECT is used to retrieve data, while FROM identifies the source and WHERE filters the rows.

End of DQL — SELECT