从表中选择另一个表中的行不匹配

I have 2 tables:

1. customer
2. customer_communication

the customer table has a unique sequence column and the customer_communication table has customer_seq which matches the sequence column in the customer table.

The rows in the customer_communication table have a datetime column, i am selecting data from both tables using these queries:

echo '<table width="100%" border="0" cellspacing="10" cellpadding="10">';
    //select from the customer_communication table
    $sql="SELECT * from customer where company_status = '' and no_communication = '' order by company ASC ";
    $rs=mysql_query($sql,$conn);
    while($result=mysql_fetch_array($rs))
    {
        $sql2="SELECT * from customer_communication WHERE customer_seq = '".$result["sequence"]."' and datetime > DATE_ADD(DATE(now()), INTERVAL 15 DAY) order by datetime ASC ";
        $rs2=mysql_query($sql2,$conn);
        if(mysql_num_rows($rs2) > 0)
        {
            echo '<tr>
            <td><a href="customer_communication.php?seq='.$result["sequence"].'">'.$result["company"].'</a></td>
            </tr>';
        }
    }
    echo '</table>';

so it selects all the rows from the customer table, then selects the rows from the customer_communication table where the customer_seq = sequence and its been 15 days from the datetime column.

how can i show all the rows from the customer table that do not exist in the customer_communication table

for example, there is sequence 1 in customer and this does not exist in the customer_seq column in the customer_communication table so i want to show this

That can be done by basic SQL. I'll leave it to you to integrate it into your php.

SELECT * FROM Customer c
WHERE NOT EXISTS
    (SELECT * FROM Customer_Communication
    WHERE Customer_seq = c.Customer_Seq);

Can you try this,

$sql="SELECT * FROM customer as a, customer_communication as b  WHERE a.company_status = '' AND a.no_communication = '' AND b.customer_seq NOT IN ( SELECT sequence FROM customer ) AND b.datetime > DATE_ADD(DATE(now()), INTERVAL 15 DAY) ORDER BY a.company ASC ";

There are multiple different ways to do this, but one way is to use a subquery to get the unique customer_seq values from customer_communication table, and then retrieve all rows from the customer table that don't have those values for sequence column.

Select * from customer c
where c.sequence not in 
(select distinct customer_seq from customer_communication)