[Parser] Support ALTER TABLE column operations

[ajitpratap0]

Summary

The parser does not fully support ALTER TABLE statements with column operations.

Current Behavior

ALTER TABLE users ADD COLUMN email VARCHAR(255)

Error: expected ADD, DROP, RENAME, or ALTER, got IDENT

Expected Behavior

Full support for ALTER TABLE column operations.

Operations to Support

-- ADD COLUMN
ALTER TABLE users ADD COLUMN email VARCHAR(255);
ALTER TABLE users ADD COLUMN created_at TIMESTAMP DEFAULT NOW();

-- DROP COLUMN
ALTER TABLE users DROP COLUMN temp_field;
ALTER TABLE users DROP COLUMN IF EXISTS old_field;

-- RENAME COLUMN
ALTER TABLE users RENAME COLUMN old_name TO new_name;

-- ALTER COLUMN (modify type)
ALTER TABLE users ALTER COLUMN age TYPE BIGINT;
ALTER TABLE users ALTER COLUMN name SET NOT NULL;
ALTER TABLE users ALTER COLUMN status SET DEFAULT 'active';
ALTER TABLE users ALTER COLUMN email DROP NOT NULL;

-- ADD CONSTRAINT
ALTER TABLE users ADD CONSTRAINT pk_users PRIMARY KEY (id);
ALTER TABLE orders ADD CONSTRAINT fk_user FOREIGN KEY (user_id) REFERENCES users(id);

-- DROP CONSTRAINT
ALTER TABLE users DROP CONSTRAINT constraint_name;

Priority

🟡 Medium - Important for DDL analysis and migration tools.

Impact

• Battle test: 2 test failures
• Important for schema evolution tracking

[ajitpratap0]

✅ Already Implemented

ALTER TABLE column operations are fully implemented in pkg/sql/parser/alter.go:

• ✅ ALTER TABLE users ADD COLUMN email VARCHAR(255) - Working
• ✅ ALTER TABLE users DROP COLUMN temp_field - Working
• ✅ ALTER TABLE users RENAME COLUMN old_name TO new_name - Working
• ✅ ALTER TABLE users ALTER COLUMN age TYPE BIGINT - Working

Comprehensive test coverage exists in parser_targeted_coverage_test.go.