mysql连接空行？

Question

我有这个疑问

SELECT cities.city, city_date.date_avail 
FROM cities, city_date 
WHERE city_date.city_id = cities.id; 

，但我想查询返回从城市所有的城市，即使他们没有与之相关的city_date？

Answer 1

看到外连接;）（右/左/ FULL）

SELECT cities.city, city_date.date_avail 
FROM cities 
LEFT OUTER JOIN city_date ON city_date.city_id = cities.id;