从MySQL联接的第一个表获取所有结果

Question

I have two tables. I need to select all the rows (only one condition hotel_id=2) from first table and selected rows (based on condition) from 2nd table. But I'm using left join only get the datas from the 2nd table.

Query

 SELECT R.name room_name,
        R.id room_id,
        UD.discount 
   FROM user_discounts UD 
   LEFT 
   JOIN rooms R 
     ON R.id = UD.room_id
  WHERE UD.user_id = 1482
    AND UD.hotel_id = 2

I need to show all rooms, but now shows the common in two tables.

Answer 1

Reverse the table relationships if you want all rooms

SELECT `R`.`name` as `room_name`, `R`.`id` as `room_id`, `UD`.`discount` as `discount`
FROM  `rooms` as `R`
LEFT JOIN `user_discounts` as `UD` ON `R`.`id`= `UD`.`room_id`
             AND`UD`.`user_id` = '1482'
             AND `UD`.`hotel_id` = '2'

But you will also need to alter the where clause as well. Instead of your original where clause they can be used as part of the conditions of the join instead.

The effect of the where clause can easily be overlooked, but if you reference a left joined table in the where clause you must also allow for the data from that table to be NULL. eg

SELECT `R`.`name` as `room_name`, `R`.`id` as `room_id`, `UD`.`discount` as `discount`
FROM  `rooms` as `R`
LEFT JOIN `user_discounts` as `UD` ON `R`.`id`= `UD`.`room_id`
WHERE (`UD`.`user_id` = '1482'
   AND `UD`.`hotel_id` = '2'
      )
   OR `UD`.`room_id` IS NULL

---

If table rooms has a hotel_id then:

SELECT `R`.`name` as `room_name`, `R`.`id` as `room_id`, `UD`.`discount` as `discount`
FROM  `rooms` as `R`
LEFT JOIN `user_discounts` as `UD` ON `R`.`id`= `UD`.`room_id`
             AND`UD`.`user_id` = '1482'
             AND `UD`.`hotel_id` = `R`.`hotel_id`
WHERE `R`.`hotel_id` = 2

Answer 2

As far as i understand you need all data from first table and only from second table which matches your where criteria.

 SELECT `R`.`name` as `room_name`, `R`.`id` as `room_id`, `UD`.`discount` as `discount`
 FROM `rooms` as `R`
 LEFT JOIN `user_discounts` as `UD`  ON `R`.`id`= `UD`.`room_id`
 AND `UD`.`user_id` = '1482'
 AND `UD`.`hotel_id` = '2'

According to your query the where part turns left join to inner join so you don't get all rows from first table, To solve this issue move your filters for second table in on clause, So only those records which match with the criteria will be returned from second table