EXTRACT function for date parts in PostgreSQL - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ query_result

intermediate

Extracting the year from a timestamp

What is the output of this query?

SELECT EXTRACT(YEAR FROM TIMESTAMP '2023-11-15 08:30:00');

PostgreSQL

SELECT EXTRACT(YEAR FROM TIMESTAMP '2023-11-15 08:30:00');

A 15

B 11

C 08

D 2023

💡 Hint

EXTRACT gets a specific part of the date/time, like year, month, or day.

The EXTRACT function returns the specified part of the date/time. Here, YEAR returns 2023 from the timestamp.

❓ query_result

intermediate

Extracting the month from a date

What does this query return?

SELECT EXTRACT(MONTH FROM DATE '2024-02-29');

PostgreSQL

SELECT EXTRACT(MONTH FROM DATE '2024-02-29');

A 29

B 2

C 2024

D 0

💡 Hint

MONTH extracts the month number from the date.

The EXTRACT function with MONTH returns the month number, which is 2 for February.

📝 Syntax

advanced

Identify the syntax error in EXTRACT usage

Which option contains a syntax error when using EXTRACT in PostgreSQL?

A SELECT EXTRACT(HOUR, TIMESTAMP '2023-05-10 12:00:00');

B SELECT EXTRACT(DAY FROM TIMESTAMP '2023-05-10 12:00:00');

C SELECT EXTRACT(MINUTE FROM TIMESTAMP '2023-05-10 12:00:00');

D SELECT EXTRACT(SECOND FROM TIMESTAMP '2023-05-10 12:00:00');

💡 Hint

Check the syntax: EXTRACT expects the part and source separated by FROM.

Option A uses a comma instead of FROM, which is invalid syntax for EXTRACT.

❓ query_result

advanced

Extracting fractional seconds from a timestamp

What is the output of this query?

SELECT EXTRACT(SECOND FROM TIMESTAMP '2023-07-20 14:23:45.678');

PostgreSQL

SELECT EXTRACT(SECOND FROM TIMESTAMP '2023-07-20 14:23:45.678');

A 0.678

B 678

C 45.678

D 45

💡 Hint

EXTRACT(SECOND) returns seconds including fractional part.

The EXTRACT function returns seconds with decimals, so 45.678 is correct.

🧠 Conceptual

expert

Understanding EXTRACT with time zone aware timestamps

Given the timestamp with time zone '2023-12-31 23:00:00+02', what does this query return?

SELECT EXTRACT(HOUR FROM TIMESTAMPTZ '2023-12-31 23:00:00+02');

PostgreSQL

SELECT EXTRACT(HOUR FROM TIMESTAMPTZ '2023-12-31 23:00:00+02');

A 23

B 21

C 1

D 0

💡 Hint

EXTRACT returns the hour part in the local time of the timestamp, ignoring time zone offset.

EXTRACT(HOUR) returns the hour part as stored, which is 23, not adjusted to UTC.

Practice

1. What does the EXTRACT function do in PostgreSQL when used with a date or timestamp?

easy

A. It adds days to the date.

B. It changes the date format to a string.

C. It retrieves a specific part like year, month, or day from the date or timestamp.

D. It deletes the time part from the timestamp.

Solution

1. Step 1: Understand the purpose of EXTRACT

   The EXTRACT function is designed to pull out specific parts such as year, month, day, hour, etc., from a date or timestamp value.

2. Step 2: Compare with other options

   Options B, C, and D describe different operations like formatting, adding days, or truncating time, which EXTRACT does not do.

3. Final Answer:

   It retrieves a specific part like year, month, or day from the date or timestamp. -> Option C

4. Quick Check:

   EXTRACT returns date parts = A [OK]

Hint: EXTRACT pulls out parts like year or month from dates [OK]

Common Mistakes:

• Thinking EXTRACT changes date format
• Confusing EXTRACT with date arithmetic
• Assuming EXTRACT removes time part

2. Which of the following is the correct syntax to extract the month from a timestamp column named created_at?

easy

A. EXTRACT(created_at, MONTH);

B. EXTRACT(MONTH FROM created_at);

C. EXTRACT('MONTH', created_at);

D. EXTRACT MONTH created_at;

Solution

1. Step 1: Recall correct EXTRACT syntax

   The correct syntax is EXTRACT(part FROM source), so to get month: EXTRACT(MONTH FROM created_at);

2. Step 2: Identify incorrect syntax forms

   Options A, B, and D use wrong argument order, quotes around part name, missing FROM keyword, or wrong punctuation.

3. Final Answer:

   EXTRACT(MONTH FROM created_at); -> Option B

4. Quick Check:

   EXTRACT(part FROM source) = C [OK]

Hint: Use EXTRACT(part FROM source) syntax exactly [OK]

Common Mistakes:

• Omitting FROM keyword
• Using quotes around part name
• Swapping argument order

3. Given the query:

SELECT EXTRACT(DAY FROM TIMESTAMP '2024-06-15 10:30:00');

What will be the output?

medium

A. 10

B. 6

C. 2024

D. 15

Solution

1. Step 1: Identify the part extracted

   The query extracts the DAY part from the timestamp '2024-06-15 10:30:00'. The day is 15.

2. Step 2: Confirm output value

   Other options represent month (6), year (2024), and hour (10), which are not extracted here.

3. Final Answer:

   15 -> Option D

4. Quick Check:

   EXTRACT(DAY) = 15 [OK]

Hint: Match EXTRACT part with date component value [OK]

Common Mistakes:

• Confusing day with month or year
• Expecting full date output
• Ignoring time part in timestamp

4. What is wrong with this query?

SELECT EXTRACT('YEAR' FROM CURRENT_DATE);

medium

A. The part name YEAR should not be in quotes.

B. CURRENT_DATE cannot be used with EXTRACT.

C. Missing parentheses around CURRENT_DATE.

D. EXTRACT requires two arguments separated by a comma.

Solution

1. Step 1: Check syntax for EXTRACT part

   The part name like YEAR should be unquoted. Quotes cause syntax error.

2. Step 2: Verify other parts of query

   CURRENT_DATE is valid input, no parentheses needed, and EXTRACT uses FROM keyword, not comma.

3. Final Answer:

   The part name YEAR should not be in quotes. -> Option A

4. Quick Check:

   Part names unquoted in EXTRACT [OK]

Hint: Do not put quotes around date parts in EXTRACT [OK]

Common Mistakes:

• Using quotes around part names
• Adding commas instead of FROM
• Thinking CURRENT_DATE needs parentheses

5. You want to find all records from a table orders where the order was placed in the first quarter (January to March) of any year. Which query correctly uses EXTRACT to filter these records?

hard

A. SELECT * FROM orders WHERE EXTRACT(QUARTER FROM order_date) = 1;

B. SELECT * FROM orders WHERE EXTRACT('MONTH' FROM order_date) < 4;

C. SELECT * FROM orders WHERE EXTRACT(MONTH, order_date) IN (1, 2, 3);

D. SELECT * FROM orders WHERE EXTRACT(MONTH order_date) BETWEEN 1 AND 3;

Solution

1. Step 1: Understand quarter extraction

   PostgreSQL supports EXTRACT(QUARTER FROM date) which returns 1 for Jan-Mar, 2 for Apr-Jun, etc.

2. Step 2: Evaluate each option

   Options A (missing FROM), B (quotes around MONTH), and C (comma instead of FROM) have syntax errors. SELECT * FROM orders WHERE EXTRACT(QUARTER FROM order_date) = 1; correctly uses EXTRACT(QUARTER FROM order_date) = 1.

3. Final Answer:

   SELECT * FROM orders WHERE EXTRACT(QUARTER FROM order_date) = 1; -> Option A

4. Quick Check:

   Use EXTRACT(QUARTER) = 1 for first quarter [OK]

Hint: Use EXTRACT(QUARTER) = 1 for first quarter filtering [OK]

Common Mistakes:

• Missing FROM keyword
• Using quotes around part names
• Using comma instead of FROM
• Not knowing EXTRACT supports QUARTER