AGE function for differences in PostgreSQL - Cheat Sheet & Quick Revision

Recall & Review

beginner

What does the AGE function in PostgreSQL do?

The AGE function calculates the difference between two dates or timestamps and returns the result as an interval showing years, months, and days.

beginner

How do you use AGE to find the difference between the current date and a past date?

Use AGE(current_date, past_date). For example, AGE(current_date, '2000-01-01') returns the time passed since January 1, 2000.

beginner

What is the output type of the AGE function in PostgreSQL?

The output is an interval type that shows the difference in years, months, days, and time components.

intermediate

What happens if you use AGE with only one argument?

AGE with one argument returns the difference between the current date/time and the given timestamp.

intermediate

Why is AGE preferred over simple subtraction for date differences?

AGE accounts for months and years properly, giving a human-readable difference, while subtraction returns just the number of days.

What does AGE('2024-06-01', '2023-06-01') return?

A 365 days

B 1 year

C 12 months

D 30 days

AGE returns the difference as an interval showing 1 year between the two dates.

Which data type does AGE return in PostgreSQL?

A Date

B Integer

C Interval

D Timestamp

AGE returns an interval type representing the difference between dates.

What is the result of AGE('2024-06-01') if today is '2024-06-10'?

A 9 days

B 10 days

C 1 day

D 0 days

AGE with one argument returns the difference from the current date/time to the given date, which is 9 days.

If you subtract two dates directly (date1 - date2), what do you get?

A Number of days as an integer

B Timestamp

C Interval with years and months

D String describing difference

Direct subtraction returns the number of days as an integer, not a detailed interval.

Why might AGE('2024-06-01', '2023-12-01') return '6 mons' instead of '182 days'?

A AGE rounds to nearest day

B AGE returns seconds

C AGE only works with years

D AGE counts months and years, not just days

AGE returns a human-readable interval counting months and years, not just total days.

Explain how the AGE function calculates the difference between two dates in PostgreSQL.

Think about how AGE shows time passed in years and months, not just days.

Describe the difference between using AGE and simple date subtraction for finding date differences.

Consider what each method returns and how it represents the difference.

Practice

1. What does the PostgreSQL AGE function return when given two timestamps?

easy

A. The number of seconds between the two timestamps as an integer

B. The difference between the two timestamps as an interval showing years, months, and days

C. The sum of the two timestamps as a new timestamp

D. The earlier of the two timestamps

Solution

1. Step 1: Understand the AGE function purpose

   The AGE function calculates the time difference between two timestamps.

2. Step 2: Identify the output format

   It returns the difference as an interval showing years, months, and days.

3. Final Answer:

   The difference between the two timestamps as an interval showing years, months, and days -> Option B

4. Quick Check:

   AGE returns interval difference [OK]

Hint: AGE shows difference as years, months, days interval [OK]

Common Mistakes:

• Thinking AGE returns seconds or integer
• Confusing AGE with addition or subtraction of timestamps
• Assuming AGE returns a timestamp

2. Which of the following is the correct syntax to get the age difference between two timestamps '2023-01-01' and '2024-06-01' in PostgreSQL?

easy

A. SELECT AGE('2024-06-01', '2023-01-01');

B. SELECT AGE('2023-01-01', '2024-06-01');

C. SELECT AGE('2024-06-01'::timestamp '2023-01-01'::timestamp);

D. SELECT AGE('2023-01-01'::timestamp, '2024-06-01'::timestamp);

Solution

1. Step 1: Check AGE function parameter order

   AGE calculates the difference as first timestamp minus second timestamp, so the later date should come first.

2. Step 2: Identify correct syntax with proper order

   Using AGE('2024-06-01', '2023-01-01') returns the positive interval difference.

3. Final Answer:

   SELECT AGE('2024-06-01', '2023-01-01'); -> Option A

4. Quick Check:

   Later date first in AGE [OK]

Hint: Put later date first in AGE function [OK]

Common Mistakes:

• Reversing the order of timestamps
• Not casting strings to timestamps when needed
• Using AGE with only one parameter incorrectly

3. What is the output of this query?

SELECT AGE('2024-06-01'::date, '2023-01-01'::date);

medium

A. 1 year 5 mons 1 day

B. 1 year 6 mons 0 days

C. 1 year 4 mons 30 days

D. 1 year 5 mons 0 days

Solution

1. Step 1: Calculate difference between dates

   From 2023-01-01 to 2024-06-01 is 1 year and 5 months exactly.

2. Step 2: Understand AGE output format

   AGE returns interval as years, months, and days; days are zero here.

3. Final Answer:

   1 year 5 mons 0 days -> Option D

4. Quick Check:

   AGE returns 1 year 5 mons 0 days [OK]

Hint: Count full years and months between dates [OK]

Common Mistakes:

• Counting months incorrectly as 6 instead of 5
• Confusing days count
• Assuming AGE returns total days only

4. Identify the error in this query:

SELECT AGE('2023-01-01');

medium

A. Incorrect date format used

B. AGE function cannot accept a single argument

C. No error; AGE accepts a single argument

D. AGE requires casting to timestamp explicitly

Solution

1. Step 1: Check AGE function argument requirements

   AGE can accept one or two arguments; with one argument it returns difference from current date.

2. Step 2: Analyze the query

   The query uses one argument but does not cast it; however, AGE accepts one argument as timestamp or date.

3. Step 3: Identify actual error

   The query is valid; PostgreSQL implicitly handles the date string and computes age to current timestamp. No error.

4. Final Answer:

   No error; AGE accepts a single argument -> Option C

5. Quick Check:

   AGE needs two timestamps or one with current date [OK]

Hint: AGE with one argument uses current date [OK]

Common Mistakes:

• Assuming AGE always needs two arguments
• Thinking date format is wrong
• Believing casting is mandatory

5. You want to find the exact age difference between a person's birthdate '1990-04-15' and today, but only in years ignoring months and days. Which query correctly uses AGE and extracts only the years part?

hard

A. SELECT EXTRACT(YEAR FROM AGE(CURRENT_DATE, '1990-04-15'));

B. SELECT EXTRACT(YEAR FROM AGE('1990-04-15', CURRENT_DATE));

C. SELECT AGE(CURRENT_DATE, '1990-04-15')::year;

D. SELECT DATE_PART('year', AGE('1990-04-15', CURRENT_DATE));

Solution

1. Step 1: Use AGE with current date and birthdate

   AGE(CURRENT_DATE, '1990-04-15') returns interval from birthdate to today.

2. Step 2: Extract only the year part

   Use EXTRACT(YEAR FROM interval) to get full years ignoring months and days.

3. Final Answer:

   SELECT EXTRACT(YEAR FROM AGE(CURRENT_DATE, '1990-04-15')); -> Option A

4. Quick Check:

   EXTRACT YEAR from AGE with later date first [OK]

Hint: Use EXTRACT(YEAR FROM AGE(later, earlier)) [OK]

Common Mistakes:

• Reversing AGE argument order
• Using wrong function like DATE_PART with reversed arguments
• Trying to cast interval directly to year