写Sql子查询

Question

我有以下3个表（组织，组织地址，地址）。每个组织都有多个地址，其中一个是主要的。如果组织有主要地址，我需要选择主要地址，否则该列可以为null或空白。

如何在mysql中编写sql子查询？ 请帮帮我，

谢谢，

Answer 1

您只需LEFT JOIN

select a.orgid, b.orgaddressid, c.address
from organization a
left join organization_address b on a.orgid = b.orgid and b.isprimaryaddress = 'YES'
left join address c on b.orgaddressid = c.addressid
where a.orgid = 1;

Answer 2

也许是这样的。我认为您需要对两个表使用LEFT JOIN：

select 
   organization.orgid, 
   organization_address.orgaddressid, 
   address.address
from 
   organization
LEFT JOIN organization_address 
    ON organization_address.orgid = organization.orgid 
    AND organization_address.isprimaryaddress = 'YES'
LEFT JOIN address 
    ON organization_address.orgaddressid = address.addressid;

Answer 3

如果右表中存在匹配的行，则LEFT JOIN选择左表中的行并匹配右表中的行。如果右表中没有任何匹配的行，则查询将为右表中的列返回NULL。

SELECT o.OrgId, oa.OrgaddressId, a.Address FROM Organization AS o 
LEFT JOIN 'Organization Address' AS oa ON oa.OrgId=o.OrgId 
LEFT JOIN Address AS a ON a.AddressId=oa.OrgAddressId
WHERE oa.IsPrimaryId='YES';

Answer 4

子查询比连接查询慢，所以我只是告诉你，请忽略子查询

如果您想获得此类结果，可以使用左连接

select a.orgid, b.orgaddressid, c.address
from organization a
left join organization_address b on a.orgid = b.orgid and b.isprimaryaddress = 'YES'
left join address c on b.orgaddressid = c.addressid
where a.orgid = 1;

左连接意味着

返回左表（table1）中的所有行，右表（table2）中的匹配行。当没有匹配时，右侧的结果为NULL。

访问Google中的加入类型，您可以获得更多想法