無法使用在where子句

Question

首先我要選擇bill_client和一列形式bill_pkg一些列嵌套查詢來選擇，這就是簡單的象下面這樣：

查詢：

SELECT c.company_id, c.client_id, p.pkg_rate 
FROM bill_client c 
JOIN bill_pkg p on c.pkg_id=p.pkg_id 
WHERE c.status=1 

我其實是想選擇與其中extract(year_month from curdate())>

SELECT c.company_id, c.client_id, p.pkg_rate 
FROM bill_client c 
JOIN bill_pkg p on c.pkg_id=p.pkg_id 
WHERE c.status=1 and extract(year_month from curdate())> 
(SELECT MAX(due_month) 
FROM bill_rent 
WHERE c.client_id=client_id and c.company_id=company_id 
GROUP BY client_id, company_id) 

但是這個查詢只返回那些ro w其中client_id與bill_rent表匹配，並非來自bill_client表的所有行。我需要檢查company_id與client_id存在但bill_rent表due_month本月列。請任何幫助。

這裏的表模式：

tabll：bill_client

CREATE TABLE `bill_client` (
`id` int(11) NOT NULL AUTO_INCREMENT, 
`client_id` varchar(30) NOT NULL, 
`client_name` varchar(50) NOT NULL, 
`address` varchar(100) NOT NULL, 
`opening_balance` decimal(10,2) NOT NULL, 
`conn_charge` decimal(13,2) NOT NULL, 
`con_date` date NOT NULL, 
`mobile` varchar(12) NOT NULL, 
`pkg_id` int(11) NOT NULL, 
`conn_type` int(11) NOT NULL, 
`status` tinyint(1) NOT NULL DEFAULT '1', 
`area_id` int(11) NOT NULL, 
`managed_by` int(11) NOT NULL, 
`remarks` varchar(100) NOT NULL, 
`photo` varchar(50) DEFAULT NULL, 
`company_id` varchar(15) NOT NULL, 
PRIMARY KEY (`id`) 
) ENGINE=InnoDB AUTO_INCREMENT=165 DEFAULT CHARSET=utf8 

表：bill_pkg

CREATE TABLE `bill_pkg` (
`id` int(11) NOT NULL AUTO_INCREMENT, 
`pkg_id` int(11) NOT NULL, 
`pkg_name` varchar(10) NOT NULL, 
`pkg_rate` decimal(10,0) NOT NULL, 
`company_id` varchar(15) NOT NULL, 
PRIMARY KEY (`id`), 
UNIQUE KEY `pkg_name` (`pkg_name`) 
) ENGINE=InnoDB AUTO_INCREMENT=20 DEFAULT CHARSET=utf8 

表：bill_rent

CREATE TABLE `bill_rent` (
`id` int(11) NOT NULL AUTO_INCREMENT, 
`company_id` varchar(15) NOT NULL, 
`client_id` varchar(10) NOT NULL, 
`due_month` varchar(6) NOT NULL, 
`pay_date` date NOT NULL, 
`amount_due` decimal(10,2) NOT NULL, 
`amount_paid` decimal(10,2) NOT NULL, 
PRIMARY KEY (`id`) 
) ENGINE=InnoDB AUTO_INCREMENT=7 DEFAULT CHARSET=utf8 

Answer 1

如果client_id與bill_rent表不匹配，MAX(due_month)將爲NULL，因此將當前月份與此進行比較將失敗。使用IFNULL(MAX(due_month), '')，這樣你總能得到一個有效的值來與之比較。

SELECT c.company_id, c.client_id, p.pkg_rate 
FROM bill_client c 
JOIN bill_pkg p on c.pkg_id=p.pkg_id 
WHERE c.status=1 and extract(year_month from curdate())> 
(SELECT IFNULL(MAX(due_month), '') 
FROM bill_rent 
WHERE c.client_id=client_id and c.company_id=company_id)