1378. Replace Employee ID With The Unique Identifier Question Link Intuition how: inner(default): combine 2 table, dropping all the null outer: conbine 2 table and use NaN to fulfill the blank, keeping all null left: remain the left table's content, if the right one can't fit the left one properly, use NaN to fulfill the right table, keeping all null right: just like left on: choose the index Pandas Code import pandas as pd def replace_employee_id(employees: pd.DataFrame, employee_uni: pd.DataFrame) -> pd.DataFrame: a=pd.merge(employees,employee_uni,how='left',on='id') return a[['unique_id', 'name']] SQL Code # Write your MySQL query statement below select unique_id, name from Employees as e left join EmployeeUNI as c on e.id = c.id