EXTRACT function for date parts in PostgreSQL - Cheat Sheet & Quick Revision

Recall & Review

beginner

What does the EXTRACT function do in PostgreSQL?

The EXTRACT function retrieves a specific part (like year, month, day) from a date or timestamp value.

beginner

Write the syntax to extract the year from a timestamp column named 'created_at'.

EXTRACT(YEAR FROM created_at)

intermediate

Which date parts can you extract using the EXTRACT function? Name at least three.

You can extract YEAR, MONTH, DAY, HOUR, MINUTE, SECOND, and more from date or timestamp values.

intermediate

What type of value does EXTRACT return?

EXTRACT returns a numeric value representing the requested date part, usually as a double precision number.

beginner

How would you extract the month from the current date in PostgreSQL?

Use EXTRACT(MONTH FROM CURRENT_DATE) to get the current month number.

What does EXTRACT(DAY FROM timestamp) return?

A The day of the week from the timestamp

B The day of the month from the timestamp

C The total number of days since 1970

D The hour part of the timestamp

EXTRACT(DAY FROM timestamp) returns the day of the month as a number.

Which of these is a valid EXTRACT syntax in PostgreSQL?

A EXTRACT(YEAR FROM timestamp)

B EXTRACT(YEAR, timestamp)

C EXTRACT timestamp YEAR

D EXTRACT FROM timestamp YEAR

The correct syntax is EXTRACT(part FROM source), for example EXTRACT(YEAR FROM timestamp).

What will EXTRACT(HOUR FROM TIMESTAMP '2024-06-01 15:30:00') return?

A 2024

B 30

C 6

D 15

It returns 15, the hour part of the timestamp.

Can EXTRACT be used on a DATE type in PostgreSQL?

A No, only on TIME

B No, only on TIMESTAMP

C Yes, it works on DATE and TIMESTAMP types

D Yes, but only for YEAR and MONTH

EXTRACT works on both DATE and TIMESTAMP types.

What is the return type of EXTRACT function?

A Double precision (numeric)

B Integer

C Boolean

D Text

EXTRACT returns a numeric value as double precision.

Explain how to use the EXTRACT function to get the month and day from a timestamp.

Think about the parts you want and the source date or timestamp.

List at least five different date parts you can extract using the EXTRACT function in PostgreSQL.

Consider parts of date and time.

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