Question

我的sql：

SELECT * FROM ex_pair
LEFT JOIN ex_currency as client
ON client_curr = currency_id
LEFT JOIN ex_currency as company
ON co_curr = currency_id

我需要获取两个currency_id的数据，但我有错误

不明确的列名：'currency_id'

有没有办法做得对，或者我需要使用两个查询？

Answer 1

您需要在联接中包含别名，如下所示：

SELECT *
FROM ex_pair
LEFT JOIN ex_currency AS client
ON client_curr = client.currency_id
LEFT JOIN ex_currency as company
ON co_curr = company.currency_id

你可能还想做一些选择*之外的事情，因为你将有两个具有相同列的表 - 比如

SELECT pair.*, company.currency_id AS company_currency_id, client.currency_id as client_currency_id, [...]
FROM ex_pair AS pair
[...]

这种方式当您明确声明要使用带有别名的ex_currency的列时，您可以更容易地知道哪个是客户端，哪个是公司。您需要对结果中所需的货币表中的每一列执行此操作，但如果通过循环遍历列表并附加别名，可以轻松地在代码中使用表结构，则可以执行此操作。

$array = [
    1=> "currency_id",
    2=> "currency_name"
];

$columns = ""
foreach($array as $column){
    $columns.= "company.".$column." AS company_".$column;
    $columns.= ",client.".$column." AS client_".$column.",";
}

$columns = rtrim($columns,',');

给你

company.currency_id AS company_currency_id,client.currency_id AS client_currency_id,company.currency_name AS company_currency_name,client.currency_name AS client_currency_name

在SELECT pair.*后添加，然后从货币表中获取列，别名，以便知道哪个是哪个。

Answer 2

您可以使用您为表格提供的别名：

SELECT client.currency_id as client_currency, company.currency_id as company_currency
FROM ex_pair 
LEFT JOIN ex_currency as client ON client_curr = client.currency_id
LEFT JOIN ex_currency as company ON co_curr = company.currency_id

双LEFT JOIN与相同的表

2 个答案: