MySQL：如果第2列中不存在值，如何更新colum1

Question

I want to write a value into a column "email_new", but only if this value does not exist in a second column called "email". This means none of the entries in the column email should contain the new mail-address.

This is for a programm to change the email-address, where the email-address has to be verified, and the column "email" allows only UNIQUE values.

"
UPDATE `table`
SET email_new = '".mysql_real_escape_string($newmail)."'
WHERE
  user = '".mysql_real_escape_string($user)."'
  && email NOT CONTAINS('".mysql_real_escape_string($newmail)."')
"

Is something like that possible?

Answer 1

$q = "
    UPDATE
        `table`
    SET
        `email_new` = '" . mysql_real_escape_string($newmail) . "'
    WHERE
        `user` = '" . mysql_real_escape_string($newmail) . "'
    AND NOT EXISTS (
        SELECT
            `email`
        FROM
            `table`
        WHERE
            `email` = '" . mysql_real_escape_string($newmail) . "'
    )
";

Answer 2

UPDATE table
SET email_new = ?
WHERE
    user = ?
    AND (SELECT COUNT(*) = 0 FROM table WHERE email = ?)

Replace ? signs by actual values.

Answer 3

you are not checking wheter the newmail is really unique:

append to your query another AND condition, something like: AND 0 = (select count(*) from table where email = mysql_real_escape_string($newmail))

Answer 4

Or simply

UPDATE `table`
SET email_new = IF (email IS NULL, ?, ?)
-- this line is if you want to update email if email is not null
, email = IF (email IS NOT NULL, ?, ?) 
WHERE ...