mysql - SQL - 从行中获取特定值，然后使用该值

Question

我有一个包含名称、等级和分数的数据表。例如，“约翰”获得了 B 级，其分值为 3.0。

如何选择低于“B”级的条目？

所以我不知何故需要做这样的事情

Value = select Point from MyTable where Grade="B"

接着

select * from MyTable where Point < value

但显然 SQL 必须是一个语句......

Answer 1

您可以嵌套选择并添加子查询：

SELECT realtable.* 
FROM (SELECT Point FROM MyTable WHERE Grade="B" LIMIT 1) subquery, MyTable realtable
WHERE subquery.Point > realtable.Point

Answer 2

Try it use in subquery as below:

select * 
from MyTable 
where Point < (select Point 
               from MyTable 
               where Grade="B")

but if you subquery returns more than one row try to use aggregate funcion for example min

select * 
from MyTable 
where Point < (select min(Point)
               from MyTable 
               where Grade="B")

or with LIMIT and join:

select * 
from MyTable mt
join (select Point from MyTable 
      where Grade="B"
      order by Point
      LIMIT 1) mt2 on mt.Point < mt2.Point