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# 1440.(Medium)计算布尔表达式的值

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.
```

来源：力扣（LeetCode）

链接：<https://leetcode-cn.com/problems/evaluate-boolean-expression>

著作权归领扣网络所有。商业转载请联系官方授权，非商业转载请注明出处。

## Solution

```sql
SELECT e.*, 
    case when operator=">" and vl.value>vr.value then 'true' 
    when operator="="  and vl.value=vr.value then 'true' 
    when operator="<" and vl.value<vr.value then 'true' 
    ELSE 'false' end as value
FROM Expressions e, Variables vl, Variables vr
WHERE e.left_operand = vl.name and e.right_operand = vr.name;
```

## Table Schema

```sql
Create Table If Not Exists Variables (name varchar(3), value int);
Create Table If Not Exists Expressions (left_operand varchar(3), operator ENUM('>', '<', '='), right_operand varchar(3));
Truncate table Variables;
insert into Variables (name, value) values ('x', '66');
insert into Variables (name, value) values ('y', '77');
Truncate table Expressions;
insert into Expressions (left_operand, operator, right_operand) values ('x', '>', 'y');
insert into Expressions (left_operand, operator, right_operand) values ('x', '<', 'y');
insert into Expressions (left_operand, operator, right_operand) values ('x', '=', 'y');
insert into Expressions (left_operand, operator, right_operand) values ('y', '>', 'x');
insert into Expressions (left_operand, operator, right_operand) values ('y', '<', 'x');
insert into Expressions (left_operand, operator, right_operand) values ('x', '=', 'x');
```


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