在while循环中仅显示未插入另一个表中的记录

Question

i have two tables, categorias and precios

in categorias im saving all the categories

in precios only a few

how do i show in a while the records missing in precios?

$query = $mysqli->query("select * from categorias"); //all data stored
while ($resultados = $query->fetch_array()) {  //here we show
 $m =$resultados['subcat']; //the subcats..

ho do i show only the categories where is not inserted in precios table?

i tried this but does not work:

$query = $mysqli->query("select * from categorias where subcat != '$pi'");

$pi is the categories inserted in precios

Answer 1

If SUBCAT is a FOREGIN KEY on PRECIOS :

SELECT * FROM CATEGORIAS WHERE SUBCAT NOT IN 
(SELECT ID FROM PRECIOS)