結合SQL查詢

Question

可能重複：
What is the difference between Join and Union?

我們如何結合單獨的查詢到單個SQL語句？例如：

SELECT packageid FROM tblhosting WHERE id='$id' AND userid='$userid' 
SELECT id FROM tblcustomfields WHERE relid=tblhosting.packageid AND fieldname='foo' 
SELECT value FROM tblcustomfieldsvalues WHERE fieldid=tblcustomfields.id AND relid='$id' 

現在，我知道嵌套queryes的，所以我可以寫這樣的：

，但我不知道是否有更好的方法來寫這個查詢？

[UPDATE2] 這裏是不變量的一個版本，希望它更清楚這次

SELECT packageid FROM tblhosting WHERE id='$id' AND userid='$userid' 
SELECT id FROM tblcustomfields WHERE relid=tblhosting.packageid AND fieldname='foo' 
SELECT value FROM tblcustomfieldsvalues WHERE fieldid=tblcustomfields.id AND relid='$id' 

這裏我只在tblcustomfieldvalues.value

Answer 1

您可以使用INNER or LEFT JOIN爲根據您的需要。

SELECT c.value 
    FROM tblhosting a INNER JOIN tblcustomfields b 
      ON b.relid = a.packageid 
     INNER JOIN tblcustomfieldsvalues c 
      ON c.fieldid = b.id 
WHERE a.id = '$id' AND 
     a.userid = '$userid' AND 
     b.fieldname = 'foo' 

Answer 2

興趣，如果你想從所有三個表中的行只有在所有三個表中存在行的情況下，這正是我所假設的，SQL看起來像這樣：

select 
    a.id, 
    b.value, 
    c.packageid 
from 
    tbcustomfields a, 
    tblcustomfieldsvalues b, 
    tblhosting c 
where 
    c.userid='$userid' 
    and a.relid=b.relid 
    and a.relid=c.packageid 
    and a.fieldname='foo' 
    and b.fieldid=c.id 

Answer 3

關於什麼：

SELECT h.packageid, cf.id, cfv.value 
FROM tblhosting h 
INNER JOIN tblcustomfields cf ON (cf.relid = h.packageid) 
INNER JOIN tblcustomfieldsvalues cfv ON (cfv.fieldid = cf.id) 
WHERE h.id = '$serviceid' 
    AND h.userid = '$userid' 
    AND cf.fieldname = 'foo' 

Answer 4

SELECT 
    tblcustomfieldvalues.value 
FROM tblhosting 
    LEFT JOIN tblcustomfields 
    ON tblcustomfields.relid = tblhosting.packageid 
     AND tblcustomfields.fieldname = 'foo' 
    LEFT JOIN tblcustomfieldsvalues 
    ON tblcustomfieldsvalues.fieldid = tblcustomfields.id 
     AND relid = '$id' 
WHERE tblhosting.id = '$id' 
    AND tblhosting.userid = '$userid' 

試試這個