Left join does not meet my requirement - mysql

Question

There are two tables over here: first is for basic details of "poll" and
second for the "answer". My requirement is like this:
i want poll records to display only for users who didn't answer the poll
I tried this query but it is not working:
select p.* from mycom_poll as p
LEFT JOIN mycom_polls_result as pr on (p.b_id != pr.b_poll_id
And pr.b_user_id !=14) where p.v_status = 'enable'
And p.v_country like '%India%'
AND p.d_activate_date <= '2015-04-17' order by p.b_id limit 1"
CREATE TABLE IF NOT EXISTS `mycom_poll` (
`b_id` bigint(20) NOT NULL AUTO_INCREMENT,
`b_user_id` bigint(20) NOT NULL DEFAULT '0',
`v_code` varchar(50) NOT NULL,
`v_question` varchar(1000) CHARACTER SET utf8 NOT NULL,
`v_country` varchar(100) NOT NULL,
`d_activate_date` date NOT NULL,
`dt_created_date` datetime NOT NULL,
`dt_updated_date` datetime NOT NULL,
`v_ip` varchar(20) NOT NULL,
`v_status` varchar(20) NOT NULL DEFAULT 'enable',
PRIMARY KEY (`b_id`)
) ENGINE=InnoDB DEFAULT CHARSET=latin1 AUTO_INCREMENT=9 ;
INSERT INTO `mycom_poll` (`b_id`, `b_user_id`, `v_code`, `v_question`, `v_country`, `d_activate_date`, `dt_created_date`, `dt_updated_date`, `v_ip`, `v_status`) VALUES(8, 0, '20150417115406-192168113C5ZJpXu2hM', 'Who is chief minstor of Gujarat in 2015 Rajyasabha?', '["Global","India","United States"]', '2015-04-15', '2015-04-17 11:56:12', '2015-04-18 07:29:11', '127.0.0.1', 'enable');
CREATE TABLE IF NOT EXISTS `mycom_polls_result` (
`b_id` bigint(20) NOT NULL AUTO_INCREMENT,
`b_poll_id` bigint(20) NOT NULL,
`b_user_id` bigint(20) NOT NULL,
`b_poll_answer` bigint(20) NOT NULL,
`dt_created_date` datetime NOT NULL,
`dt_updated_date` datetime NOT NULL,
`v_ip` varchar(20) NOT NULL,
`v_status` varchar(20) NOT NULL DEFAULT 'enable',
PRIMARY KEY (`b_id`),
KEY `b_poll_id` (`b_poll_id`,`b_user_id`),
KEY `b_poll_id_2` (`b_poll_id`,`b_user_id`,`b_poll_answer`),
KEY `b_poll_id_3` (`b_poll_id`,`b_user_id`,`b_poll_answer`),
KEY `b_user_id` (`b_user_id`)
) ENGINE=InnoDB DEFAULT CHARSET=latin1 AUTO_INCREMENT=3 ;
INSERT INTO `mycom_polls_result` (`b_id`, `b_poll_id`, `b_user_id`, `b_poll_answer`, `dt_created_date`, `dt_updated_date`, `v_ip`, `v_status`) VALUES
(1, 8, 14, 64, '2015-04-18 13:27:50', '2015-04-18 13:27:50', '127.0.0.1', 'enable');

Answer 1

Your query:
select p.* from mycom_poll as p
LEFT JOIN mycom_polls_result as pr on (p.b_id != pr.b_poll_id
And pr.b_user_id !=14) where p.v_status = 'enable'
And p.v_country like '%India%'
AND p.d_activate_date <= '2015-04-17' order by p.b_id limit 1
First I am going to point out the mistake of this query, what you are doing here is, you are left joining two tables on id of first table not equals to id of second table which will join all the table data whose ids are not same, when you do a left join, you always get the data of left table and if data not present in right table you get null.
So in where clause you should check any if the field of second table is null or not like this.
select p.* from mycom_poll as p
LEFT JOIN mycom_polls_result as pr on p.b_id = pr.b_poll_id
where pr.b_user_id !=14 p.v_status = 'enable'
And p.v_country like '%India%' AND p.d_activate_date <= '2015-04-17'
and pr.b_poll_id is null order by p.b_id limit 1
I request you to understand left join first.

Answer 2

At the moment you are using LEFT JOIN which is correct to force all rows of the mycom_poll table to be considered, but you're joining on p.b_id != pr.b_poll_id which will associate each poll row in mycom_poll with all the unrelated results rows in mycom_polls_result that don't belong to it.
You probably need to join on p.b_id = pr.b_poll_id, and then require rows with pr.b_poll_id IS NULL in your WHERE clause.
You also seem to be trying to restrict results to pr.b_user_id !=14, which is fine, but that needs to be stated along with other filtering conditions in the WHERE clause. The LEFT JOIN ON clause is just going to be used to declare the column relationships between your tables.
SELECT p.*
FROM mycom_poll as p
LEFT JOIN mycom_polls_result as pr
ON p.b_id = pr.b_poll_id
WHERE
pr.b_user_id !=14
AND p.v_status = 'enable'
AND p.v_country like '%India%'
AND p.d_activate_date <= '2015-04-17'
AND pr.b_poll_id IS NULL
ORDER BY p.b_id LIMIT 1
HTH