feat(parser): support '?' positional placeholders (fix #522)

pkg/sql/parser/expressions_literal.go

@@ -254,9 +254,14 @@ func (p *Parser) parsePrimaryExpression() (ast.Expression, error) {
 		return &ast.LiteralValue{Value: value, Type: "bool"}, nil
 	}
 
-	if p.isType(models.TokenTypePlaceholder) {
-		// Handle SQL placeholders (e.g., $1, $2 for PostgreSQL; @param for SQL Server)
+	if p.isType(models.TokenTypePlaceholder) || p.isType(models.TokenTypeQuestion) {
+		// Handle SQL placeholders: $1, ? (JDBC), @param. `?` is also a JSON key
+		// existence operator in PostgreSQL (data ? 'key'); that case is parsed in
+		// binary-operator position, so a bare `?` at operand position is a placeholder.
 		value := p.currentToken.Token.Value
+		if value == "" {
+			value = "?"
+		}
 		p.advance()
 		return &ast.LiteralValue{Value: value, Type: "placeholder"}, nil
 	}

pkg/sql/parser/question_placeholder_test.go

@@ -0,0 +1,61 @@
+// Copyright 2026 GoSQLX Authors
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//     http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+package parser_test
+
+import (
+	"testing"
+
+	"github.com/ajitpratap0/GoSQLX/pkg/gosqlx"
+	"github.com/ajitpratap0/GoSQLX/pkg/sql/keywords"
+)
+
+// Regression: `?` positional placeholders (JDBC) must parse and round-trip.
+// Previously `WHERE a = ?` failed with "unexpected token: QUESTION".
+// Postgres JSON existence `data ? 'key'` is untouched (binary-operator path).
+func TestQuestionPlaceholder(t *testing.T) {
+	tests := []struct {
+		name string
+		sql  string
+	}{
+		{"single", `SELECT * FROM omi_reporting WHERE category = ?`},
+		{"multiple", `SELECT * FROM omi_reporting WHERE a > ? AND b < ? AND category IN (?, ?)`},
+		{"in having group", `SELECT category, count() FROM omi_reporting WHERE next_verification_date >= ? GROUP BY category`},
+	}
+	for _, tt := range tests {
+		t.Run(tt.name, func(t *testing.T) {
+			tree, err := gosqlx.ParseWithDialect(tt.sql, keywords.DialectClickHouse)
+			if err != nil {
+				t.Fatalf("parse failed: %v", err)
+			}
+			if got := tree.SQL(); got != tt.sql {
+				t.Errorf("SQL() = %q, want %q", got, tt.sql)
+			}
+		})
+	}
+}
+
+// TestQuestionJSONOperator guards against breaking Postgres JSON existence
+// operator `?` when treating `?` as a placeholder.
+func TestQuestionJSONOperatorStillWorks(t *testing.T) {
+	sql := `SELECT * FROM users WHERE data ? 'key'`
+	// PostgreSQL dialect.
+	tree, err := gosqlx.ParseWithDialect(sql, keywords.DialectPostgreSQL)
+	if err != nil {
+		t.Fatalf("parse json operator: %v", err)
+	}
+	if got := tree.SQL(); got != sql {
+		t.Errorf("SQL() = %q, want %q", got, sql)
+	}
+}