为什么 字段SUM 的结果不正确

tb1
Name Salary(INT)  Dept.
Bob 1000      Sale
Tom 2000     Finance
John 3000     Sale
Winson 4000  Saletb2
Name Bonus(INT)
Bob 100
Tom 200

SELEC
        tb1.Name, SUM(CASE WHEN tb1.Dept='Sale' THEN 0.67
        WHEN tb1.Dept='Finance' THEN 0.88 ELSE 1 END         
*(tb1.Salarytb1.Salary + COALESCE(tb2.Bonus,0))) Total
From
        tb1
LEFT JOIN
        tb2
ON
       tb1.Name = tb2.Name   group by tb1.Name order by Total