mysql - 在 MySQL 查询中使用列名作为表

Question

我有一个数据库，其中一个表中的列是我需要在其中查找相应记录的表名。

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| TABLE questions                                                                                                                                                                                 |
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| Field                   | Type                                                                                                                                                                  |
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| question_id             | int(11)                                                                                                                                                               |
| question_response_table | enum('question_responses_datetime','question_responses_int','question_responses_float','question_responses_bool','question_responses_text','question_responses_enum') |
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question_response_table 中的每个值列是另一个保存用户响应的表(原始数据库设计要求将响应按数据类型分隔到单独的表中)。

我有一个查询，除了用户的响应之外，它还提供了我需要的一切。但出于性能原因，我真的很想将该响应附加到同一查询上(数据库每年都会显着增长)。这是我想做的事情的一个例子:

SELECT cr.category_id, cr.category_response_id, r.response 
FROM category_responses AS cr 
JOIN response_key_questions AS rkq ON cr.category_id = rkq.category_id 
JOIN questions AS q ON q.question_id = rkq.question_id
JOIN {q.question_response_table} AS r ON r.category_response_id = cr.category_response_id 
WHERE cr.belongs_to = 4 AND cr.reporting_year_id = 1 AND cr.date_retired IS NULL 
ORDER BY cr.category_id

Answer 1

这会变得很丑陋。最好创建一个 View ，将 response 列添加到 cr 表中。

SELECT cr.category_id, cr.category_response_id,
    CASE 
    WHEN q.question_response_table = '...' THEN
        (SELECT response
         FROM ... r
         WHERE r.category_response_id = cr.category_response_id)
    ... 
    ELSE '' 
    END AS response
FROM category_responses AS cr 
JOIN response_key_questions AS rkq ON cr.category_id = rkq.category_id 
JOIN questions AS q ON q.question_id = rkq.question_id
WHERE cr.belongs_to = 4 AND cr.reporting_year_id = 1 AND cr.date_retired IS NULL 
ORDER BY cr.category_id