SQL INSERT, UPDATE, and DELETE Explained

Quick answer

INSERT adds new rows. UPDATE changes existing rows. DELETE removes rows.

These are the main SQL commands used to modify data inside a table. They are essential, but they also require more caution than a normal SELECT query because they change the database.

In this guide

Sample table

Imagine you have a table called employees:

How INSERT works

INSERT adds a new row to a table.

INSERT INTO employees (employee_id, name, department, salary, city)
VALUES (4, 'Dana', 'IT', 85000, 'Denver');

That query adds a brand-new employee row for Dana.

Why listing columns is a good habit

You can sometimes write an INSERT without explicitly listing column names, but beginners should avoid that. Listing the columns makes the query easier to read and much safer if the table structure changes later.

What the table would look like after the INSERT

How UPDATE works

UPDATE changes values in existing rows.

UPDATE employees
SET city = 'Austin'
WHERE employee_id = 2;

This changes Ben’s city from Dallas to Austin.

You can update other columns too

UPDATE employees
SET salary = 60000
WHERE name = 'Chris';

This changes Chris’s salary to 60000.

You can update multiple columns at once

UPDATE employees
SET department = 'Finance',
    city = 'Boston'
WHERE employee_id = 1;

How DELETE works

DELETE removes rows from a table.

DELETE FROM employees
WHERE employee_id = 3;

This removes Chris’s row from the table.

Unlike UPDATE, which changes the values inside a row, DELETE removes the whole row entirely.

Why WHERE matters

The WHERE clause tells SQL which rows should be updated or deleted. Without it, SQL may apply the command to every row in the table.

Dangerous UPDATE example

UPDATE employees
SET city = 'Miami';

That would change the city for every employee.

Dangerous DELETE example

DELETE FROM employees;

That would delete every row in the table.

Safe workflow before changing data

A smart habit is to preview the rows first with a SELECT query using the same filter you plan to use in your UPDATE or DELETE.

SELECT *
FROM employees
WHERE employee_id = 2;

If that preview shows the exact row you want, then you can safely move on to:

UPDATE employees
SET city = 'Austin'
WHERE employee_id = 2;

Common mistakes

• Forgetting the WHERE clause in an UPDATE
• Forgetting the WHERE clause in a DELETE
• Inserting values in the wrong column order
• Trying to insert a duplicate primary key value
• Updating more rows than intended because the filter was too broad
• Deleting a row when you really only meant to change one column

Practice questions

Bottom line

INSERT adds rows. UPDATE changes rows. DELETE removes rows.

These commands are core SQL skills, but they should always be used carefully because they change the real data in your tables.

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