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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
Output
Try It Yourself
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.
-
Join
Expressions
withVariables
to Retrieve Operand Values:- Left Operand: Associate each expression's
left_operand
with its correspondingvalue
from theVariables
table. - Right Operand: Similarly, associate each expression's
right_operand
with its correspondingvalue
from theVariables
table.
- Left Operand: Associate each expression's
-
Evaluate Each Boolean Expression:
- Use a
CASE
statement to compare the retrieved operand values based on the specifiedoperator
(<
,>
,=
). - Assign
'true'
if the condition is met; otherwise, assign'false'
.
- Use a
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 correspondingvalue
from theVariables
table, aliased asleft_value
for clarity.
- Retrieves the
FROM Expressions e1 JOIN Variables v1 ON e1.left_operand = v1.name
:- Performs an inner join between the
Expressions
table (e1
) and theVariables
table (v1
) based on theleft_operand
matching thename
inVariables
.
- Performs an inner join between the
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 theright_operand
from theVariables
table, aliased asright_value
for clarity.
- Retrieves the
JOIN Variables v2 ON e1.right_operand = v2.name
:- Performs an inner join between the
Expressions
table (e1
) and theVariables
table (v2
) based on theright_operand
matching thename
inVariables
.
- Performs an inner join between the
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 theleft_value
is less than theright_value
, the expression evaluates to'true'
.
- If the operator is
WHEN e1.operator = '>' AND v1.value > v2.value THEN 'true'
:- If the operator is
'>'
and theleft_value
is greater than theright_value
, the expression evaluates to'true'
.
- If the operator is
WHEN e1.operator = '=' AND v1.value = v2.value THEN 'true'
:- If the operator is
'='
and theleft_value
is equal to theright_value
, the expression evaluates to'true'
.
- If the operator is
ELSE 'false'
:- In all other cases, the expression evaluates to
'false'
.
- In all other cases, the expression evaluates to
AS value
:- Aliases the result of the
CASE
statement asvalue
to represent the outcome of the boolean expression.
- Aliases the result of the
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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