Join two tables where one table name is inside the other table - mysql

Question

I am registering various properties in the property table. Depending on the type of property, three new tables namely apartment, villa, land is used to store specific data. So what I have is the a master table which holds property URL, Property type, Ref table name (any of the three sub table name - apartment, villa, land) and the ref id which is the primary autoincrement key of the sub tables.
I tried the three seperated select query and Union of the result. The drawback is that the results will be mostly from the first table name uses among the three.
function getMyProperty() {
$tables = $this->getPropertyTables();
$result = array();
foreach ($tables as $key => $table) {
$this->db->select('a.main_heading, a.sub_heading, a.location, a.about_property, a.property_price, a.available_from, p.property_url');
$this->db->from('property p');
$this->db->join($table . ' a', 'p.ref_id = a.id');
$this->db->where('p.posted_by', $this->session->user_id);
$this->db->where('p.ref_table', $this->db->dbprefix($table));
$query = $this->db->get();
$res = $query->result();
$result = array_merge($result, $res);
}
return $result;
}
DB SCHEMA
CREATE TABLE `apartment` (
`id` int(11) NOT NULL,
`main_heading` varchar(256) NOT NULL,
`sub_heading` text NOT NULL,
`build_up_area` int(11) NOT NULL,
`carpet_area` int(11) NOT NULL,
`no_of_bedrooms` int(11) NOT NULL,
`bathrooms` int(11) NOT NULL,
`available_from` date NOT NULL,
`furnishing` varchar(256) NOT NULL,
`facing` varchar(100) NOT NULL,
`flooring` varchar(256) NOT NULL,
`parking` varchar(256) NOT NULL,
`width_of_facing_road` varchar(100) NOT NULL,
`property_age` int(11) NOT NULL,
`property_price` decimal(16,2) NOT NULL DEFAULT '0.00',
`address` text NOT NULL,
`about_property` text NOT NULL,
`location` varchar(256) NOT NULL,
`amenities` text NOT NULL,
`owner_name` varchar(256) NOT NULL,
`owner_email` varchar(256) NOT NULL,
`owner_phone` varchar(100) NOT NULL,
`active` enum('Y','N') NOT NULL DEFAULT 'Y'
) ENGINE=InnoDB DEFAULT CHARSET=latin1 ROW_FORMAT=COMPACT;
INSERT INTO `apartment` (`id`, `main_heading`, `sub_heading`, `build_up_area`, `carpet_area`, `no_of_bedrooms`, `bathrooms`, `available_from`, `furnishing`, `facing`, `flooring`, `parking`, `width_of_facing_road`, `property_age`, `property_price`, `address`, `about_property`, `location`, `amenities`, `owner_name`, `owner_email`, `owner_phone`, `active`) VALUES
(2, 'My Appartment', 'Place', 255, 400, 4, 4, '2019-08-08', 'semi', 'north', 'vitrified', '2', '15', 15, '15.00', 'ghkgkgk', 'jkhjkhjk', 'Kochi', '', 'Agent', 'abc#edg.com', '9876543210', 'Y'),
(3, 'My Appartment 2', 'Test', 255, 400, 4, 4, '2019-08-08', 'semi', 'north', 'vitrified', '2', '15', 15, '15.00', 'ghkgkgk', 'jkhjkhjk', 'Kochi', '', 'Agent', 'abc#edg.com', '9876543210', 'Y');
CREATE TABLE `land` (
`id` int(11) NOT NULL,
`main_heading` varchar(256) NOT NULL,
`sub_heading` text NOT NULL,
`plot_area` int(11) NOT NULL,
`gated_colony` int(11) NOT NULL,
`open_sides` int(11) NOT NULL,
`available_from` date NOT NULL,
`dimensions` varchar(256) NOT NULL,
`facing` varchar(100) NOT NULL,
`boundary_wall` varchar(256) NOT NULL,
`parking` varchar(256) NOT NULL,
`width_of_facing_road` varchar(100) NOT NULL,
`property_age` int(11) NOT NULL,
`property_price` decimal(16,2) NOT NULL DEFAULT '0.00',
`address` text NOT NULL,
`about_property` text NOT NULL,
`location` varchar(256) NOT NULL,
`owner_name` varchar(256) NOT NULL,
`owner_email` varchar(256) NOT NULL,
`owner_phone` varchar(100) NOT NULL,
`active` tinyint(4) NOT NULL
) ENGINE=InnoDB DEFAULT CHARSET=latin1 ROW_FORMAT=COMPACT;
CREATE TABLE `property` (
`property_id` int(11) NOT NULL,
`posted_by` int(11) NOT NULL,
`post_type` enum('RENT','SELL','LEASE') NOT NULL,
`property_type` enum('VILLA','APARTMENT','LAND') NOT NULL,
`property_url` varchar(50) NOT NULL,
`ref_table` varchar(50) NOT NULL DEFAULT '',
`ref_id` int(11) NOT NULL DEFAULT '0',
`posted_on` date NOT NULL
) ENGINE=InnoDB DEFAULT CHARSET=latin1 ROW_FORMAT=COMPACT;
INSERT INTO `property` (`property_id`, `posted_by`, `post_type`, `property_type`, `property_url`, `ref_table`, `ref_id`, `posted_on`) VALUES
(4, 1, 'SELL', 'VILLA', 'lyohlp', 'villa', 2, '2019-08-05'),
(5, 1, 'SELL', 'APARTMENT', 'cvbdit', 'apartment', 2, '2019-08-05'),
(6, 2, 'SELL', 'APARTMENT', 'qwerty', 'apartment', 3, '2019-08-05'),
(7, 3, 'RENT', 'VILLA', 'asdfgh', 'villa', 3, '2019-08-05');
CREATE TABLE `villa` (
`id` int(11) NOT NULL,
`main_heading` varchar(256) NOT NULL,
`sub_heading` text NOT NULL,
`build_up_area` int(11) NOT NULL,
`carpet_area` int(11) NOT NULL,
`no_of_bedrooms` int(11) NOT NULL,
`bathrooms` int(11) NOT NULL,
`available_from` date NOT NULL,
`furnishing` varchar(256) NOT NULL,
`facing` varchar(100) NOT NULL,
`flooring` varchar(256) NOT NULL,
`total_area` varchar(256) NOT NULL,
`width_of_facing_road` varchar(100) NOT NULL,
`property_age` int(11) NOT NULL,
`property_price` decimal(16,2) NOT NULL DEFAULT '0.00',
`address` text NOT NULL,
`about_property` text NOT NULL,
`location` varchar(256) NOT NULL,
`amenities` text NOT NULL,
`owner_name` varchar(256) NOT NULL,
`owner_email` varchar(256) NOT NULL,
`owner_phone` varchar(100) NOT NULL,
`active` enum('Y','N') NOT NULL DEFAULT 'Y'
) ENGINE=InnoDB DEFAULT CHARSET=latin1 ROW_FORMAT=COMPACT;
INSERT INTO `villa` (`id`, `main_heading`, `sub_heading`, `build_up_area`, `carpet_area`, `no_of_bedrooms`, `bathrooms`, `available_from`, `furnishing`, `facing`, `flooring`, `total_area`, `width_of_facing_road`, `property_age`, `property_price`, `address`, `about_property`, `location`, `amenities`, `owner_name`, `owner_email`, `owner_phone`, `active`) VALUES
(2, 'My Villa', 'Kakkanad', 54, 5, 4, 4, '2019-08-06', 'semi', 'west', 'not-vitrified', '111', '10', 0, '12.00', 'fhgfgh', 'ghfghf', 'Kochi', 'car_parking,water_supply,garden,fitness_center,shower,fridge', 'dfdfdf', 'abc#edg.com', '9876543210', 'Y'),
(3, 'My Villa 2', 'Place', 54, 5, 4, 4, '2019-08-06', 'semi', 'west', 'not-vitrified', '111', '10', 0, '12.00', 'fhgfgh', 'ghfghf', 'Kochi', 'car_parking,water_supply,garden,fitness_center,shower,fridge', 'dfdfdf', 'abc#edg.com', '9876543210', 'Y');
ALTER TABLE `apartment`
ADD PRIMARY KEY (`id`);
ALTER TABLE `land`
ADD PRIMARY KEY (`id`);
ALTER TABLE `property`
ADD PRIMARY KEY (`property_id`),
ADD KEY `ref_id` (`ref_id`);
ALTER TABLE `villa`
ADD PRIMARY KEY (`id`);
ALTER TABLE `apartment`
MODIFY `id` int(11) NOT NULL AUTO_INCREMENT, AUTO_INCREMENT=4;
ALTER TABLE `land`
MODIFY `id` int(11) NOT NULL AUTO_INCREMENT;
ALTER TABLE `property`
MODIFY `property_id` int(11) NOT NULL AUTO_INCREMENT, AUTO_INCREMENT=8;
ALTER TABLE `villa`
MODIFY `id` int(11) NOT NULL AUTO_INCREMENT, AUTO_INCREMENT=4;
I expect result based on the decreasing of post date irrespective of the order of the table I provided.

Answer 1

As we know only one of the type can be mapped with the property.
So data will be generated from one table(which store details of that property type) only, other tables will produce null.
A great way to pick data from multiple columns if only one of them is not null, is using CONCAT_WS or COALESCE.
SELECT
P.property_id, P.property_url,
CONCAT_WS('', V.main_heading, A.main_heading, L.main_heading) AS main_heading,
CONCAT_WS('', V.sub_heading, A.sub_heading, L.sub_heading) AS sub_heading,
CONCAT_WS('', V.location, A.location, L.location) AS location,
CONCAT_WS('', V.about_property, A.about_property, L.about_property) AS about_property,
CONCAT_WS('', V.property_price, A.property_price, L.property_price) AS property_price,
CONCAT_WS('', V.available_from, A.available_from, L.available_from) AS available_from,
P.posted_on
FROM property P
LEFT JOIN villa V ON P.ref_table = 'villa' AND P.ref_id = V.id
LEFT JOIN apartment A ON P.ref_table = 'apartment' AND P.ref_id = A.id
LEFT JOIN land L ON P.ref_table = 'land' AND P.ref_id = L.id
ORDER BY P.posted_on DESC;
Using COALESCE at place of CONCAT_WS would be like:
COALESCE(V.main_heading, A.main_heading, L.main_heading) AS main_heading
Reference: MySQL Function: COALESCE, MySQL Function: CONCAT_WS

Answer 2

You can integrate the following query in CI instead of looping in php. It provides single set array.
SELECT
a.*,
b.id,
c.id,
d.id
FROM
`property` AS a
LEFT JOIN `apartment` AS b
ON a.`ref_id` = b.id
AND a.`ref_table` = 'apartment'
LEFT JOIN `land` AS c
ON a.`ref_id` = c.id
AND a.`ref_table` = 'land'
LEFT JOIN `villa` AS d
ON a.`ref_id` = d.id
AND a.`ref_table` = 'villa'
ORDER BY a.`posted_by` DESC ;
Here, joining tables respective of constant table name and other fields will return null.Those conditions can map in PHP code.
Note : add necessary columns in select what you needed.
add this condition too : $this->db->where('p.posted_by', $this->session->user_id); in query