LEFT/RIGHT JOIN不返回空行 [英] LEFT/RIGHT JOIN not returning empty row

问题描述

我有一个查询，该查询在2个表上使用自联接以返回单行结果.我的问题是，如果seat1为空，我什么也得不到.

I have a query which uses a self join on 2 tables to return a single row of results. The problem I have is that if seat1 is empty, I don't get anything returned.

SELECT seat1.seat_type_id
     , seat1.seat_type_qty
     , seat2.seat_type_id
     , seat2.seat_type_qty
  FROM jos_sv_apptpro2_requests AS R 
  LEFT JOIN jos_sv_apptpro2_seat_counts AS seat1 ON R.id_requests = seat1.request_id 
  LEFT JOIN jos_sv_apptpro2_seat_counts AS seat2 ON R.id_requests = seat2.request_id
 WHERE (seat1.seat_type_id = 6 AND seat2.seat_type_id = 7)
   AND R.id_requests = 8703
   AND R.resource = 3

这应该返回:

seat_type_id 6
seat_type_qty 0 <= this is the empty row
seat_type_id1 7
seat_type_qty 1

我现在有:

SELECT
IFNULL(seat1.seat_type_qty,0) AS seat_type_qty,
IFNULL(seat2.seat_type_qty,0) AS seat_type_qty
FROM jos_sv_apptpro2_requests AS R 
LEFT JOIN jos_sv_apptpro2_seat_counts AS seat1 
ON R.id_requests = seat1.request_id AND seat1.seat_type_id = 6
LEFT JOIN jos_sv_apptpro2_seat_counts AS seat2 
ON R.id_requests = seat2.request_id AND seat2.seat_type_id = 7
WHERE R.id_requests = 8696

重新编辑！ -我的错误是，我在PHP中两次返回seat_type_qty作为对象.全部排序-谢谢！

REEDIT! - My mistake, I'm returning seat_type_qty twice as an object in my PHP. All sorted now - THANKS!

推荐答案

WHERE seat1.seat_type_id = 6 AND seat2.seat_type_id = 7

删除所有带有NULL值的行.您应该将这些条件移到JOIN条件，以便RDBMS达到您的期望:

is what removes all rows with NULL values. You should move those condition to the JOIN criteria so that the RDBMS does what you are expecting:

LEFT JOIN jos_sv_apptpro2_seat_counts AS seat1 ON R.id_requests = seat1.request_id 
                                              AND seat1.seat_type_id = 6
LEFT JOIN jos_sv_apptpro2_seat_counts AS seat2 ON R.id_requests = seat2.request_id
                                              AND seat2.seat_type_id = 7

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