SQL联接：联接两个表以将第二个表的结果转换为grep结果，然后

Question

I want to grep the Employee Unique Tax ID from first Tables to second starting like:

select * from where Employee_ID as 'var1' Join on Employee_Catagory as 'var2' Join On Employee_Pos. as var3 . 

To get the Employee Unique_Tax_ID as $res I want to have it in a Sigle SQL Statement. The var1 -var3 value is not important because that can I change with PHP in that file with functions.

Answer 1

Simple LEFT JOIN should do:

select t1.employee_id,
    t1.employee_category,
    t1.employee_pos,
    t2.unique_tax_id
from table1 t1
left join table2 t2 on t1.system_id = t2.system_id