如何在 MySQL 中创建/使用变量

我知道在 MySQL 中你可以做类似的事情SELECT foo AS bar FROM TABLE t;我正在尝试做一些事情来达到以下效果SELECT command1 FROM table1 AS foo, command2 FROM table2 AS bar, (foo-bar) AS difference;但 MySQL 不允许我这样做。有没有办法在 MySQL 中做到这一点，或者我需要一些扩展？提前致谢！

Schema

create table t1
(   id int auto_increment primary key,
    thing varchar(100) not null
);

create table t2
(   id int auto_increment primary key,
    descr varchar(100) not null
);

insert t1(thing) values ('cat'),('dog');
insert t2(descr) values ('meow'),('likes bones');

Query

select t1.thing,t2.descr into @var1,@var2 
from t1 
join t2 
on t2.id=t1.id 
where t1.id=2; -- to avoid Error 1172: Result consisted of more than one row

查看变量

select @var1 as thing,@var2 as descr;
+-------+-------------+
| thing | descr       |
+-------+-------------+
| dog   | likes bones |
+-------+-------------+

像上面这样选择变量的重要部分是将结果集限制为最多 1 行。