我目前的查询得到了什么：
+---------------+--------------------+-------+--------+--------+
| Name | email | Apple | Orange | Banana |
+---------------+--------------------+-------+--------+--------+
| NULL | NULL | YES | YES | YES |
| Molly Weasley | mweasley@gmail.com | YES | YES | YES |
| Viktor Krum | vkrum@gmail.com | YES | YES | NULL |
+---------------+--------------------+-------+--------+--------+
我想要的是什么：
+---------------+--------------------+-------+--------+--------+--------------+
| Name | email | Apple | Orange | Banana | Info Entered |
+---------------+--------------------+-------+--------+--------+--------------+
| Severus Snape | ssnape@gmail.com | | | | No |
| Molly Weasley | mweasley@gmail.com | YES | YES | YES | Yes |
| Viktor Krum | vkrum@gmail.com | YES | YES | | Yes |
+---------------+--------------------+-------+--------+--------+--------------+
基本上，Severus Snape尚未回答调查问卷（他在答案表中没有记录）但我还是喜欢给他看（因为他收到了问题表中所示的调查问卷）。苹果，橙子和香蕉对他来说都是空白的（由于某些原因，它一直显示为YES）。
我使用的查询：
SELECT CONCAT(people.firstName, ' ', people.lastName) AS Name,
people.email,
MAX(CASE WHEN questions.fruit = 'apple' THEN 'YES' END) AS Apple,
MAX(CASE WHEN questions.fruit = 'orange' THEN 'YES' END) AS Orange,
MAX(CASE WHEN questions.fruit = 'banana' THEN 'YES' END) AS Banana
FROM answers
LEFT JOIN questions ON answers.questionID = questions.questionID
LEFT JOIN people ON questions.person = people.person AND questions.questionnaire = '24'
GROUP BY CONCAT(people.firstName, ' ', people.lastName), people.email
ORDER BY Name
示例数据：
人员表：
+-----------+----------+--------------------+--------+
| firstName | lastName | email | person |
+-----------+----------+--------------------+--------+
| Harry | Potter | hpotter@gmail.com | 1 |
| Ron | Weasley | rweasley@gmail.com | 2 |
| Hermione | Granger | hgranger@gmail.com | 3 |
| Severus | Snape | ssnape@gmail.com | 4 |
| Viktor | Krum | vkrum@gmail.com | 5 |
| Molly | Weasley | mweasley@gmail.com | 6 |
| Oliver | Wood | owood@gmail.com | 7 |
| Remus | Loopin | rlupin@gmail.com | 8 |
+-----------+----------+--------------------+--------+
问题表：
+---------------+--------+------------+--------+
| questionnaire | person | questionID | fruit |
+---------------+--------+------------+--------+
| 23 | 1 | 55 | apple |
| 23 | 1 | 56 | orange |
| 23 | 1 | 57 | banana |
| 23 | 2 | 58 | apple |
| 23 | 2 | 59 | orange |
| 23 | 2 | 60 | banana |
| 23 | 3 | 61 | apple |
| 23 | 3 | 62 | orange |
| 23 | 3 | 63 | banana |
| 24 | 4 | 64 | apple |
| 24 | 4 | 65 | orange |
| 24 | 4 | 66 | banana |
| 24 | 5 | 67 | apple |
| 24 | 5 | 68 | orange |
| 24 | 5 | 69 | banana |
| 24 | 6 | 70 | apple |
| 24 | 6 | 71 | orange |
| 24 | 6 | 72 | banana |
+---------------+--------+------------+--------+
答案表：
+----------+------------+---------------------+-----------------+
| answerID | questionID | info | dateAnswered |
+----------+------------+---------------------+-----------------+
| 40 | 59 | some info here | 5/26/2017 19:23 |
| 41 | 59 | | 5/26/2017 18:30 |
| 42 | 59 | | 5/26/2017 18:29 |
| 43 | 66 | | 5/26/2017 18:36 |
| 44 | 66 | | 5/26/2017 20:28 |
| 45 | 70 | | 5/26/2017 20:28 |
| 46 | 55 | more info here | 5/26/2017 20:29 |
| 47 | 71 | | 5/26/2017 20:29 |
| 48 | 67 | | 5/26/2017 20:31 |
| 49 | 64 | | 5/26/2017 18:37 |
| 50 | 55 | | 5/26/2017 18:46 |
| 51 | 72 | | 5/26/2017 18:46 |
| 52 | 72 | another bit of info | 5/26/2017 18:46 |
| 53 | 72 | and more info | 5/26/2017 18:46 |
| 54 | 61 | | 5/26/2017 18:29 |
| 55 | 61 | | 5/26/2017 18:30 |
| 56 | 68 | the info | 5/26/2017 18:36 |
| 57 | 59 | | 5/26/2017 19:22 |
| 58 | 66 | | 5/26/2017 19:37 |
| 59 | 61 | | 5/26/2017 19:37 |
| 60 | 61 | | 5/26/2017 18:33 |
| 61 | 68 | this info | 5/26/2017 18:38 |
| 62 | 68 | | 5/26/2017 19:33 |
| 63 | 68 | some more info | 5/26/2017 19:42 |
| 64 | 68 | | 5/26/2017 19:56 |
| 65 | 60 | | 5/26/2017 20:03 |
+----------+------------+---------------------+-----------------+
答案 0 :(得分：1)
尝试更改顺序：
SELECT
CONCAT(people.firstName, ' ', people.lastName) AS Name,
people.email,
MAX(CASE WHEN questions.fruit = 'apple' AND answers.questionID IS NOT NULL THEN 'YES' END) AS Apple,
MAX(CASE WHEN questions.fruit = 'orange' AND answers.questionID IS NOT NULL THEN 'YES' END) AS Orange,
MAX(CASE WHEN questions.fruit = 'banana' AND answers.questionID IS NOT NULL THEN 'YES' END) AS Banana
FROM people
JOIN questions ON questions.person = people.person AND questions.questionnaire = '24'
LEFT JOIN answers ON answers.questionID = questions.questionID
GROUP BY CONCAT(people.firstName, ' ', people.lastName), people.email
ORDER BY Name