1440. Evaluate Boolean Expression

Level: Medium

Question:

Table Variables:

+---------------+---------+
| Column Name   | Type    |
+---------------+---------+
| name          | varchar |
| value         | int     |
+---------------+---------+
name is the primary key for this table.
This table contains the stored variables and their values.

Table Expressions:

+---------------+---------+
| Column Name   | Type    |
+---------------+---------+
| left_operand  | varchar |
| operator      | enum    |
| right_operand | varchar |
+---------------+---------+
(left_operand, operator, right_operand) is the primary key for this table.
This table contains a boolean expression that should be evaluated.
operator is an enum that takes one of the values ('<', '>', '=')
The values of left_operand and right_operand are guaranteed to be in the Variables table.

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.

Example 1:

Input: 
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             |
+--------------+----------+---------------+
Output:
 
+--------------+----------+---------------+-------+
| left_operand | operator | right_operand | value |
+--------------+----------+---------------+-------+
| x            | >        | y             | false |
| x            | <        | y             | true  |
| x            | =        | y             | false |
| y            | >        | x             | true  |
| y            | <        | x             | false |
| x            | =        | x             | true  |
+--------------+----------+---------------+-------+
Explanation:
 
As shown, you need to find the value of each boolean expression in the table using the variables table.

My Solution:

JOIN the tables based on operands. Since we have two operands, we need to join the variable table twice. Then, we can use CASE to specify the operator and return the proper result.

select left_operand, operator, right_operand, 
(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
 inner join variables v1 on left_operand=v1.name
 inner join variables v2 on right_operand=v2.name