Question

我有一个名为properties（p）的表和另一个名为certificates（c）的表。可以为每个属性分配多个证书，或者根本不分配证书。我需要生成一个使用连接的查询，并且只显示每个属性的证书表中的一个证书。显示的一个证书必须是具有最新到期日期的证书。证书表中有一个名为＆＃39; certificate_expiry_date＆＃39;的字段。简单连接将是p.property_id = c.certificate_property，但目前输出所有证书。

我的查询尝试

这是我到目前为止的查询;

SELECT DISTINCT t.tenancy_property, t.*, p.*, c.* FROM tenancy t
INNER JOIN property p
on t.tenancy_property = p.property_id
LEFT JOIN
(
    SELECT *
    FROM certificate
    WHERE certificate_expiry_date > CURDATE()
    ORDER BY certificate_expiry_date DESC
    LIMIT 1
) c ON p.property_id = c.certificate_property
WHERE t.tenancy_type='1' AND p.property_mains_gas_supply='1' AND p.property_availability='2' ORDER BY t.tenancy_id DESC LIMIT {$startpoint} , {$per_page}

此查询执行正常，但似乎没有考虑证书表上的左连接。

表certificate的表结构

CREATE TABLE IF NOT EXISTS `certificate` (
  `certificate_id` int(11) NOT NULL AUTO_INCREMENT,
  `certificate_property` int(11) DEFAULT NULL,
  `certificate_type` tinyint(4) DEFAULT NULL,
  `certificate_reference` varchar(255) COLLATE utf8_bin DEFAULT NULL,
  `certificate_start_date` date DEFAULT NULL,
  `certificate_expiry_date` date DEFAULT NULL,
  `certificate_notes` text COLLATE utf8_bin,
  `certificate_renewal_instructed` tinyint(4) DEFAULT NULL,
  `certificate_renewal_contractor` int(11) DEFAULT NULL,
  PRIMARY KEY (`certificate_id`)
) ENGINE=MyISAM  DEFAULT CHARSET=utf8 COLLATE=utf8_bin AUTO_INCREMENT=219 ;

Answer 1

使用您更新的信息更新了答案

这样的东西？

（注意：此答案假设每个属性至少有一个证书，否则子查询qMostRecentExpire可能会失败）

select
    p.property_id
    , p.* 
    , ( select 
            c.certificate_id 
        from
            certificates    as c 
        where
            c.certificate_property = p.property_id    -- all the cert of this property 
            and c.certificate_expiry_date < CURDATE() -- cert has expired
        order by c.certificate_expiry_date desc       
        limit 1                                       -- most recent one 
      ) as qMostRecentExpire
from
    properties  as p

在知道某些属性可能没有证书后更新了答案

select
    p.property_id
    , p.* 
    , ( select 
            c.certificate_id 
        from
            certificates    as c 
        where
            c.certificate_property = p.property_id    -- all the cert of this property 
            and c.certificate_expiry_date < CURDATE() -- cert has expired
        order by c.certificate_expiry_date desc       
        limit 1                                       -- most recent one 
    ) as qMostRecentExpire
from
    properties      as p
    , certificates  as c                              -- inner join : properties that
where                                                 -- has not cert will be dropped
    p.property_id = c.certificate_property

Answer 2

如果我们只需要从certificates表中返回一列或两列，我们有时可以在SELECT列表中使用相关子查询。

这种方法对大型表有一些性能影响;但对于某些用例，如果有适当的索引，这可能是一种可行的方法。

 SELECT p.id
      , p.somecol

      , ( SELECT c.col
            FROM certificate c
           WHERE c.property_id = p.id
           ORDER BY c.date_added DESC, c.id DESC
           LIMIT 1
        ) AS most_recent_cert_col

      , ( SELECT c.date_added
            FROM certificate c
           WHERE c.property_id = p.id
           ORDER BY c.date_added DESC, c.id DESC
           LIMIT 1
        ) AS most_recent_cert_date_added

   FROM property p 
  WHERE ... 
 ORDER BY ...

MySQL - 向左连接添加子句

2 个答案: