查询多个JOIN

Question

我想创建一个可以在以后使用的联系人表。所以结构：

table contact: id, contact_type
table contact_meta : id, contact_id, meta_key, meta_value

这个查询是不好的做法？ （超过10个加入？）

select c.id, 
    cm1.meta_value as name,
    cm2.meta_value as email,
    cm3.meta_value as bussiness_phone
    from contact as c
    left join contact_meta as cm1 on (cm1.contact_id = c.id)
    left join contact_meta as cm2 on (cm2.contact_id = c.id)
    left join contact_meta as cm3 on (cm3.contact_id = c.id)
    where c.contact_type = 'supplier' and 
    cm1.meta_key = 'name' and 
    cm2.meta_key = 'email' and
    cm3.meta_key = 'bussiness_phone' ORDER BY c.id DESC

Answer 1

查询不是“不良做法”，而是将额外条件移到它们所属的连接子句中：

select c.id, 
  cm1.meta_value as name,
  cm2.meta_value as email,
  cm3.meta_value as bussiness_phone
from contact as c
left join contact_meta as cm1 on cm1.contact_id = c.id and cm1.meta_key = 'name'
left join contact_meta as cm2 on cm2.contact_id = c.id and cm2.meta_key = 'email'
left join contact_meta as cm3 on cm3.contact_id = c.id and cm3.meta_key = 'bussiness_phone'
order by c.id desc

请注意删除不必要的括号。