BETWEEN for range filtering in PostgreSQL - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ query_result

intermediate

Output of BETWEEN with inclusive range

Given a table products with a column price, what rows will this query return?

SELECT * FROM products WHERE price BETWEEN 10 AND 20;

PostgreSQL

CREATE TABLE products (id INT, name TEXT, price INT);
INSERT INTO products VALUES (1, 'Pen', 5), (2, 'Notebook', 10), (3, 'Backpack', 15), (4, 'Calculator', 20), (5, 'Chair', 25);

A Rows with price less than 20

B Rows with price 10, 15, and 20

C Rows with price 10 and 20 only

D Rows with price 11 to 19 only

💡 Hint

BETWEEN includes both the start and end values.

The BETWEEN operator in SQL is inclusive, so it includes rows where price is exactly 10 and exactly 20, as well as any values in between.

📝 Syntax

intermediate

Correct syntax for BETWEEN with dates

Which of the following queries correctly filters rows with order_date between January 1, 2023 and January 31, 2023 (inclusive)?

A SELECT * FROM orders WHERE order_date BETWEEN '01/01/2023' AND '01/31/2023';

B SELECT * FROM orders WHERE order_date BETWEEN 2023-01-01 AND 2023-01-31;

C SELECT * FROM orders WHERE order_date BETWEEN '2023-01-01' AND '2023-01-31';

D SELECT * FROM orders WHERE order_date BETWEEN '2023/01/01' AND '2023/01/31';

💡 Hint

Date literals in PostgreSQL should be in ISO format and enclosed in single quotes.

Option C uses the correct ISO date format 'YYYY-MM-DD' with quotes, which PostgreSQL recognizes as dates. Other options either lack quotes or use non-standard formats.

❓ optimization

advanced

Optimizing BETWEEN for indexed columns

You have a large table sales with an index on sale_date. Which query is most efficient to find sales between '2023-01-01' and '2023-01-31'?

A SELECT * FROM sales WHERE sale_date BETWEEN '2023-01-01' AND '2023-01-31';

B SELECT * FROM sales WHERE sale_date >= '2023-01-01' AND sale_date <= '2023-01-31';

C SELECT * FROM sales WHERE sale_date > '2023-01-01' AND sale_date < '2023-01-31';

D SELECT * FROM sales WHERE sale_date IN ('2023-01-01', '2023-01-31');

💡 Hint

BETWEEN is often optimized by the database engine for range queries on indexed columns.

BETWEEN is equivalent to >= and <= but is clearer and often optimized by PostgreSQL to use the index efficiently. Option A excludes boundary dates, and D only matches exact dates.

🔧 Debug

advanced

Why does this BETWEEN query return no rows?

Given the table events with a start_time column of type TIMESTAMP, why does this query return no rows?

SELECT * FROM events WHERE start_time BETWEEN '2023-03-10' AND '2023-03-11';

Assume there are rows with start_time on '2023-03-11 15:00:00'.

A Because the query syntax is invalid for TIMESTAMP columns

B Because BETWEEN does not work with TIMESTAMP columns

C Because the dates are in the wrong format

D Because '2023-03-11' is treated as '2023-03-11 00:00:00', excluding later times that day

💡 Hint

Consider how date strings without time default in TIMESTAMP comparisons.

The string '2023-03-11' is interpreted as midnight at the start of that day, so rows with times later on '2023-03-11' are excluded. To include the whole day, the end boundary should be '2023-03-11 23:59:59' or use < '2023-03-12'.

🧠 Conceptual

expert

BETWEEN behavior with NULL values

Consider a table employees with a column age that can be NULL. What will this query return?

SELECT * FROM employees WHERE age BETWEEN 30 AND 40;

Specifically, what happens to rows where age is NULL?

A Rows with NULL age are excluded because NULL comparisons return UNKNOWN

B Rows with NULL age are included because BETWEEN treats NULL as a wildcard

C The query returns an error due to NULL values in BETWEEN

D Rows with NULL age are included only if 30 or 40 is NULL

💡 Hint

Remember how SQL treats NULL in comparisons.

In SQL, any comparison with NULL returns UNKNOWN, which is treated as false in WHERE clauses. Therefore, rows with NULL in the age column are excluded from the result.

Practice

1. What does the BETWEEN operator do in PostgreSQL?

easy

A. Filters rows where a column's value is NULL.

B. Filters rows where a column's value is exactly equal to a single value.

C. Filters rows where a column's value is within a specified range, inclusive.

D. Filters rows where a column's value is outside a specified range.

Solution

1. Step 1: Understand the purpose of BETWEEN

   The BETWEEN operator checks if a value lies between two given values, including both ends.

2. Step 2: Compare with other filtering options

   Unlike equality or NULL checks, BETWEEN covers a range inclusively.

3. Final Answer:

   Filters rows where a column's value is within a specified range, inclusive. -> Option C

4. Quick Check:

   BETWEEN means inclusive range filter [OK]

Hint: BETWEEN includes both range ends, like a closed interval [OK]

Common Mistakes:

• Thinking BETWEEN excludes the range ends
• Confusing BETWEEN with equality (=)
• Assuming BETWEEN works only with numbers

2. Which of the following is the correct syntax to select rows where the column age is between 18 and 30 (inclusive)?

easy

A. SELECT * FROM users WHERE age BETWEEN 18 TO 30;

B. SELECT * FROM users WHERE age BETWEEN 18 AND 30;

C. SELECT * FROM users WHERE age >= 18 AND <= 30;

D. SELECT * FROM users WHERE age IN BETWEEN (18, 30);

Solution

1. Step 1: Recall correct BETWEEN syntax

   The correct syntax uses BETWEEN followed by the lower and upper bounds separated by AND.

2. Step 2: Check each option

   SELECT * FROM users WHERE age BETWEEN 18 AND 30; uses BETWEEN 18 AND 30 correctly; others have syntax errors or wrong keywords.

3. Final Answer:

   SELECT * FROM users WHERE age BETWEEN 18 AND 30; -> Option B

4. Quick Check:

   BETWEEN ... AND ... is correct syntax [OK]

Hint: BETWEEN always uses AND, not TO or commas [OK]

Common Mistakes:

• Using TO instead of AND
• Using incomplete comparison operators
• Misplacing parentheses or commas

3. Given the table orders with a column order_date of type DATE, what will this query return?

SELECT order_id FROM orders WHERE order_date BETWEEN '2023-01-01' AND '2023-01-31';

medium

A. Orders placed exactly on January 1, 2023, only.

B. All orders placed before January 1, 2023.

C. All orders placed after January 31, 2023.

D. All orders placed from January 1, 2023, to January 31, 2023, inclusive.

Solution

1. Step 1: Understand the BETWEEN date range

   The query filters orders where order_date is between '2023-01-01' and '2023-01-31', including both dates.

2. Step 2: Interpret the result set

   It returns all orders placed on any date from January 1 to January 31, 2023, inclusive.

3. Final Answer:

   All orders placed from January 1, 2023, to January 31, 2023, inclusive. -> Option D

4. Quick Check:

   BETWEEN includes both start and end dates [OK]

Hint: BETWEEN dates includes both start and end days [OK]

Common Mistakes:

• Assuming BETWEEN excludes the end date
• Confusing BETWEEN with less than or greater than only
• Thinking it returns only one date

4. Identify the error in this query:

SELECT * FROM products WHERE price BETWEEN 100, 500;

medium

A. The BETWEEN clause uses a comma instead of AND between values.

B. The price column cannot be used with BETWEEN.

C. BETWEEN requires parentheses around the values.

D. The query is correct and will run without errors.

Solution

1. Step 1: Check BETWEEN syntax

   BETWEEN requires two values separated by AND, not a comma.

2. Step 2: Identify the syntax error

   The query uses a comma between 100 and 500, which is invalid syntax.

3. Final Answer:

   The BETWEEN clause uses a comma instead of AND between values. -> Option A

4. Quick Check:

   BETWEEN needs AND, not commas [OK]

Hint: BETWEEN always uses AND, never commas [OK]

Common Mistakes:

• Using commas instead of AND
• Adding unnecessary parentheses
• Assuming BETWEEN works with any separator

5. You want to find all employees hired between January 1, 2020, and December 31, 2022, but exclude those hired exactly on January 1, 2020. Which query achieves this?

hard

A. All of the above queries will work correctly.

B. SELECT * FROM employees WHERE hire_date > '2020-01-01' AND hire_date <= '2022-12-31';

C. SELECT * FROM employees WHERE hire_date BETWEEN '2020-01-02' AND '2022-12-31';

D. SELECT * FROM employees WHERE hire_date BETWEEN '2020-01-01' AND '2022-12-31' AND hire_date <> '2020-01-01';

Solution

1. Step 1: Understand the requirement

   We want employees hired after January 1, 2020, up to December 31, 2022, excluding January 1, 2020 itself.

2. Step 2: Analyze each query

   SELECT * FROM employees WHERE hire_date BETWEEN '2020-01-01' AND '2022-12-31' AND hire_date <> '2020-01-01'; uses BETWEEN including '2020-01-01' but excludes it with <> condition.
   SELECT * FROM employees WHERE hire_date > '2020-01-01' AND hire_date <= '2022-12-31'; uses > and <= to exclude '2020-01-01' naturally.
   SELECT * FROM employees WHERE hire_date BETWEEN '2020-01-02' AND '2022-12-31'; shifts the BETWEEN range to start from '2020-01-02', excluding '2020-01-01'.

3. Step 3: Confirm all queries meet the condition

   All three queries exclude '2020-01-01' and include dates up to '2022-12-31'.

4. Final Answer:

   All of the above queries will work correctly. -> Option A

5. Quick Check:

   Multiple ways to exclude start date in range [OK]

Hint: Exclude start date by > or adjust BETWEEN start or add <> [OK]

Common Mistakes:

• Assuming BETWEEN excludes start date by default
• Using BETWEEN without excluding the start date
• Confusing BETWEEN with exclusive range