没有价值的联接SQL表

Question

I am doing a table join

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;

My problem is that my table looks like this:

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

So sometimes the rows will have the value of city as 0. However, with the table join if the city=0, id 3 would not be shown in the results. As there is no city id of 0. Is there a way to rectify this using SQL?

Answer 1

As eggyal suggested, your query could look something like this:

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

Using a CASE statement works but I think COALESCE is a little cleaner.

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;

As eggyal said, it's best to use NULL values but if you're stuck using zeros you could add a 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;