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Grokking SQL for Tech InterviewsInterview Problems: Medium12. Evaluate Boolean Expression

Problem

Table Variables:

+---------------+---------+
| Column Name   | Type    |
+---------------+---------+
| name          | varchar |
| value         | int     |
+---------------+---------+
In the `Variables` table, `name` is the primary key.
This table contains the stored variables and their values.

Table Expressions:

+---------------+---------+
| Column Name   | Type    |
+---------------+---------+
| left_operand  | varchar |
| operator      | enum    |
| right_operand | varchar |
+---------------+---------+
In the `Expressions` table, (left_operand, operator, right_operand) is the primary key.
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.

Problem Definition

Evaluate the boolean expressions in the Expressions table.

Return the result table in the any order.

Example

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Output

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Try It Yourself

Database Exercise

Database Schema:

-- Database schema would be rendered here

Exercise Script:

-- Exercise script would be rendered here

Available actions: Execute

Solution

To evaluate the boolean expressions in the Expressions table based on the values of variables in the Variables table, we need to systematically process and compare the values of the operands using the specified operators. This involves joining the tables to retrieve the necessary values and then applying conditional logic to determine the outcome of each expression.

  1. Join Expressions with Variables to Retrieve Operand Values:

    • Left Operand: Associate each expression’s left_operand with its corresponding value from the Variables table.
    • Right Operand: Similarly, associate each expression’s right_operand with its corresponding value from the Variables table.

  2. Evaluate Each Boolean Expression:

    • Use a CASE statement to compare the retrieved operand values based on the specified operator (<, >, =).
    • Assign 'true' if the condition is met; otherwise, assign 'false'.

SQL Query

SELECT e1.left_operand,
       e1.operator,
       e1.right_operand,
       CASE
           WHEN e1.operator = '<' AND v1.value < v2.value THEN 'true'
           WHEN e1.operator = '>' AND v1.value > v2.value THEN 'true'
           WHEN e1.operator = '=' AND v1.value = v2.value THEN 'true'
           ELSE 'false'
       END AS value
FROM   Expressions e1
       JOIN Variables v1 ON e1.left_operand = v1.name
       JOIN Variables v2 ON e1.right_operand = v2.name;

Step-by-Step Approach

Step 1: Join Expressions with Variables to Retrieve Left Operand Values

Associate each expression’s left_operand with its corresponding value from the Variables table. This allows us to access the numerical value needed for evaluation.

SQL Query:

SELECT e1.left_operand,
       e1.operator,
       e1.right_operand,
       v1.value AS left_value
FROM   Expressions e1
       JOIN Variables v1 ON e1.left_operand = v1.name;

Explanation:

  • SELECT e1.left_operand, e1.operator, e1.right_operand, v1.value AS left_value:

    • Retrieves the left_operand, operator, right_operand, and the corresponding value from the Variables table, aliased as left_value for clarity.

  • FROM Expressions e1 JOIN Variables v1 ON e1.left_operand = v1.name:

    • Performs an inner join between the Expressions table (e1) and the Variables table (v1) based on the left_operand matching the name in Variables.

Output After Step 1:

+--------------+----------+---------------+------------+
| left_operand | operator | right_operand | left_value |
+--------------+----------+---------------+------------+
| x            | >        | y             | 66         |
| x            | <        | y             | 66         |
| x            | =        | y             | 66         |
| y            | >        | x             | 77         |
| y            | <        | x             | 77         |
| x            | =        | x             | 66         |
+--------------+----------+---------------+------------+

Step 2: Join Expressions with Variables to Retrieve Right Operand Values

Associate each expression’s right_operand with its corresponding value from the Variables table. This complements the left_value obtained in Step 1, enabling complete evaluation of the expression.

SQL Query:

SELECT e1.left_operand,
       e1.operator,
       e1.right_operand,
       v1.value AS left_value,
       v2.value AS right_value
FROM   Expressions e1
       JOIN Variables v1 ON e1.left_operand = v1.name
       JOIN Variables v2 ON e1.right_operand = v2.name;

Explanation:

  • v2.value AS right_value:

    • Retrieves the value corresponding to the right_operand from the Variables table, aliased as right_value for clarity.

  • JOIN Variables v2 ON e1.right_operand = v2.name:

    • Performs an inner join between the Expressions table (e1) and the Variables table (v2) based on the right_operand matching the name in Variables.

Output After Step 2:

+--------------+----------+---------------+------------+-------------+
| left_operand | operator | right_operand | left_value | right_value |
+--------------+----------+---------------+------------+-------------+
| x            | >        | y             | 66         | 77          |
| x            | <        | y             | 66         | 77          |
| x            | =        | y             | 66         | 77          |
| y            | >        | x             | 77         | 66          |
| y            | <        | x             | 77         | 66          |
| x            | =        | x             | 66         | 66          |
+--------------+----------+---------------+------------+-------------+

Step 3: Evaluate Each Boolean Expression Using CASE Statement

Determine whether each expression evaluates to 'true' or 'false' based on the comparison of left_value and right_value using the specified operator.

SQL Query:

SELECT e1.left_operand,
       e1.operator,
       e1.right_operand,
       v1.value AS left_value,
       v2.value AS right_value,
       CASE
           WHEN e1.operator = '<' AND v1.value < v2.value THEN 'true'
           WHEN e1.operator = '>' AND v1.value > v2.value THEN 'true'
           WHEN e1.operator = '=' AND v1.value = v2.value THEN 'true'
           ELSE 'false'
       END AS value
FROM   Expressions e1
       JOIN Variables v1 ON e1.left_operand = v1.name
       JOIN Variables v2 ON e1.right_operand = v2.name;

Explanation:

  • CASE Statement:

    • WHEN e1.operator = '<' AND v1.value < v2.value THEN 'true':

      • If the operator is '<' and the left_value is less than the right_value, the expression evaluates to 'true'.

    • WHEN e1.operator = '>' AND v1.value > v2.value THEN 'true':

      • If the operator is '>' and the left_value is greater than the right_value, the expression evaluates to 'true'.

    • WHEN e1.operator = '=' AND v1.value = v2.value THEN 'true':

      • If the operator is '=' and the left_value is equal to the right_value, the expression evaluates to 'true'.

    • ELSE 'false':

      • In all other cases, the expression evaluates to 'false'.

  • AS value:

    • Aliases the result of the CASE statement as value to represent the outcome of the boolean expression.

Output After Step 3:

+--------------+----------+---------------+------------+-------------+--------+
| left_operand | operator | right_operand | left_value | right_value | value  |
+--------------+----------+---------------+------------+-------------+--------+
| x            | >        | y             | 66         | 77          | false  |
| x            | <        | y             | 66         | 77          | true   |
| x            | =        | y             | 66         | 77          | false  |
| y            | >        | x             | 77         | 66          | true   |
| y            | <        | x             | 77         | 66          | false  |
| x            | =        | x             | 66         | 66          | true   |
+--------------+----------+---------------+------------+-------------+--------+

Final 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   |
+--------------+----------+---------------+--------+
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