feat(writer): 🎉 Implement add column statements for schema evolution

* Add functionality to generate ALTER TABLE ADD COLUMN statements for existing tables.
* Introduce tests for generating and writing add column statements.
* Enhance schema evolution capabilities when new columns are added.

pkg/writers/pgsql/writer.go

@@ -168,6 +168,13 @@ func (w *Writer) GenerateSchemaStatements(schema *models.Schema) ([]string, erro
 		statements = append(statements, stmts...)
 	}
 
+	// Phase 3.5: Add missing columns (for existing tables)
+	addColStmts, err := w.GenerateAddColumnStatements(schema)
+	if err != nil {
+		return nil, fmt.Errorf("failed to generate add column statements: %w", err)
+	}
+	statements = append(statements, addColStmts...)
+
 	// Phase 4: Primary keys
 	for _, table := range schema.Tables {
 		// First check for explicit PrimaryKeyConstraint
@@ -351,6 +358,68 @@ func (w *Writer) GenerateSchemaStatements(schema *models.Schema) ([]string, erro
 	return statements, nil
 }
 
+// GenerateAddColumnStatements generates ALTER TABLE ADD COLUMN statements for existing tables
+// This is useful for schema evolution when new columns are added to existing tables
+func (w *Writer) GenerateAddColumnStatements(schema *models.Schema) ([]string, error) {
+	statements := []string{}
+
+	statements = append(statements, fmt.Sprintf("-- Add missing columns for schema: %s", schema.Name))
+
+	for _, table := range schema.Tables {
+		// Sort columns by sequence or name for consistent output
+		columns := make([]*models.Column, 0, len(table.Columns))
+		for _, col := range table.Columns {
+			columns = append(columns, col)
+		}
+		sort.Slice(columns, func(i, j int) bool {
+			if columns[i].Sequence != columns[j].Sequence {
+				return columns[i].Sequence < columns[j].Sequence
+			}
+			return columns[i].Name < columns[j].Name
+		})
+
+		for _, col := range columns {
+			colDef := w.generateColumnDefinition(col)
+
+			// Generate DO block that checks if column exists before adding
+			stmt := fmt.Sprintf("DO $$\nBEGIN\n"+
+				"  IF NOT EXISTS (\n"+
+				"    SELECT 1 FROM information_schema.columns\n"+
+				"    WHERE table_schema = '%s'\n"+
+				"      AND table_name = '%s'\n"+
+				"      AND column_name = '%s'\n"+
+				"  ) THEN\n"+
+				"    ALTER TABLE %s.%s ADD COLUMN %s;\n"+
+				"  END IF;\n"+
+				"END;\n$$",
+				schema.Name, table.Name, col.Name,
+				schema.SQLName(), table.SQLName(), colDef)
+			statements = append(statements, stmt)
+		}
+	}
+
+	return statements, nil
+}
+
+// GenerateAddColumnsForDatabase generates ALTER TABLE ADD COLUMN statements for the entire database
+func (w *Writer) GenerateAddColumnsForDatabase(db *models.Database) ([]string, error) {
+	statements := []string{}
+
+	statements = append(statements, "-- Add missing columns to existing tables")
+	statements = append(statements, fmt.Sprintf("-- Database: %s", db.Name))
+	statements = append(statements, "-- Generated by RelSpec")
+
+	for _, schema := range db.Schemas {
+		schemaStatements, err := w.GenerateAddColumnStatements(schema)
+		if err != nil {
+			return nil, fmt.Errorf("failed to generate add column statements for schema %s: %w", schema.Name, err)
+		}
+		statements = append(statements, schemaStatements...)
+	}
+
+	return statements, nil
+}
+
 // generateCreateTableStatement generates CREATE TABLE statement
 func (w *Writer) generateCreateTableStatement(schema *models.Schema, table *models.Table) ([]string, error) {
 	statements := []string{}
@@ -373,7 +442,7 @@ func (w *Writer) generateCreateTableStatement(schema *models.Schema, table *mode
 		columnDefs = append(columnDefs, "  "+def)
 	}
 
-	stmt := fmt.Sprintf("CREATE TABLE %s.%s (\n%s\n)",
+	stmt := fmt.Sprintf("CREATE TABLE IF NOT EXISTS %s.%s (\n%s\n)",
 		schema.SQLName(), table.SQLName(), strings.Join(columnDefs, ",\n"))
 	statements = append(statements, stmt)
 
@@ -458,6 +527,11 @@ func (w *Writer) WriteSchema(schema *models.Schema) error {
 		return err
 	}
 
+	// Phase 3.5: Add missing columns (priority 120)
+	if err := w.writeAddColumns(schema); err != nil {
+		return err
+	}
+
 	// Phase 4: Create primary keys (priority 160)
 	if err := w.writePrimaryKeys(schema); err != nil {
 		return err
@@ -499,6 +573,44 @@ func (w *Writer) WriteTable(table *models.Table) error {
 	return w.WriteSchema(schema)
 }
 
+// WriteAddColumnStatements writes ALTER TABLE ADD COLUMN statements for a database
+// This is used for schema evolution/migration when new columns are added
+func (w *Writer) WriteAddColumnStatements(db *models.Database) error {
+	var writer io.Writer
+	var file *os.File
+	var err error
+
+	// Use existing writer if already set (for testing)
+	if w.writer != nil {
+		writer = w.writer
+	} else if w.options.OutputPath != "" {
+		// Determine output destination
+		file, err = os.Create(w.options.OutputPath)
+		if err != nil {
+			return fmt.Errorf("failed to create output file: %w", err)
+		}
+		defer file.Close()
+		writer = file
+	} else {
+		writer = os.Stdout
+	}
+
+	w.writer = writer
+
+	// Generate statements
+	statements, err := w.GenerateAddColumnsForDatabase(db)
+	if err != nil {
+		return err
+	}
+
+	// Write each statement
+	for _, stmt := range statements {
+		fmt.Fprintf(w.writer, "%s;\n\n", stmt)
+	}
+
+	return nil
+}
+
 // writeCreateSchema generates CREATE SCHEMA statement
 func (w *Writer) writeCreateSchema(schema *models.Schema) error {
 	if schema.Name == "public" {
@@ -564,6 +676,35 @@ func (w *Writer) writeCreateTables(schema *models.Schema) error {
 	return nil
 }
 
+// writeAddColumns generates ALTER TABLE ADD COLUMN statements for missing columns
+func (w *Writer) writeAddColumns(schema *models.Schema) error {
+	fmt.Fprintf(w.writer, "-- Add missing columns for schema: %s\n", schema.Name)
+
+	for _, table := range schema.Tables {
+		// Sort columns by sequence or name for consistent output
+		columns := getSortedColumns(table.Columns)
+
+		for _, col := range columns {
+			colDef := w.generateColumnDefinition(col)
+
+			// Generate DO block that checks if column exists before adding
+			fmt.Fprintf(w.writer, "DO $$\nBEGIN\n")
+			fmt.Fprintf(w.writer, "  IF NOT EXISTS (\n")
+			fmt.Fprintf(w.writer, "    SELECT 1 FROM information_schema.columns\n")
+			fmt.Fprintf(w.writer, "    WHERE table_schema = '%s'\n", schema.Name)
+			fmt.Fprintf(w.writer, "      AND table_name = '%s'\n", table.Name)
+			fmt.Fprintf(w.writer, "      AND column_name = '%s'\n", col.Name)
+			fmt.Fprintf(w.writer, "  ) THEN\n")
+			fmt.Fprintf(w.writer, "    ALTER TABLE %s.%s ADD COLUMN %s;\n",
+				schema.SQLName(), table.SQLName(), colDef)
+			fmt.Fprintf(w.writer, "  END IF;\n")
+			fmt.Fprintf(w.writer, "END;\n$$;\n\n")
+		}
+	}
+
+	return nil
+}
+
 // writePrimaryKeys generates ALTER TABLE statements for primary keys
 func (w *Writer) writePrimaryKeys(schema *models.Schema) error {
 	fmt.Fprintf(w.writer, "-- Primary keys for schema: %s\n", schema.Name)