Three-valued logic (TRUE, FALSE, UNKNOWN) in SQL - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to select rows where the condition is TRUE in three-valued logic.

SQL

SELECT * FROM employees WHERE active IS [1];

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ATRUE

BNULL

CUNKNOWN

DFALSE

Attempts:

3 left

💡 Hint

Common Mistakes

Using NULL instead of TRUE

Using = TRUE instead of IS TRUE

In SQL three-valued logic, to check if a condition is true, use IS TRUE.

2fill in blank

medium

Complete the code to select rows where the condition is UNKNOWN in three-valued logic.

SQL

SELECT * FROM orders WHERE shipped_date IS [1];

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AFALSE

BUNKNOWN

CTRUE

DNULL

Attempts:

3 left

💡 Hint

Common Mistakes

Using IS NULL instead of IS UNKNOWN

Using IS FALSE instead of IS UNKNOWN

In SQL, IS UNKNOWN checks if the condition evaluates to unknown (NULL).

3fill in blank

hard

Fix the error in the WHERE clause to correctly filter rows where the condition is FALSE.

SQL

SELECT * FROM products WHERE discontinued IS [1];

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ATRUE

BUNKNOWN

CFALSE

DNULL

Attempts:

3 left

💡 Hint

Common Mistakes

Using = FALSE instead of IS FALSE

Using IS NULL instead of IS FALSE

To filter rows where a condition is false in three-valued logic, use IS FALSE.

4fill in blank

hard

Fill both blanks to complete the query that selects rows where the condition is either TRUE or UNKNOWN.

SQL

SELECT * FROM customers WHERE status IS [1] OR status IS [2];

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ATRUE

BFALSE

CUNKNOWN

DNULL

Attempts:

3 left

💡 Hint

Common Mistakes

Using IS NULL instead of IS UNKNOWN

Using IS FALSE instead of IS UNKNOWN

This query selects rows where status is either TRUE or UNKNOWN, using IS TRUE and IS UNKNOWN.

5fill in blank

hard

Fill all three blanks to create a CASE expression that returns 'Yes', 'No', or 'Unknown' based on a three-valued logic condition.

SQL

SELECT CASE WHEN approved IS [1] THEN 'Yes' WHEN approved IS [2] THEN 'No' ELSE [3] END AS approval_status FROM applications;

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ATRUE

BFALSE

C'Unknown'

D'Pending'

Attempts:

3 left

💡 Hint

Common Mistakes

Using NULL instead of a string for ELSE

Mixing TRUE and FALSE in wrong order

The CASE expression checks if approved is TRUE or FALSE, returning 'Yes' or 'No'. Otherwise, it returns 'Unknown'.