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[英]mySQL SELECT using LEFT JOIN, returning value or NULL WHERE column is specified in right table
[英]select LEFT/RIGHT JOIN using WHERE and return NULL's
我需要將以下兩個工作查詢轉換為單個查詢,但我嘗試的所有內容因各種原因而死於我。 我的最終結果是嘗試列出所有手頭的軟件,並顯示已安裝的軟件以及未針對要查詢的特定PC安裝的軟件。 對於已安裝的軟件,請列出名稱,否則顯示名稱為NULL。 我在where子句中嘗試了一些子選擇語句,它給了我一個沒有錯誤的結果,但沒有給出正確的結果。 任何幫助表示贊賞。
qry1
SELECT device_software.sw_id
FROM Software_device LEFT JOIN Device ON Software_device.d_id = Device.d_id
WHERE Device.d_id = 1;
qry2
SELECT Software.name, Software.sw_id, qry1.sw_id
FROM software LEFT JOIN qry1 ON software.sw_id = qry1.sw_id;
設備表
------------------
| name | d_id |
------------------
| PC1 | 1 |
| PC2 | 2 |
| PC3 | 3 |
------------------
軟件表
------------------
| name | sw_id |
------------------
| SW_a | A |
| SW_b | B |
| SW_c | C |
| SW_d | D |
------------------
Software_Device表(多對多)
------------------
| d_id | sw_id |
------------------
| 1 | A |
| 1 | B |
| 2 | A |
| 2 | B |
| 2 | C |
------------------
結果我正在尋找...(在PC1上安裝和卸載的軟件)
---------------------------------
| Sotfware | pc_id | name |
---------------------------------
| SW_a | 1 | PC1 |
| SW_b | 1 | PC1 |
| SW_c | NULL | NULL |
| SW_d | NULL | NULL |
---------------------------------
我列出了mysql和sql標簽,因為我覺得它不重要,但是萬一它確實如此,我正在使用mysql。
SELECT
s.name AS software,
IF((SELECT COUNT(sw.d_id) FROM software_device sw WHERE sw.sw_id = s.sw_id AND d_id = 1) > 0, 1, NULL) AS pc_id,
(SELECT d.name FROM device d INNER JOIN software_device sw ON d.d_id = sw.d_id WHERE sw.sw_id = s.sw_id AND d.d_id = 1) AS name
FROM
software s
ORDER BY s.name
編輯2:也許不是最有效/美麗,但它的工作原理
根據您的要求,這將為您提供正確的結果:
select software.name as Software,
device.d_id as pc_id,
device.name as name
from software
left join device_software
on device_software.sw_id = software.sw_id
left join device
on device_software.d_id = device.d_id
and device.d_id = 1
只是注意:我更喜歡將主鍵作為表中的第一列。 我還建議用復數(設備)命名你的表
如果RomanKonz的答案肯定很接近,只需將“where”移動到“on”,讓左邊的連接正常工作。
select software.name as Software,
device.d_id as pc_id,
device.name as name
from software
left join device_software
on device_software.sw_id = software.sw_id
and **device_software**.d_id = 1
left join device
on device_software.d_id = device.d_id
;
以下應該做你需要的:
select s.`name` as `Software`, d.`d_id`, d.`name` as `Device`
from software s
left outer join software_device sd
on sd.`sw_id` = s.`sw_id` and sd.`d_id` = 1
left outer join device d using (`d_id`)
order by s.`name`
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