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嗨我試圖查詢是要顯示每個4表中的4個值。MySQL選擇連接空值
select i.name, c.name, s.name, d.name
from instructor i
join course c
on i.pid = c.instructor
join course_taken ct
on c.id = ct.cid
join student s
on s.id = ct.sid
join department d
on s.major = d.id or s.major is null
where i.name = 'lee';
一切都很好,除了零部分。
表結構
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| student | course | instructor | department |
--------------------------------------------------
| name | name | id | id |
--------------------------------------------------
| id | id | name | name |
--------------------------------------------------
| major |instructor| department | |
--------------------------------------------------
結果
--------------------------------------------------
| i.name | c.name | s.name | d.name |
--------------------------------------------------
| kim | math | jack | cs |
--------------------------------------------------
| kim | math | john | cs | --> THIS VALUE IS
--------------------------------------------------
| kim | math | john | ss | --> NULL AND SHOULD BE PRINTED IN NULL
--------------------------------------------------
| kim |math | json | ss |
--------------------------------------------------
如何打印空當 'CS' 和 'SS' 在 「約翰」 是NULL?
謝謝你讓我知道該功能。我試圖隱藏主要(CS或SS如果NULL)。我嘗試從教師i'選擇i.name,c.name,s.name,coalesce(d.name,'NULL') ,沒有工作。仍然在主要列中獲得'cs'和'ss':( – Gomtting
@Gomtting,請參閱編輯答案,如果有幫助 – Rahul
哦,對不起,我想我沒有意識到我從上面選擇的名稱..我編輯表我需要主要的名字,即使它是NULL,就像'cs/cs/ss/ss'到'cs/NULL/NULL/ss'。 – Gomtting