ขอตัวอย่างการ join 3 ตารางแบบนี้หน่อยครับ ผล คือ ตาราง ผลลัพธื

SELECT tb1.id_customer,tb1.name,tb2.num_a FROM  T_customer tb1
INNER JOIN T_A tb2  ON tb1.id_customer=tb2.id_customer
LEFT JOIN T_B tb3 ON tb1.id_cutomer=tb3.id_customer

SELECT @rowno:=@rowno+1 id_run,id_customer, name, num_ 
FROM (
   select id_customer, num_a from table_a
   union all
   select id_customer, num_b from table_b
) as uni 
, tablename as tb
, (select @rowno:=0) as rn
where tb.id_customer = uni.id_customer