정의되지 않은 색인이 항상 나타나는 이유는 무엇입니까? 해결 방법은 무엇입니까?

Question

<?php

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	include("connect/connect.php");

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	$xh=$_GET['xh'];

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	$sql="select xh,xm,xb,zymc from xsjbxxb where xh=".$xh;/lect host,user from user

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	$result=mysqli_query($conn,$sql);

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Answer 1

xh 필드가 기본 키인가요?