How to check id of one table to another table?

Question

How to find whether distinct value id of one table exits to another tables or not using php.

For example: Let table1 have id (not primary or unique) 4 , 5 , 6 , 4 , 6 , how to check if id of table2 have 4 , 5 , 6 or not ?

$query_check= "select distinct(project_code) from table1";
$projects_check = mysql_query($query_check);

while ($result_check = mysql_fetch_array($projects_check)){
    $all[] = $result_check[0];  
}

$query = "select id from table2";
$projects = mysql_query($query);
while ($result = mysql_fetch_array($projects)){
    foreach($all as $all_val){
        $pro_code = $result[0];
        if($pro_code != $all_val){ }
    }
}

Answer 1

This SQL may help:

SELECT table2.id 
FROM table2
WHERE table2.id NOT IN (
    SELECT DISTINCT table1.id
    FROM table1
)

Answer 2

SELECT table2.id 
FROM table2
WHERE table2.id IN (
    SELECT DISTINCT table1.id
    FROM table1
)

Answer 3

To get a list of all the project codes from table1 that are ids in table2:-

SELECT DISTINCT table1.project_code  
FROM table1
INNER JOIN table2
ON table1.project_code = table2.id

To get a list of all the project codes from table1 that are NOT ids in table2

SELECT DISTINCT table1.project_code  
FROM table1
LEFT OUTER JOIN table2
ON table1.project_code = table2.id
WHERE table2.id IS NULL

If it is the entries on 2 that you want that are not on 1

SELECT DISTINCT table2.id  
FROM table2
LEFT OUTER JOIN table1
ON table1.project_code = table2.id
WHERE table1.project_code IS NULL