calculate sum of variables in a loop

I have the following task:

I have many equal tables of different people (>500) which contain working hours. The following is a sample:

"table01"

date | hours

"table02"

date | hours "table03"

date | hours .....

I have one table with relationship between tables to "cost-Center":

"table_cost-center"

table01 | 0010

table02 | 0030

table03 | 0020

table04 | 0040

table05 | 0060

......

I have to calculate the sum of hours between date 1 and date 2 of each cost-center in a loop. I need to display each sum of every single cost-center and calculate the total sum of all cost-centers together.

My Idea:

$whours= mysql_query ("SELECT sum(hours) FROM `$cost-center_tabelle[$z]` WHERE `$cost-center_table[$z]`.date BETWEEN '$date2' AND '$date1'