Medium之1440.计算布尔表达式的值

灰太狼 2022-08-31 04:49 294阅读 0赞

**表 Variables:**

<table> 
 <thead> 
  <tr> 
   <th>Column Name</th> 
   <th>Type</th> 
  </tr> 
 </thead> 
 <tbody> 
  <tr> 
   <td>name</td> 
   <td>varchar</td> 
  </tr> 
  <tr> 
   <td>value</td> 
   <td>int</td> 
  </tr> 
 </tbody> 
</table>

name 是该表主键.  
该表包含了存储的变量及其对应的值.

**表 Expressions:**

<table> 
 <thead> 
  <tr> 
   <th>Column Name</th> 
   <th>Type</th> 
  </tr> 
 </thead> 
 <tbody> 
  <tr> 
   <td>left_operand</td> 
   <td>varchar</td> 
  </tr> 
  <tr> 
   <td>operator</td> 
   <td>enum</td> 
  </tr> 
  <tr> 
   <td>right_operand</td> 
   <td>varchar</td> 
  </tr> 
 </tbody> 
</table>

(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.

### 问题 ###

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.

### 示例 ###

**Variables 表:**

<table> 
 <thead> 
  <tr> 
   <th>name</th> 
   <th>value</th> 
  </tr> 
 </thead> 
 <tbody> 
  <tr> 
   <td>x</td> 
   <td>66</td> 
  </tr> 
  <tr> 
   <td>y</td> 
   <td>77</td> 
  </tr> 
 </tbody> 
</table>

**Expressions 表:**

<table> 
 <thead> 
  <tr> 
   <th>left_operand</th> 
   <th>operator</th> 
   <th>right_operand</th> 
  </tr> 
 </thead> 
 <tbody> 
  <tr> 
   <td>x</td> 
   <td>&gt;</td> 
   <td>y</td> 
  </tr> 
  <tr> 
   <td>x</td> 
   <td>&lt;</td> 
   <td>y</td> 
  </tr> 
  <tr> 
   <td>x</td> 
   <td>=</td> 
   <td>y</td> 
  </tr> 
  <tr> 
   <td>y</td> 
   <td>&gt;</td> 
   <td>x</td> 
  </tr> 
  <tr> 
   <td>y</td> 
   <td>&lt;</td> 
   <td>x</td> 
  </tr> 
  <tr> 
   <td>x</td> 
   <td>=</td> 
   <td>x</td> 
  </tr> 
 </tbody> 
</table>

**Result 表:**

<table> 
 <thead> 
  <tr> 
   <th>left_operand</th> 
   <th>operator</th> 
   <th>right_operand</th> 
   <th>value</th> 
  </tr> 
 </thead> 
 <tbody> 
  <tr> 
   <td>x</td> 
   <td>&gt;</td> 
   <td>y</td> 
   <td>false</td> 
  </tr> 
  <tr> 
   <td>x</td> 
   <td>&lt;</td> 
   <td>y</td> 
   <td>true</td> 
  </tr> 
  <tr> 
   <td>x</td> 
   <td>=</td> 
   <td>y</td> 
   <td>false</td> 
  </tr> 
  <tr> 
   <td>y</td> 
   <td>&gt;</td> 
   <td>x</td> 
   <td>true</td> 
  </tr> 
  <tr> 
   <td>y</td> 
   <td>&lt;</td> 
   <td>x</td> 
   <td>false</td> 
  </tr> 
  <tr> 
   <td>x</td> 
   <td>=</td> 
   <td>x</td> 
   <td>true</td> 
  </tr> 
 </tbody> 
</table>

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.

来源：力扣（LeetCode）  
链接：https://leetcode-cn.com/problems/evaluate-boolean-expression  
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### 解答 ###

select e.*,
        case when operator = '>' and v1.value>v2.value then 'true'
            when operator = '<' and v1.value<v2.value then 'true'
            when operator = '=' and v1.value=v2.value then 'true'
            else 'false'
        end as value
    from expressions e
        left join 
        variables v1 on e.left_operand = v1.name
        left join 
        variables v2 on e.right_operand = v2.name