Question

我们正在尝试从MySQL数据库中的几个表中进行选择。

我们的查询是：

SELECT r.reviews_id, 
       r.customers_name,
       r.date_added,
       rd.reviews_text,
       r.reviews_rating,
       c.customers_email_address
FROM reviews r, reviews_description rd, customers c
WHERE r.customers_id = c.customers_id
  AND r.reviews_id = rd.reviews_id
  AND r.products_id = '74'
  AND r.approved='1'
ORDER BY LENGTH(rd.reviews_text) DESC

但是，表reviews在字段customers_id

上也有NULL条目

命令WHERE r.customers_id = c.customers_id仅将其限制为具有customers_id

的字段

我想知道我们如何选择NULL字段？

我们尝试了WHERE (r.customers_id = c.customers_id) OR (r.customers_id IS NULL) - 但这不起作用。

提前感谢您的帮助。

Answer 1

不要使用旧的隐式连接语法。使用显式连接 - 在本例中为left join

select r.reviews_id, r.customers_name, r.date_added, rd.reviews_text, r.reviews_rating, 
       c.customers_email_address 
FROM reviews r
INNER JOIN reviews_description rd ON r.reviews_id = rd.reviews_id
LEFT JOIN customers c ON r.customers_id = c.customers_id 
WHERE r.products_id = '74' 
AND r.approved='1' 
ORDER BY LENGTH(rd.reviews_text) DESC

Answer 2

应该是这样的：

WHERE (r.customers_id = c.customers_id = 0 OR r.customers_id IS NULL)

而不是：

WHERE (r.customers_id = c.customers_id) OR (r.customers_id IS NULL)

MySQL SELECT查询中还包括NULL行

2 个答案: