想不通的MySQL聲明

Question

我的SQL語句看起來像這樣

SELECT q.content 
FROM questions AS q, user_question_assoc AS uqa 
WHERE uqa.user_id=? AND (q.course_id = uqa.course_id 
OR q.lesson_id=uqa.lesson_id 
OR q.question_id=uqa.question_id) 

聲明的最後一部分是不是因爲我需要

WHERE uqa.user_id=? AND (q.course_id = uqa.course_id 
    OR q.lesson_id=uqa.lesson_id 
    OR q.question_id=uqa.question_id) 

其實我想弄清楚1，僅在MySQL聲明從以下（我試過在人類的語言來解釋））

如果uqa.question_id不爲0（或NULL）則

WHERE uqa.user_id=? AND (q.course_id = uqa.course_id 
     AND q.lesson_id=uqa.lesson_id 
     AND q.question_id=uqa.question_id) 

如果uqa.lesson_id不爲0（或NULL），但uqa.question_id是0，那麼

WHERE uqa.user_id=? AND (q.course_id = uqa.course_id 
     AND q.lesson_id=uqa.lesson_id) 

如果uqa.course_id不爲0（或NULL），但uqa.question_id和uqa.lesson_id是0，那麼

WHERE uqa.user_id=? AND q.course_id = uqa.course_id 

有什麼建議？ THX提前

Answer 1

我認爲你可以使用情況下，如：

WHERE (CASE WHEN xx is not null then xid when yyx is null the yid end) = (CASE WHEN ww is null then someother id else id end) 

我不知道MySQL的語法，但是這是它如何能在Oracle或MS SQL進行

Answer 2

一些如果您使用OR子句，則數據庫（如MySQL）不會使用索引。使用UNION而是可以與大表性能要好得多：

SELECT 
    1 AS id, 
    q.content 
FROM 
    questions AS q 
INNER JOIN 
    user_question_assoc AS uqa ON 
     q.course_id = uqa.course_id AND 
     q.lesson_id = uqa.lesson_id AND 
     q.question_id = uqa.question_id 
WHERE 
    uqa.user_id  = ? AND 
    uqa.question_id <> 0 
UNION 
SELECT 
    2 AS id, 
    q.content 
FROM 
    questions AS q 
INNER JOIN 
    user_question_assoc AS uqa ON 
     q.course_id = uqa.course_id AND 
     q.lesson_id = uqa.lesson_id 
WHERE 
    uqa.user_id  = ? AND 
    uqa.question_id = 0 
UNION 
SELECT 
    3 AS id, 
    q.content 
FROM 
    questions AS q 
INNER JOIN 
    user_question_assoc AS uqa ON 
     q.course_id = uqa.course_id 
WHERE 
    uqa.user_id  = ? AND 
    uqa.question_id = 0 AND 
    uqa.lession_id = 0 

Answer 3

首先，你應該這樣做使用適當的聯接語法：

SELECT q.content 
FROM questions q join 
    user_question_assoc uqa 
    on (q.course_id = uqa.course_id OR 
     q.lesson_id=uqa.lesson_id OR 
     q.question_id=uqa.question_id) 
WHERE uqa.user_id=? 

現在試試這個：

SELECT q.content 
FROM questions q join 
    user_question_assoc uqa 
    on q.course_id = uqa.course_id AND 
     (q.lesson_id=uqa.lesson_id OR coalesce(uqa.lesson_id, 0) = 0) AND 
     (q.question_id=uqa.question_id or coalesce(uqa.question_id, 0) = 0) 
WHERE uqa.user_id=?