1440. Evaluate Boolean Expression.sql

```
Write an SQL query to evaluate the boolean expressions in Expressions table.

Return the result table in any order.

The query result format is in the following example.

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

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

Result table:
+--------------+----------+---------------+-------+
| left_operand | operator | right_operand | value |
+--------------+----------+---------------+-------+
| x            | >        | y             | false |
| x            | <        | y             | true  |
| x            | =        | y             | false |
| y            | >        | x             | true  |
| y            | <        | x             | false |
| x            | =        | x             | true  |
+--------------+----------+---------------+-------+
As shown, you need find the value of each boolean exprssion in the table using the variables table.
```

SELECT 
	e.left_operand, 
  e.operator, 
  e.right_operand,
  (CASE 
		WHEN e.operator = '>' AND v1.value > v2.value THEN 'true'
  	WHEN e.operator = '<' AND v1.value < v2.value THEN 'true'
  	WHEN e.operator = '=' AND v1.value = v2.value THEN 'true'
  	ELSE 'false'
	END) AS value
FROM Expressions e
JOIN Variables v1 ON e.left_operand = v1.name
JOIN Variables v2 ON e.right_operand = v2.name;