Arjun learns to read error messages

ERROR 1045 (28000): Access denied for user 'appuser'@'localhost' (using password: YES)

This means: wrong password, user does not exist, or user does not have permission.

-- Check if user exists:
SELECT user, host FROM mysql.user WHERE user = 'appuser';

-- Reset password:
ALTER USER 'appuser'@'localhost' IDENTIFIED BY 'new_password';

-- Grant permissions:
GRANT ALL PRIVILEGES ON myapp_db.* TO 'appuser'@'localhost';
FLUSH PRIVILEGES;

-- Check what permissions a user has:
SHOW GRANTS FOR 'appuser'@'localhost';

ERROR 1064 (42000): You have an error in your SQL syntax near 'FORM users' at line 1

MySQL tells you WHERE the error is. Look just before that position.
Common causes:
- Misspelled keyword (FORM instead of FROM)
- Missing comma between column names
- Missing quote around a string value
- Missing semicolon at end

-- Debug tip: run simpler parts of the query first:
SELECT * FROM users;                  -- does the table exist?
SELECT id FROM users;                 -- does the column exist?
SELECT id FROM users WHERE id = 1;   -- does WHERE work?

ERROR 1062 (23000): Duplicate entry 'rahul@example.com' for key 'email'

You tried to INSERT a value that already exists in a UNIQUE column.

-- Find the existing row:
SELECT * FROM users WHERE email = 'rahul@example.com';

-- Options:
INSERT IGNORE INTO users ...           -- skip if duplicate
INSERT ... ON DUPLICATE KEY UPDATE ... -- update if duplicate

ERROR 1452 (23000): Cannot add or update a child row: a foreign key constraint fails

You tried to insert an order with user_id = 999 but no user with id 999 exists.

-- Check what value you are inserting:
SELECT * FROM users WHERE id = 999;   -- does this user exist?

-- Temporarily disable foreign key checks (only for bulk imports):
SET FOREIGN_KEY_CHECKS = 0;
-- do your import
SET FOREIGN_KEY_CHECKS = 1;

ERROR 1406 (22001): Data too long for column 'name' at row 1

The value you tried to insert is longer than the column allows.

-- Check the column definition:
DESCRIBE users;

-- Fix: increase the column size:
ALTER TABLE users MODIFY COLUMN name VARCHAR(200);

ERROR 1054 (42S22): Unknown column 'usr.name' in 'field list'

You used a column name or alias that does not exist or is misspelled.

DESCRIBE users;     -- see actual column names
SHOW TABLES;        -- confirm table exists

ERROR 2002 (HY000): Can't connect to local MySQL server through socket '/var/run/mysqld/mysqld.sock'

MySQL is not running.

sudo systemctl status mysql
sudo systemctl start mysql
sudo journalctl -u mysql -n 50   -- see why it failed to start

-- Find the error log location:
SHOW VARIABLES LIKE 'log_error';

-- Read it:
sudo tail -50 /var/log/mysql/error.log

Step 1: Read the full error message. Note the error number.
Step 2: Check what line/position MySQL says the error is at.
Step 3: Run a simpler version of the query to isolate the problem.
Step 4: Check DESCRIBE tablename to confirm columns and types.
Step 5: Check if referenced data exists (for foreign key errors).
Step 6: Check SHOW PROCESSLIST for lock or connection issues.

SHOW PROCESSLIST;           -- see all active connections and their queries
SHOW VARIABLES LIKE '%timeout%';    -- connection and query timeout settings