比较mysql的两个表通过PHP

Question

说我有两个MySQL表：

country 
---- 
username  Country 
John   USA 
Sarah   Canada 

fav_color 
---- 
username  Color 
John   Blue 
Sarah   Green 

如何选择莎拉最喜欢的颜色，但只有当她是在加拿大？

喜欢的东西：

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

Answer 1

选择对两个表，并指定要通过用户名匹配记录：

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

Answer 2

有关使用WHERE EXISTS如何？

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

Answer 3

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

Answer 4

一个漂亮的直线前进加盟;

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

演示here。