index.js

// 開始一個 transaction
await conn.query("BEGIN");
// 先 select 使用者指定的 seat，並確認 isbooked 的狀態是 false (FOR UPDATE 是關鍵，等等會說明)
const sql = "SELECT * FROM seats where id = $1 and isbooked = 0 FOR UPDATE";
const result = await conn.query(sql, [id]);

if (result.rowCount === 0) {
  res.send({"error": "Seat already booked"})
  return;
} 
// 執行更新
const sqlU= "update seats set isbooked = 1, name = $2 where id = $1"
const updateResult = await conn.query(sqlU,[id, name]);
        
// 結束 transaction
await conn.query("COMMIT");