> Both tables use id field as their primary keys. Managers are also employees, so the two tables share the ids. Since managers have more attributes, their infor

> Both tables use id field as their primary keys. Managers are also employees, so the two tables share the ids. Since managers have more attributes, their information is stored in a separate table. > Now we want to find the total income (including the allowance) of each employee (including every manager). > A JOIN operation is necessary for SQL to do it: > SELECT employee.id, employee.name, employy.salary+manager.allowance > FROM employee > LEFT JOIN manager ON employee.id=manager.id > But for two tables having a one-to-one relationship, we can treat them like one table: > SELECT id,name,salary+allowance > FROM employee What about employees who aren't managers? I assume they have no entry in the manager table. The SQL would ignore them, because it's a left join, which is not what was asked for. Does the proposed query do the same? What happens if there is also salesperson table id allowance ? Which table is joined? This language seems a little half-baked.