是否可以将此子查询转换为联接？

Question

如果可能，我想用联接替换子查询。

SELECT `fftenant_farmer`.`person_ptr_id`, `fftenant_surveyanswer`.`text_value`
FROM `fftenant_farmer`
INNER JOIN `fftenant_person` 
ON (`fftenant_farmer`.`person_ptr_id` = `fftenant_person`.`id`) 
LEFT OUTER JOIN `fftenant_surveyanswer` 
ON fftenant_surveyanswer.surveyquestion_id = 1
AND fftenant_surveyanswer.`surveyresult_id` IN (SELECT y.`surveyresult_id` FROM `fftenant_farmer_surveyresults` y WHERE y.farmer_id = `fftenant_farmer`.`person_ptr_id`)

我试过了：

SELECT `fftenant_farmer`.`person_ptr_id`, `fftenant_surveyanswer`.`text_value`#, T5.`text_value`
FROM `fftenant_farmer`
INNER JOIN `fftenant_person` 
ON (`fftenant_farmer`.`person_ptr_id` = `fftenant_person`.`id`) 
LEFT OUTER JOIN `fftenant_farmer_surveyresults` 
ON (`fftenant_farmer`.`person_ptr_id` = `fftenant_farmer_surveyresults`.`farmer_id`) 
LEFT OUTER JOIN `fftenant_surveyanswer` 
ON (`fftenant_farmer_surveyresults`.`surveyresult_id` = `fftenant_surveyanswer`.`surveyresult_id`) 
AND fftenant_surveyanswer.surveyquestion_id = 1 

但这给了我每个农民一份调查结果的记录。 我只希望第一个查询返回的每个农民一条记录。

在大多数RDBM上，联接可能更快，但是我问这个问题的真正原因是我似乎无法制定联接来替换子查询，并且我想知道是否有可能。

Answer 1

您可以使用DISTINCT或GROUP BY ，如mvds和Brilliand所建议的那样，但是我认为，如果将最后一个联接更改为内部联接，则它更接近查询的设计意图，但可以提高其优先级：

SELECT farmer.person_ptr_id, surveyanswer.text_value
  FROM fftenant_farmer AS farmer
 INNER
  JOIN fftenant_person AS person
    ON person.id = farmer.person_ptr_id
  LEFT
 OUTER
  JOIN
(      fftenant_farmer_surveyresults AS farmer_surveyresults
 INNER
  JOIN fftenant_surveyanswer AS surveyanswer
    ON surveyanswer.surveyresult_id = farmer_surveyresults.surveyresult_id
   AND surveyanswer.surveyquestion_id = 1
)
    ON farmer_surveyresults.farmer_id = farmer.person_ptr_id

从广义上讲，这最终会得到与DISTINCT或GROUP BY方法相同的结果，但是恕我直言， 这是一种更原则性，较少临时性的方法。

Answer 2

使用SELECT DISTINCT或GROUP BY删除重复的条目。

尽量减少尝试：

SELECT DISTINCT `fftenant_farmer`.`person_ptr_id`, `fftenant_surveyanswer`.`text_value`#, T5.`text_value`
FROM `fftenant_farmer`
INNER JOIN `fftenant_person` 
ON (`fftenant_farmer`.`person_ptr_id` = `fftenant_person`.`id`) 
LEFT OUTER JOIN `fftenant_farmer_surveyresults` 
ON (`fftenant_farmer`.`person_ptr_id` = `fftenant_farmer_surveyresults`.`farmer_id`) 
LEFT OUTER JOIN `fftenant_surveyanswer` 
ON (`fftenant_farmer_surveyresults`.`surveyresult_id` = `fftenant_surveyanswer`.`surveyresult_id`) 
AND fftenant_surveyanswer.surveyquestion_id = 1

Answer 3

我问这个问题的真正原因是我似乎无法制定联接来替换子查询，并且我想知道是否有可能

然后考虑一个更简单的示例，例如

SELECT * 
  FROM T1
 WHERE id IN (SELECT id FROM T2);

这被称为半连接，并且如果需要，可以使用（带有其他SELECT ） JOIN和SELECT子句重写为a）仅从“外部”表中进行项目，b）仅返回DISTINCT行：

SELECT DISTINCT T1.* 
  FROM T1
       JOIN T2 USING (id);