将多个SQL查询合并的两种方式

说明：单个简单查询是非常容易的，但是为了避免多次访问访问数据库，我们会尽可能通过表关联将业务所需要的字段值一次性查出来。而有时候不太清楚表之间的关联关系（这取决于对业务的熟悉程度），或者实际情况就是少了一张关联表，导致我们不能通过关联表来查到我们所需要的字段值。这时就可以通过以下两种方式，将多个查询合并在一起。

以RBAC所需要的五张表为例，如果需要查询某人具备的角色、权限，可通两张关联表来实现

select t1.username, t3.name as role_name, t5.name as permission_name
from i_users t1left join i_user_roles_mapping t2 on t1.id = t2.user_nameleft join i_roles t3 on t2.role_name = t3.idleft join i_role_permissions_mapping t4 on t3.id = t4.role_nameleft join i_permissions t5 on t4.permission_name = t5.id
where t1.username = 'lisi'

但假设现在没有这两张mapping表，需要将以下三个查询合并成一个

select username from i_users where username = 'lisi';
select name as role_name from i_roles where id = '2';
select name as permission_name from i_permissions where id = '5';

如果用union all是一列，不是一行

select username from i_users where username = 'lisi'
union all
select name as role_name from i_roles where id = '2'
union all
select name as permission_name from i_permissions where id = '5';

这时就可以用下面这两种方式

方式一

通过子查询的方式，如下：

select t1.username, t2.name, t3.name
from (select * from i_users where username = 'lisi') t1,(select *  from i_roles where id = '2') t2,(select * from i_permissions where id = '5') t3;

方式二

使用with as，如下：

with t1 as (select * from i_users where username = 'lisi'
),
t2 as (select *  from i_roles where id = '2'
),
t3 as (select * from i_permissions where id = '5'
)
select t1.username, t2.name as role_name, t3.name as permission_name from t1, t2, t3;

以上两种方式的查询效果，等同于前面的关联查询，如下：