1440. 计算布尔表达式的值(常看看)

SQL架构

表 Variables:

+---------------+---------+
| Column Name   | Type    |
+---------------+---------+
| name          | varchar |
| value         | int     |
+---------------+---------+
name 是该表主键.
该表包含了存储的变量及其对应的值.

表 Expressions:

+---------------+---------+
| Column Name   | Type    |
+---------------+---------+
| left_operand  | varchar |
| operator      | enum    |
| right_operand | varchar |
+---------------+---------+
(left_operand, operator, right_operand) 是该表主键.
该表包含了需要计算的布尔表达式.
operator 是枚举类型, 取值于('<', '>', '=')
left_operand 和 right_operand 的值保证存在于 Variables 表单中.

写一个 SQL 查询,  以计算表 Expressions 中的布尔表达式.

返回的结果表没有顺序要求.

查询结果格式如下例所示.

Variables 表:
+------+-------+
| name | value |
+------+-------+
| x    | 66    |
| y    | 77    |
+------+-------+

Expressions 表:
+--------------+----------+---------------+
| left_operand | operator | right_operand |
+--------------+----------+---------------+
| x            | >        | y             |
| x            | <        | y             |
| x            | =        | y             |
| y            | >        | x             |
| y            | <        | x             |
| x            | =        | x             |
+--------------+----------+---------------+

Result 表:
+--------------+----------+---------------+-------+
| left_operand | operator | right_operand | value |
+--------------+----------+---------------+-------+
| x            | >        | y             | false |
| x            | <        | y             | true  |
| x            | =        | y             | false |
| y            | >        | x             | true  |
| y            | <        | x             | false |
| x            | =        | x             | true  |
+--------------+----------+---------------+-------+
如上所示, 你需要通过使用 Variables 表来找到 Expressions 表中的每一个布尔表达式的值.

select
s1.left_operand,s1.operator,s1.right_operand,
if(case
when s1.operator = '>' then s1.value > v2.value 
when s1.operator = '<' then s1.value < v2.value #  用 case 创造 x > Y ...
when s1.operator = '=' then s1.value = v2.value 
end = 1,'true','false') value
from
( 
select
e.left_operand,e.operator,e.right_operand, v.value
from
Expressions e  left join variables v 
on e.left_operand = v.name
) s1 left join variables v2
 on s1.right_operand = v2.name 

case 放列名的用法  case  operand
when '>' ...
when '<' ...     :

select e.left_operand,e.operator,e.right_operand,
case e.operator
    when '>' then if(v1.value>v2.value,'true','false')
    when '<' then if(v1.value<v2.value,'true','false')
    else  if(v1.value=v2.value,'true','false')
end value
    
from Expressions e
left join Variables v1 on v1.name = e.left_operand 
left join Variables v2 on v2.name = e.right_operand