从 MySQL 中的键值对表中检索一行数据

Question

我有两张桌子，一张叫customer，一张叫customer_attributes。

这个想法是客户表包含核心客户数据，并且可以自定义应用程序以支持其他属性，具体取决于它的使用方式。

customer_attributes 有以下 3 列：

customerID
key1
value1

我是否可以检索完整的行，如果指定了任何附加属性，如果没有，则默认为 NULL？我正在使用以下查询，但它仅在 customer_attributes 表中存在这两个属性时才有效。

SELECT `customer`.*, `ca1`.`value1` AS `wedding_date`, `ca2`.`value1` AS `test` 
FROM `customer` 
LEFT JOIN `customer_attributes` AS `ca1` ON customer.customerID = ca1.customerID 
LEFT JOIN `customer_attributes` AS `ca2` ON customer.customerID = ca2.customerID 
WHERE (customer.customerID = '58029') 
   AND (ca1.key1 = 'wedding_date') 
   AND (ca2.key1 = 'test')

在这种情况下，我感兴趣的两个属性称为“wedding_date”和“test”

Answer 1

试试这个：

SELECT `customer`.*, `ca1`.`value1` AS `wedding_date`, `ca2`.`value1` AS `test` 
FROM `customer` 
LEFT JOIN `customer_attributes` AS `ca1` ON customer.customerID = ca1.customerID  AND ca1.key1='wedding_date'
LEFT JOIN `customer_attributes` AS `ca2` ON customer.customerID = ca2.customerID AND ca2.key1='test'
WHERE (customer.customerID = '58029') 

将 ca1/ca2 上的 2 个 WHERE 条件移动到 JOIN 条件中应该对其进行排序

Answer 2

仅返回行的原因是 WHERE 子句中的测试。任何没有正确 key1 的行都会被完全忽略 - 否定您的 LEFT JOIN。

您可以将 key1 测试移至您的 JOIN 条件

SELECT `customer`.*, `ca1`.`value1` AS `wedding_date`, `ca2`.`value1` AS `test` 
FROM `customer` 
LEFT JOIN `customer_attributes` AS `ca1` ON customer.customerID = ca1.customerID AND ca1.key1 = 'wedding_date'
LEFT JOIN `customer_attributes` AS `ca2` ON customer.customerID = ca2.customerID AND ca2.key1 = 'test'
WHERE (customer.customerID = '58029') 

Answer 3

使用 LEFT OUTER JOIN 谓词的“关键”测试，如下所示：

SELECT `customer`.*, `ca1`.`value1` AS `wedding_date`, `ca2`.`value1` AS `test` 
FROM `customer` 
LEFT JOIN `customer_attributes` AS `ca1` ON customer.customerID = ca1.customerID 
   AND (ca1.key1 = 'wedding_date') 
LEFT JOIN `customer_attributes` AS `ca2` ON customer.customerID = ca2.customerID 
   AND (ca2.key1 = 'test')
WHERE (customer.customerID = '58029')