[WIP] Implement MERGE statement execution logic

[Copilot]

Thanks for assigning this issue to me. I'm starting to work on it and will keep this PR's description up to date as I form a plan and make progress.

Original prompt

---

This section details on the original issue you should resolve

<issue_title>UPSERT/MERGE Statement Not Implemented</issue_title>
<issue_description>## Summary
The MERGE statement syntax is parsed by AlaSQL (src/75merge.js) but the execution logic is not implemented. The execute() method simply returns 1 without performing any insert, update, or delete operations. Additionally, PostgreSQL-style INSERT ... ON CONFLICT (UPSERT) syntax is not supported.

Current Behavior

// MERGE statement is parsed but does nothing:
alasql(`
  MERGE INTO target AS t
  USING source AS s
  ON t.id = s.id
  WHEN MATCHED THEN UPDATE SET t.value = s.value
  WHEN NOT MATCHED THEN INSERT (id, value) VALUES (s.id, s.value)
`);

// Returns: 1 (but no actual operation performed)

// PostgreSQL UPSERT syntax not supported:
alasql(`
  INSERT INTO users (id, name, email) 
  VALUES (1, 'John', '[email protected]')
  ON CONFLICT (id) DO UPDATE SET email = EXCLUDED.email
`);

// Error: Parser doesn't recognize ON CONFLICT syntax

Expected Behavior - MERGE Statement

Example 1: Basic MERGE with INSERT and UPDATE

// Setup
alasql('CREATE TABLE target (id INT, name STRING, value INT)');
alasql('CREATE TABLE source (id INT, name STRING, value INT)');
alasql('INSERT INTO target VALUES (1, "Alice", 100), (2, "Bob", 200)');
alasql('INSERT INTO source VALUES (2, "Bob", 250), (3, "Charlie", 300)');

// Merge operation
alasql(`
  MERGE INTO target AS t
  USING source AS s
  ON t.id = s.id
  WHEN MATCHED THEN 
    UPDATE SET t.value = s.value
  WHEN NOT MATCHED THEN 
    INSERT (id, name, value) VALUES (s.id, s.name, s.value)
`);

// Expected result in target table:
// [
//   {id: 1, name: 'Alice', value: 100},   // Unchanged (not in source)
//   {id: 2, name: 'Bob', value: 250},     // Updated (matched)
//   {id: 3, name: 'Charlie', value: 300}  // Inserted (not matched)
// ]

Example 2: MERGE with DELETE

alasql(`
  MERGE INTO target AS t
  USING source AS s
  ON t.id = s.id
  WHEN MATCHED THEN 
    UPDATE SET t.value = s.value
  WHEN NOT MATCHED BY SOURCE THEN 
    DELETE
`);

// Expected: Rows in target not in source are deleted

Example 3: MERGE with Conditional Logic

alasql(`
  MERGE INTO inventory AS inv
  USING shipment AS ship
  ON inv.product_id = ship.product_id
  WHEN MATCHED AND ship.quantity > 0 THEN
    UPDATE SET inv.stock = inv.stock + ship.quantity
  WHEN MATCHED AND ship.quantity = 0 THEN
    DELETE
  WHEN NOT MATCHED THEN
    INSERT (product_id, stock) VALUES (ship.product_id, ship.quantity)
`);

Expected Behavior - PostgreSQL UPSERT (ON CONFLICT)

Example 1: INSERT with UPDATE on Conflict

alasql('CREATE TABLE users (id INT PRIMARY KEY, name STRING, email STRING, updated_at DATE)');

alasql(`
  INSERT INTO users (id, name, email, updated_at) 
  VALUES (1, 'John', '[email protected]', NOW())
  ON CONFLICT (id) 
  DO UPDATE SET 
    email = EXCLUDED.email,
    updated_at = NOW()
`);

// If id=1 exists: Updates email and updated_at
// If id=1 doesn't exist: Inserts new row

Example 2: INSERT with DO NOTHING on Conflict

alasql(`
  INSERT INTO users (id, name, email) 
  VALUES (1, 'John', '[email protected]')
  ON CONFLICT (id) DO NOTHING
`);

// If id=1 exists: No action taken
// If id=1 doesn't exist: Inserts new row

Example 3: Conditional UPSERT

alasql(`
  INSERT INTO users (id, name, email, login_count) 
  VALUES (1, 'John', '[email protected]', 1)
  ON CONFLICT (id) 
  DO UPDATE SET 
    login_count = users.login_count + 1,
    last_login = NOW()
  WHERE users.email = EXCLUDED.email
`);

Use Cases

1. Synchronizing Data

// Sync external data with local cache
alasql(`
  MERGE INTO local_cache AS local
  USING external_data AS ext
  ON local.product_id = ext.product_id
  WHEN MATCHED THEN UPDATE SET local.price = ext.price, local.updated = NOW()
  WHEN NOT MATCHED THEN INSERT VALUES (ext.product_id, ext.price, NOW())
`);

2. Maintaining User Sessions

// Insert new session or update existing
alasql(`
  INSERT INTO sessions (user_id, session_token, last_seen)
  VALUES (?, ?, NOW())
  ON CONFLICT (user_id)
  DO UPDATE SET session_token = EXCLUDED.session_token, last_seen = NOW()
`, [userId, token]);

3. Deduplication

// Keep only the latest record
alasql(`
  INSERT INTO unique_events (event_id, data, timestamp)
  VALUES (?, ?, NOW())
  ON CONFLICT (event_id) DO NOTHING
`, [eventId, data]);

Implementation Status

Currently Parsed (src/75merge.js)

• ✅ MERGE INTO ... USING ... ON ...
• ✅ WHEN MATCHED / NOT MATCHED clauses
• ✅ BY TARGET / BY SOURCE qualifiers
• ✅ INSERT / UPDATE / DELETE actions
• ✅ toString() method generates SQL

Not Implemented

• ❌ Execute method (returns hardcoded 1)
• ❌ Actual merge logic
• ❌ ON CONFLICT syntax (not in parser)
• ❌ DO UPDATE / DO NOTH...

• Fixes UPSERT/MERGE Statement Not Implemented #2407

---

💬 We'd love your input! Share your thoughts on Copilot coding agent in our 2 minute survey.