1

可能重复：
Join 和 Union 有什么区别？

我们如何将单独的查询组合成单个 sql 语句？例子：

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'

现在，我知道嵌套查询，所以我可以这样写：

但我想知道是否有更好的方法来编写这个查询？

[Update2] 这里是一个没有变量的版本，希望这次更清楚

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'

这里我只感兴趣tblcustomfieldvalues.value

4

4 回答 4

1

关于什么：

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'
于 2012-06-18T23:06:19.143 回答
1

您可以INNER or LEFT JOIN根据需要使用它。

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'
于 2012-06-18T23:42:11.810 回答
0

如果您只想从所有三个表中获取所有三个表中存在行的行，这就是我假设您想要的，SQL 如下所示：

select
    a.id,
    b.value,
    c.packageid
from
    tbcustomfields a,
    tblcustomfieldsvalues b,
    tblhosting c
where
    c.userid='$userid'
    and a.relid=b.relid
    and a.relid=c.packageid
    and a.fieldname='foo'
    and b.fieldid=c.id
于 2012-06-18T23:05:54.297 回答
0 
SELECT
  tblcustomfieldvalues.value
FROM tblhosting
  LEFT JOIN tblcustomfields
    ON tblcustomfields.relid = tblhosting.packageid
      AND tblcustomfields.fieldname = 'foo'
  LEFT JOIN tblcustomfieldsvalues
    ON tblcustomfieldsvalues.fieldid = tblcustomfields.id
      AND relid = '$id'
WHERE tblhosting.id = '$id'
    AND tblhosting.userid = '$userid'

尝试这个

于 2012-06-25T12:43:28.883 回答