👉 When MySQL is installed, it automatically creates some databases. These databases are not for your application data. They are used by MySQL itself to work properly.

When you run:

SHOW DATABASES;

You usually see something like:

information_schema
mysql
performance_schema
sys

Let’s understand each one clearly.

👉 A read-only system database that stores metadata (Data about your databases)

• List of databases
• List of tables
• List of columns
• Index information
• Constraints

📌 Example:

SELECT table_name
FROM information_schema.tables
WHERE table_schema = 'sakila';

• ❌ You cannot insert data
• ❌ You cannot delete tables
• ✅ You only query it

🧠 Think of it as MySQL’s “dictionary”

👉 The most critical internal database It stores user accounts and permissions.

• Users and passwords
• Privileges
• Roles
• Authentication info

📌 Example:

SELECT user, host FROM mysql.user;

• ⚠️ Do NOT modify tables manually
• ❌ Do not store application data here
• 🔐 Used by MySQL for security

🧠 Think of it as MySQL’s “security office”

👉 A database used to monitor MySQL performance.

• Slow queries
• Locks
• Threads
• Memory usage
• Query execution stats

📌 Example:

SELECT * FROM performance_schema.events_statements_summary_by_digest;

• Used by DBAs
• Helps tune performance
• Not required for daily app work

🧠 Think of it as MySQL’s “monitoring dashboard”

👉 A helper database built on top of performance_schema Provides easy-to-read views.

performance_schema tables are:

• Large
• Complex
• Hard to query

sys simplifies them.

SELECT * FROM sys.user_summary;

This gives human-readable performance info.

• Contains views, not real tables
• Safe to query
• Meant for diagnostics

🧠 Think of it as MySQL’s “report generator”

👉 In your own database

Example:

CREATE DATABASE app_db;
USE app_db;

Then create tables there.

❌ Creating tables in mysql ❌ Deleting sys database ❌ Trying to insert rows into information_schema ❌ Treating system databases like normal databases

System databases are for MySQL User databases are for YOU