How to fix ERROR 1212: Incorrect table definition; all MERGE tables must be in the same database in MySQL

-- List all tables in database_a:
USE database_a;
SHOW TABLES;

-- List all tables in database_b:
USE database_b;
SHOW TABLES;

-- Check the full names with database prefixes:
SELECT TABLE_SCHEMA, TABLE_NAME FROM INFORMATION_SCHEMA.TABLES
WHERE TABLE_NAME IN ('table1', 'table2', 'table3');

-- Use database where tables reside:
USE correct_database;

-- Check table structures:
DESC table1;
DESC table2;
DESC table3;

-- Compare column definitions more precisely:
SELECT COLUMN_NAME, COLUMN_TYPE, IS_NULLABLE, COLUMN_KEY, EXTRA
FROM INFORMATION_SCHEMA.COLUMNS
WHERE TABLE_SCHEMA = 'correct_database' AND TABLE_NAME IN ('table1', 'table2')
ORDER BY TABLE_NAME, ORDINAL_POSITION;

-- Compare indexes:
SHOW INDEX FROM table1;
SHOW INDEX FROM table2;

-- Option 1: Move tables to the target database using RENAME:
RENAME TABLE database_a.table1 TO target_db.table1;
RENAME TABLE database_b.table2 TO target_db.table2;

-- Option 2: Copy tables and drop originals (if RENAME fails due to cross-filesystem):
CREATE TABLE target_db.table1 LIKE database_a.table1;
INSERT INTO target_db.table1 SELECT * FROM database_a.table1;
DROP TABLE database_a.table1;

-- Verify the move:
USE target_db;
SHOW TABLES;

USE target_database;

CREATE TABLE merged_table (
  id INT NOT NULL AUTO_INCREMENT PRIMARY KEY,
  data VARCHAR(255),
  created_at TIMESTAMP DEFAULT CURRENT_TIMESTAMP,
  INDEX(id)
) ENGINE=MERGE UNION=(table1, table2, table3) INSERT_METHOD=LAST;

-- Verify the MERGE table was created:
SHOW CREATE TABLE merged_table\G
DESC merged_table;

-- Query all rows across merged tables:
SELECT COUNT(*) FROM merged_table;
SELECT * FROM merged_table LIMIT 10;

-- Insert into the MERGE table (INSERT_METHOD=LAST sends to the last table):
INSERT INTO merged_table (data) VALUES ('test entry');

-- Query to verify the insert:
SELECT * FROM table3 WHERE data = 'test entry';

-- Update and delete operations:
UPDATE merged_table SET data = 'updated' WHERE id = 1;
DELETE FROM merged_table WHERE id = 1;

-- If any operation fails, check table structure differences and error log.

// BAD - references tables from different databases:
const query = `
  CREATE TABLE merge_table (
    id INT PRIMARY KEY,
    data VARCHAR(255)
  ) ENGINE=MERGE UNION=(db_a.table1, db_b.table2) INSERT_METHOD=LAST;
`;

// GOOD - all tables from same database:
const query = `
  CREATE TABLE single_db.merge_table (
    id INT PRIMARY KEY,
    data VARCHAR(255)
  ) ENGINE=MERGE UNION=(table1, table2, table3) INSERT_METHOD=LAST;
`;