题目介绍: 计算布尔表达式的值 evaluate-boolean-expression
难度中等
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. ```
sql
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