SQL选择语句

我需要做一个sql SELECT语句，只有在满足条件的情况下才能连接表（或从连接表中获取值），这可能吗

    $sql = "SELECT orders.id_o, orders.user_id,orders.price, users.username 
    FROM orders Left JOIN users 
    ON orders.user_id = users.id  
    ORDER BY order_date ASC";

我有一个订单表，其中有用户ID，但我也有随机生成的ID，用于作为来宾订购的用户。我想用这个ID加入orders表上的users表，但是当这个ID是随机生成的时候，我只想从order表返回值，因为用户表中没有这个ID的记录

    $sql = "SELECT orders.id_o, orders.user_id,orders.price, users.username 
    FROM orders Left JOIN users 
    ON orders.user_id = users.id  
    ORDER BY order_date ASC";

我只有这个，它将在两个表中都存在user_id的位置写入行

$sql = "SELECT orders.id_o, orders.user_id, orders.price, users.username 
FROM orders JOIN users 
ON orders.user_id = users.id  
ORDER BY order_date ASC";

    $sql = "SELECT orders.id_o, orders.user_id,orders.price, users.username 
    FROM orders Left JOIN users 
    ON orders.user_id = users.id  
    ORDER BY order_date ASC";

---

这正是左连接的作用。要回答注释中的后续问题，可以使用

coalesce

替换从左连接返回的

null

s：

SELECT    orders.id_o, orders.user_id, orders.price, 
          COALESCE(users.username, 'Guest')
FROM      orders
LEFT JOIN users ON orders.user_id = users.id  
ORDER BY  order_date ASC

    $sql = "SELECT orders.id_o, orders.user_id,orders.price, users.username 
    FROM orders Left JOIN users 
    ON orders.user_id = users.id  
    ORDER BY order_date ASC";

---

几乎，只需更改为

left join

    $sql = "SELECT orders.id_o, orders.user_id,orders.price, users.username 
    FROM orders Left JOIN users 
    ON orders.user_id = users.id  
    ORDER BY order_date ASC";

---

下面的语句应该可以做到这一点：

    $sql = "SELECT orders.id_o, orders.user_id,orders.price, users.username 
    FROM orders Left JOIN users 
    ON orders.user_id = users.id  
    ORDER BY order_date ASC";

SELECT
 orders.id_o,
 orders.user_id,
 orders.price,
 users.username 

FROM orders

LEFT JOIN users 
  ON orders.user_id = users.id  

---

按顺序排序\u日期ASC

左连接，而不是内部连接标题是否能获得一些创造性？可能重复，您可以重复Hammerthanks，它工作得很好。是否可以为未返回任何内容的字段设置默认值？（例如，如果随机生成的ID username字段为空，我想将默认值设置为Guest）@rtom一个优雅的方法是使用

coalesce

。请参阅我编辑的答案。

    $sql = "SELECT orders.id_o, orders.user_id,orders.price, users.username 
    FROM orders Left JOIN users 
    ON orders.user_id = users.id  
    ORDER BY order_date ASC";