ref : http://www.isriya.com/node/1370/3-tables-inner-join
SELECT * FROM y INNER JOIN x ON x.a=y.a INNER JOIN z ON x.a = z.a WHERE condition
ถ้ากลัวงงก็ใส่วงเล็บครอบได้
SELECT * FROM (y INNER JOIN x ON x.a=y.a) INNER JOIN z ON x.a = z.a WHERE condition
