How to find out the missing data

For example, I currently have 999 pieces of data with IDs ranging from 1 to 1000. How can I use SQL statements to find out what the missing ID is?
PS: What if two pieces of data are missing?

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For example, I currently have 999 pieces of data with IDs ranging from 1 to 1000. How can I use SQL statements to find out what the missing ID is?
PS: What if two pieces of data are missing?

<code>(1+2+...+1000) - sum(id)  不就行了吗？</code>

<code>第一个
select t.id from (select @rownum:=@rownum+1 no,id  from table, 
(SELECT @rownum:=0) r)t where t.no - t.id = 1 limit 1
第二个
select t.id from (select @rownum:=@rownum+1 no,id  from table, 
(SELECT @rownum:=0) r)t where t.no - t.id = 2 limit 1</code>

It is not recommended to use SQL statements to perform such complex operations. Instead of increasing the burden on the database, it is better to let PHP bear it~

<code><?php
$arr = [1,4,8,9,10,20]; // 假设这是从数据库中读取出来的ID数组
$res = [];
for ($i = 0; $i < count($arr)-1; $i++) {
    if (($num = $arr[$i+1] - $arr[$i]) > 1) {
        for ($j = 1; $j < $num; $j++) {
            $res[] = $arr[$i] + $j;
        }
    }
}

var_dump($res);</code>