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11. MySQL Part 2: UPDATE, ALTER, DELETE, Keys, Constraints and Users

11.3 ALTER TABLE: Add, Modify, Rename and Drop Columns

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Table banavlyavar requirement badalte – navin column, mothi size, naav badal. Tyasathi ALTER TABLE.

ALTER TABLE students ADD COLUMN phone VARCHAR(15);                    -- add at the end
ALTER TABLE students ADD COLUMN gender CHAR(1) AFTER name;            -- add after a column
ALTER TABLE students ADD COLUMN batch VARCHAR(20) FIRST;              -- add as first column
ALTER TABLE students MODIFY COLUMN phone VARCHAR(20) NOT NULL DEFAULT '';   -- change type/rules
ALTER TABLE students CHANGE COLUMN phone mobile VARCHAR(20);          -- rename + redefine
ALTER TABLE students RENAME COLUMN mobile TO contact_no;              -- rename only (MySQL 8 / MariaDB 10.5+)
ALTER TABLE students DROP COLUMN batch;                               -- remove column and its data
ALTER TABLE students ADD INDEX idx_city (city);                       -- add an index
ALTER TABLE courses RENAME TO course_list;                            -- rename table
RENAME TABLE course_list TO courses;                                  -- same, other syntax
DESCRIBE students;
Want to Use
New column ADD COLUMN
Change type, size, NULL, DEFAULT MODIFY COLUMN
Rename and change definition CHANGE COLUMN old new type
Rename only RENAME COLUMN old TO new
Remove column DROP COLUMN

Ravindra Bagale's Tip

If you shrink a column with MODIFY (VARCHAR(100) → VARCHAR(10)), long data gets cut off or you get an error – many students only find this out later. Before shrinking a column, check the longest value with SELECT MAX(LENGTH(name)) FROM students;. And take a backup before running ALTER on a live table!

Practice task

Add phone after email, change it to VARCHAR(20), rename it to mobile, then drop it. Run DESCRIBE after each step.

Extended teaching notes

Why this lesson matters

Use this lesson to strengthen systems you own: understand the risk, practise safely in a lab, then write and apply the fix.

Apply the fix across stacks

Same class of bug, same idea in every language.

Java / Spring Boot

Parameterise queries, encode output, CSRF tokens, limited Actuator, dependency scanning.

ASP.NET Core

EF Core / parameterised SQL, Razor encoding, HTTPS, Data Protection.

Node.js / Express

helmet, parameterised SQL, npm audit, avoid eval and shell concat.

Python / Django / Flask / FastAPI

ORM, autoescape, pip-audit, Bandit; never shell=True with user input.

PHP / Laravel

PDO/Eloquent, Blade escaping, composer audit.

Go / Ruby

database/sql placeholders / ActiveRecord; html/template / ERB escaping; govulncheck / Brakeman.

nginx / Apache / Tomcat

Hide versions, TLS, security headers, least-open ports.

MySQL / PostgreSQL / MongoDB / Redis

Private networks, auth on, least-privilege users, no 0.0.0.0/0.

AWS / Azure / GCP

Roles not long-lived keys, private data stores, cloud audit logs on.

Mini case study

Suppose we are on a purple-team exercise for an Indian mid-size company (sample data). Red notes a technique from this lesson against a lab clone. Blue must show the control that stops it, the log that would have fired, and the ticket that hardens production. That is the standard this course trains you for.

Defender's worksheet (do this today)

  1. Write the lesson title, date and the exact lab target you are allowed to use.
  2. List three owned systems (VMs, apps, or cloud accounts) where this lesson applies.
  3. Run one safe check from the lesson on a lab system only. Paste the command and output.
  4. Write a two-sentence fix you would give a teammate.
  5. Name one related public advisory (CERT-In, CISA, NVD, or a vendor bulletin) with its date.

Indian workplace (sample data)

Suppose we are a junior analyst at Infosys, TCS, Wipro or a fintech SOC in Bengaluru/Pune. Turn this lesson into one checklist item with an owner, a due date and a place to store evidence (ticket + screenshot).

Global practice

Compare your notes with OWASP, CIS Benchmarks, NIST CSF/IR guidance or a cloud Well-Architected security question. Write five lines on what overlaps.

Everyday analogy

Explain the risk and the fix to a family member without jargon. If they can repeat the warning, you understand it well enough to teach juniors.

Key terms

  • Control: a safeguard (patch, config, process, detection).
  • Finding: a weakness backed by evidence.
  • Remediation: the fix plus how you verified it.
  • Scope: systems you have written permission to touch.
  • Least privilege: only the access the task needs.
  • Defence in depth: several layers so one failure does not end the story.

Common mistakes

  • Skipping written permission "just for a quick scan".
  • Practising on hostel, office or public targets without authorisation.
  • Collecting tool output without writing the fix.
  • Copying attack steps onto production to "verify". Defenders verify with versions, configs and patches.
  • Publishing screenshots that contain secrets, cookies or internal hostnames.

Recap

  • Evidence, fix, owner, date.
  • Own lab only.
  • Fix > spectacle.