Troubleshooting

​

Symptoms

Error de conexión: Connection refused

Error: Communications link failure

​

Root Causes

1. MySQL Server Not Running
   • The MySQL service is stopped
   • The server crashed or failed to start
2. Wrong Server Address
   • Selected server is not running MySQL
   • Firewall blocking port 3306
3. Port Already in Use
   • Another service is using port 3306
   • MySQL is running on a different port

​

Solutions

sudo systemctl status mysql
# or
sudo service mysql status

net start | find "MySQL"

sudo systemctl start mysql  # Linux
net start MySQL80           # Windows

netstat -an | grep 3306
# or
lsof -i :3306

mysql -h localhost -u root -p

​

Symptoms

Error: Access denied for user 'username'@'localhost' (using password: YES)

​

Root Causes

1. Invalid Username
   • Username does not exist in MySQL
   • Typo in username field
2. Incorrect Password
   • Wrong password entered
   • Password contains special characters not properly entered
3. User Permissions
   • User exists but doesn’t have required privileges
   • User is restricted to specific hosts

​

Solutions

mysql -h localhost -u [username] -p
# Enter password when prompted

SHOW GRANTS FOR 'username'@'localhost';

ALTER USER 'username'@'localhost' IDENTIFIED BY 'new_password';
FLUSH PRIVILEGES;

​

Symptoms

No se encontraron bases de datos

​

Root Causes

1. No User Databases
   • Only system databases exist (information_schema, mysql, performance_schema, sys)
   • All user databases were dropped
2. Insufficient Privileges
   • User cannot execute SHOW DATABASES
   • User has access but grants are not visible

​

Solutions

SHOW DATABASES;

CREATE DATABASE test_db;

SHOW GRANTS FOR CURRENT_USER();

GRANT SHOW DATABASES ON *.* TO 'username'@'localhost';
FLUSH PRIVILEGES;

​

Symptoms

Error: Connection timed out

​

Root Causes

1. Network Issues
   • Slow or unreliable network connection
   • Remote server not responding
2. Server Overload
   • MySQL server is under heavy load
   • Too many concurrent connections
3. Firewall/Proxy
   • Firewall dropping packets
   • Proxy interfering with connection

​

Solutions

ping [server_address]
telnet [server_address] 3306

SHOW VARIABLES LIKE 'max_connections';
SHOW STATUS LIKE 'Threads_connected';

SET GLOBAL max_connections = 200;

String url = "jdbc:mysql://" + host + ":3306/" + database 
           + "?useSSL=false&connectTimeout=10000";

​

Symptoms

Error: SSL connection error

​

Root Cause

​

Solution

SHOW VARIABLES LIKE 'require_secure_transport';

String url = "jdbc:mysql://" + host + ":3306/" + database + "?useSSL=false";

String url = "jdbc:mysql://" + host + ":3306/" + database 
           + "?useSSL=true&requireSSL=true";

SET GLOBAL require_secure_transport = OFF;

​

Symptoms

Ingrese una consulta SQL

​

Root Cause

​

Solution

SELECT * FROM [table_name] LIMIT 100

​

Symptoms

Error en la consulta: You have an error in your SQL syntax...

​

Root Causes

1. Invalid SQL Syntax
   • Missing keywords (SELECT, FROM, WHERE, etc.)
   • Incorrect column/table names
   • Unmatched quotes or parentheses
2. Reserved Keyword Usage
   • Using reserved words without backticks
   • Example: SELECT order FROM orders (should be `order`)
3. Missing Semicolon (rare)
   • Some queries require explicit termination

​

Solutions

mysql -h localhost -u [username] -p [database] -e "[your_query]"

SELECT `order`, `date`, `table` FROM `orders`;

​

Symptoms

Error en la consulta: Table 'database.tablename' doesn't exist

Error en la consulta: Unknown column 'column_name' in 'field list'

​

Root Causes

1. Typo in Name
   • Incorrect spelling of table or column
   • Wrong case (if database is case-sensitive)
2. Wrong Database
   • Connected to different database than intended
   • Table exists but in another schema
3. Table Dropped
   • Table was deleted after table list was populated
   • Table list needs refreshing

​

Solutions

Conectado a: [database_name]

DESCRIBE table_name;

SHOW COLUMNS FROM table_name;

​

Symptoms

Consulta ejecutada correctamente

​

Root Causes

1. No Matching Rows
   • WHERE clause filters out all rows
   • Table is empty
2. Successful Modification
   • INSERT/UPDATE/DELETE executed on empty table
   • Query affected 0 rows

​

Verification

Filas afectadas: 0

SELECT * FROM [table] LIMIT 100

SELECT * FROM users WHERE id = 999;  -- May return nothing
SELECT * FROM users;                 -- Returns all rows

​

Symptoms

Error en la consulta: Duplicate entry '...' for key 'PRIMARY'

Error en la consulta: Cannot add or update a child row: a foreign key constraint fails

​

Root Causes

1. Primary Key Violation
   • Attempting to insert duplicate primary key
   • Unique constraint violation
2. Foreign Key Violation
   • Referencing non-existent parent row
   • Attempting to delete parent row with children
3. NOT NULL Violation
   • Inserting NULL into NOT NULL column
   • Missing required columns in INSERT

​

Solutions

SELECT * FROM table WHERE id = [your_id];

INSERT IGNORE INTO users (id, name) VALUES (1, 'John');

REPLACE INTO users (id, name) VALUES (1, 'John');

SELECT * FROM parent_table WHERE id = [foreign_key_value];

​

Symptoms

​

Root Causes

1. Large Result Set
   • SELECT without LIMIT on huge table
   • Returning millions of rows
2. Missing Index
   • Query scans entire table without index
   • JOIN without proper indexing
3. Expensive Operations
   • Complex calculations or aggregations
   • Large GROUP BY or ORDER BY operations

​

Solutions

SELECT * FROM large_table LIMIT 1000;

EXPLAIN SELECT * FROM users WHERE email = '[email protected]';

CREATE INDEX idx_email ON users(email);

​

Symptoms

​

Root Cause

• Database list loading
• Database connection
• Query execution
• Table refresh

​

Normal vs. Problem

• Controls are grayed out
• Operation completes within seconds
• Interface unlocks automatically

• Operation takes very long time
• Interface never unlocks
• Application must be force-closed

​

Solutions

​

Symptoms

​

Root Cause

1. After initial connection (Controller.java:172-173)
2. When “Refrescar tablas” is clicked (Controller.java:64)

​

Solution

​

Symptoms

​

Root Causes

1. Empty Result Set
   • Query returned 0 rows (see “Query Returns No Results” above)
2. Modification Query
   • INSERT/UPDATE/DELETE shows affected row count instead of results
   • Source: Model.java:124, Model.java:147-148
3. Previous Results Not Cleared
   • Old results still displayed from previous query

​

Solutions

​

Symptoms

Error al desconectar: ...

​

Root Causes

1. Confirmation Cancelled
   • User clicked “No” on confirmation dialog
   • Expected behavior, not an error
2. Connection Already Closed
   • Connection was terminated externally
   • SQLException during disconnect

​

Solutions

¿Desea desconectarse y cambiar de base de datos?

​

Symptoms

ClassNotFoundException: com.mysql.jdbc.Driver

No suitable driver found for jdbc:mysql://...

​

Root Cause

​

Solutions

https://dev.mysql.com/downloads/connector/j/

java -cp ".:mysql-connector-j-8.0.33.jar" -jar mysql-editor.jar

try {
    Class.forName("com.mysql.cj.jdbc.Driver");
    System.out.println("Driver loaded successfully");
} catch (ClassNotFoundException e) {
    System.err.println("Driver not found: " + e.getMessage());
}

​

Symptoms

Loading class 'com.mysql.jdbc.Driver'. This is deprecated.

Public Key Retrieval is not allowed

​

Root Causes

1. Old Driver with New MySQL
   • MySQL Connector/J 5.x with MySQL 8.0
   • Deprecated driver class
2. Authentication Plugin Mismatch
   • MySQL 8.0 uses caching_sha2_password
   • Old drivers don’t support this

​

Solutions

https://dev.mysql.com/downloads/connector/j/8.0.html

String url = "jdbc:mysql://" + host + ":3306/" + database 
           + "?useSSL=false&allowPublicKeyRetrieval=true";

ALTER USER 'username'@'localhost' 
IDENTIFIED WITH mysql_native_password BY 'password';
FLUSH PRIVILEGES;