Question

我正在做一个表连接

SELECT b.type AS business, 
       b.business AS business,
       b.city AS city 
FROM businesses as b 
   JOIN cities AS c ON n.city = c.id;

我的问题是我的桌子看起来像这样：

id    type   business    city
 1     1       2           4
 2     4       5           2
 3     2       3           0

因此有时行的值将为city的值为0.但是，如果city = 0，则使用表连接，结果中不会显示id 3。由于没有城市ID为0.有没有办法使用SQL来纠正这个问题？

Answer 1

正如eggyal建议的那样，您的查询可能如下所示：

SELECT b.type AS business, 
       b.business AS business,
       CASE WHEN c.city IS  NULL THEN 'N/A' ELSE c.city END AS city 
FROM businesses as b 
  LEFT JOIN cities AS c ON b.city = c.id;

Answer 2

使用CASE语句有效，但我认为COALESCE更清晰。

SELECT b.type AS business, 
       b.business AS business,
       COALESCE(c.city,'N/A') AS city 
FROM businesses as b 
  LEFT JOIN cities AS c ON b.city = c.id;

正如eggyal所说，最好使用NULL值，但如果您使用零，则可以添加NULLIF

SELECT b.type AS business, 
       b.business AS business,
       COALESCE(NULLIF(c.city,0),'N/A') AS city 
FROM businesses as b 
  LEFT JOIN cities AS c ON b.city = c.id;

加入没有值的SQL表

2 个答案: