Question

我有以下三个表

表a（id，name）作为主键

表b（id，name，country）作为主键，c_id作为外键

表c（id）作为主键，另一列是类型

我想从这些表中选择ID，名称和类型，其中country =＆＃39; CA＆＃39;

我的数据库是MySQL，我写的SQL如下：

select n.activity_date,n.advertiser_id,c.type as type
from  a JOIN b as n
    on (a.id=b.id,a.name=b.name)
    JOIN c
    on n.c_id=c.id
where b.country='CA'
limit 10;

错误是

ERROR 1064 (42000): You have an error in your SQL syntax; check the manual that corresponds to your MySQL server version for the right syntax to use near 'as n
on (a.activity_date=d.activity_date,a.advertiser_id=d.advertiser_id)
JOIN t' at line 2

Answer 1

您必须将AND用于多个条件连接，而不是逗号。

select n.activity_date,n.advertiser_id,c.type as type
from  a 
    JOIN b as n on a.id=n.id AND a.name=n.name
    JOIN c on n.c_id=c.id
where n.country='CA'
limit 10;

Answer 2

您的查询应该是

select n.activity_date,n.advertiser_id,c.type as type
from  a JOIN b as n
    -- because of the alias name you've got to reference table b as n
    -- and you've omitted the AND operator
    on (a.id=n.id AND a.name=n.name)  
    JOIN c
    on n.c_id=c.id
where n.country='CA'  -- you've got to use n instead of b
limit 10;

Answer 3

您的on不正确。您不要将这些子句与,分开。你必须使用布尔逻辑，这意味着and和or，例如：

from  a JOIN b as n
    on (a.id=b.id AND a.name=b.name)
                  ^^^^---- no commas here

如何使用多个主键连接三个表

3 个答案: