在 SELECT Case 语句中组合行

Question

我有这个问题

SELECT questions.question_id, 
       questions.question, 
       questions.answer_id, 
       nlp_terms.word,
       ( CASE 
           WHEN answers.id = questions.answer_id THEN answers.answer 
         END ) AS answer, 
       ( CASE 
           WHEN answers.id != questions.answer_id THEN answers.answer 
         END ) AS incorrect_answer_1, 
       ( CASE 
           WHEN answers.id != questions.answer_id THEN answers.answer 
         END ) AS incorrect_answer_2 
FROM   questions 
       JOIN answers 
         ON answers.question_id = questions.question_id 
       JOIN nlp_terms 
         ON questions.question_id = nlp_terms.question_id
WHERE questions.question_id = '1'

此查询输出：

╔═════════════╦═══════════════════════════════════════════╦═══════════╦═══════════╦════════╦════════════════════╦════════════════════╗
║ question_id ║                 question                  ║ answer_id ║   word    ║ answer ║ incorrect_answer_1 ║ incorrect_answer_2 ║
╠═════════════╬═══════════════════════════════════════════╬═══════════╬═══════════╬════════╬════════════════════╬════════════════════╣
║        1369 ║ What continent is the country Lesotho in? ║      4106 ║ What      ║ NULL   ║ South America      ║ South America      ║
║        1369 ║ What continent is the country Lesotho in? ║      4106 ║ continent ║ NULL   ║ South America      ║ South America      ║
║        1369 ║ What continent is the country Lesotho in? ║      4106 ║ is        ║ NULL   ║ South America      ║ South America      ║
║        1369 ║ What continent is the country Lesotho in? ║      4106 ║ the       ║ NULL   ║ South America      ║ South America      ║
║        1369 ║ What continent is the country Lesotho in? ║      4106 ║ country   ║ NULL   ║ South America      ║ South America      ║
║        1369 ║ What continent is the country Lesotho in? ║      4106 ║ Lesotho   ║ NULL   ║ South America      ║ South America      ║
║        1369 ║ What continent is the country Lesotho in? ║      4106 ║ in?       ║ NULL   ║ South America      ║ South America      ║
║        1369 ║ What continent is the country Lesotho in? ║      4106 ║ What      ║ Africa ║ NULL               ║ NULL               ║
║        1369 ║ What continent is the country Lesotho in? ║      4106 ║ continent ║ Africa ║ NULL               ║ NULL               ║
║        1369 ║ What continent is the country Lesotho in? ║      4106 ║ is        ║ Africa ║ NULL               ║ NULL               ║
║        1369 ║ What continent is the country Lesotho in? ║      4106 ║ the       ║ Africa ║ NULL               ║ NULL               ║
║        1369 ║ What continent is the country Lesotho in? ║      4106 ║ country   ║ Africa ║ NULL               ║ NULL               ║
║        1369 ║ What continent is the country Lesotho in? ║      4106 ║ Lesotho   ║ Africa ║ NULL               ║ NULL               ║
║        1369 ║ What continent is the country Lesotho in? ║      4106 ║ in?       ║ Africa ║ NULL               ║ NULL               ║
║        1369 ║ What continent is the country Lesotho in? ║      4106 ║ What      ║ NULL   ║ Australia          ║ Australia          ║
║        1369 ║ What continent is the country Lesotho in? ║      4106 ║ continent ║ NULL   ║ Australia          ║ Australia          ║
║        1369 ║ What continent is the country Lesotho in? ║      4106 ║ is        ║ NULL   ║ Australia          ║ Australia          ║
║        1369 ║ What continent is the country Lesotho in? ║      4106 ║ the       ║ NULL   ║ Australia          ║ AustraliA          ║
║        1369 ║ What continent is the country Lesotho in? ║      4106 ║ country   ║ NULL   ║ Australia          ║ Australia          ║
║        1369 ║ What continent is the country Lesotho in? ║      4106 ║ Lesotho   ║ NULL   ║ Australia          ║ Australia          ║
║        1369 ║ What continent is the country Lesotho in? ║      4106 ║ in?       ║ NULL   ║ Australia          ║ Australia          ║
╚═════════════╩═══════════════════════════════════════════╩═══════════╩═══════════╩════════╩════════════════════╩════════════════════╝

如您所见，它引入了我想要的所有数据，只是我的 CASE 语句采用了自己的一行而不是分组。

我有很多 GROUP BY 操作，但无法让它像我预期的输出：

╔═════════════╦═══════════════════════════════════════════╦═══════════╦═══════════╦════════╦════════════════════╦════════════════════╗
║ question_id ║                 question                  ║ answer_id ║   word    ║ answer ║ incorrect_answer_1 ║ incorrect_answer_2 ║
╠═════════════╬═══════════════════════════════════════════╬═══════════╬═══════════╬════════╬════════════════════╬════════════════════╣
║        1369 ║ What continent is the country Lesotho in? ║      4106 ║ What      ║ Africa ║ South America      ║ Australia          ║
║        1369 ║ What continent is the country Lesotho in? ║      4106 ║ continent ║ Africa ║ South America      ║ Australia          ║
║        1369 ║ What continent is the country Lesotho in? ║      4106 ║ is        ║ Africa ║ South America      ║ Australia          ║
║        1369 ║ What continent is the country Lesotho in? ║      4106 ║ the       ║ Africa ║ South America      ║ Australia          ║
║        1369 ║ What continent is the country Lesotho in? ║      4106 ║ country   ║ Africa ║ South America      ║ Australia          ║
║        1369 ║ What continent is the country Lesotho in? ║      4106 ║ Lesotho   ║ Africa ║ South America      ║ Australia          ║
║        1369 ║ What continent is the country Lesotho in? ║      4106 ║ in?       ║ Africa ║ South America      ║ Australia          ║
╚═════════════╩═══════════════════════════════════════════╩═══════════╩═══════════╩════════╩════════════════════╩════════════════════╝

为了获得预期的输出，我缺少什么？

Answer 1

我认为这会产生您想要的结果：

SELECT q.question_id, q.question, q.answer_id, t.word,
       MAX(CASE WHEN a.id = q.answer_id THEN a.answer 
           END) AS answer, 
       MAX(CASE WHEN a.id <> q.answer_id THEN a.answer 
           END) AS incorrect_answer_1, 
       MIN(CASE WHEN a.id <> q.answer_id THEN a.answer 
           END) AS incorrect_answer_2 
FROM questions q JOIN
     answers a
     ON a.question_id = q.question_id JOIN
     nlp_terms t
     ON q.question_id = t.question_id
WHERE q.question_id = 1
GROUP BY q.question_id, q.question, q.answer_id, t.word;

Answer 2

您是否尝试过“WHERE questions.question_id = '1' AND answer IS NOT NULL”

参考：http://www.dofactory.com/sql/where-isnull