尝试检查id是否存在

Question

您好，我想尝试检查ID是否存在，我应该在if语句上放些什么，我希望它仅检查并插入数据库中是否不存在，然后插入。如果存在的话，什么也不做。

function exists($date_, $account, $dimension)
{
    $sql = "SELECT account FROM ".TB_PREF."budget WHERE account=".db_escape($account)
    ." AND tran_date='$date_' AND
        id=".db_escape($dimension)." AND 
    $result = db_query($sql, "Cannot retreive transaction");

    return (db_num_rows($result) > 0);
}

function add($date_, $account, $dimension, $dimension2, $amount)
{
    $date = date2sql($date_);

    if (exists_gl_budget($date, $account, $dimension, $dimension2))

    else
        $sql = "INSERT INTO ".TB_PREF."budget(tran_date,
            account, id) VALUES ('$date',
            ".db_escape($account).", ".db_escape($dimension).", "
            .db_escape($dimension2).", ".db_escape($amount).", '')";

    db_query($sql, " not be saved");
}