leetcode-SQL-1440. 计算布尔表达式的值

+---------------+---------+
| 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.

+---------------+---------+
| 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.

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 
    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 as value
from Expressions as e
left join Variables as v1 on v1.name=e.left_operand
left join Variables as v2 on v2.name=e.right_operand;