通过php与mysql比较两个表

Question

说我有这两个mysql表：

country
----
username        Country
John            USA
Sarah           Canada

fav_color
----
username        Color
John            Blue
Sarah           Green

如何选择Sarah最喜欢的颜色，但仅限于她在加拿大的情况下？

就像是：

SELECT color FROM fav_color WHERE name='sarah' AND username=(a canadian username)

Answer 1

一个相当简单的JOIN；

SELECT color FROM fav_color f
JOIN country c ON c.username=f.username
WHERE c.username='Sarah'
  AND c.country='Canada';

演示在这里 。

Answer 2

在两个表上选择，然后指定要按用户名匹配记录：

SELECT color FROM fav_color, country 
    WHERE fav_color.username='Sarah' AND
    fav_color.username = country.username AND
    country = 'Canada';

Answer 3

使用WHERE EXISTS怎么样？

SELECT color FROM fav_color
    WHERE EXISTS
        (SELECT username FROM country
             WHERE country.username = fav_color.username
             AND country.Country = 'Canada')

Answer 4

select color from fav_color where 
    name='sarah' and 
    name in (
        select name from country where country='canada'
    )