Interval type for durations in PostgreSQL

Introduction

The interval type helps you store and work with durations of time, like hours, days, or months. It makes it easy to add or subtract time periods in your data.

You want to record how long a task took, like 3 hours and 15 minutes.

You need to calculate the difference between two dates or times.

You want to add a duration to a date, like adding 2 weeks to a start date.

You want to store recurring time periods, like a subscription length of 1 month.

You want to compare durations, like checking if one event lasted longer than another.

Syntax

PostgreSQL

INTERVAL '[quantity] [unit]'

The quantity is a number (can be decimal).

The unit can be seconds, minutes, hours, days, weeks, months, years, etc.

Examples

This represents a duration of 3 hours and 15 minutes.

PostgreSQL

INTERVAL '3 hours 15 minutes'

This represents a duration of 1 day.

PostgreSQL

INTERVAL '1 day'

This represents a duration of 2 months and 10 days.

PostgreSQL

INTERVAL '2 months 10 days'

This represents a duration of 45 seconds.

PostgreSQL

INTERVAL '45 seconds'

Sample Program

This query shows today's date, adds 1 month and 5 days to it, and also shows a duration of 2 hours and 30 minutes.

PostgreSQL

SELECT CURRENT_DATE AS today,
       (CURRENT_DATE + INTERVAL '1 month 5 days')::date AS future_date,
       INTERVAL '2 hours 30 minutes' AS duration;

Important Notes

You can add or subtract intervals from dates or timestamps easily.

Intervals can be combined by adding them together.

Be careful with months and years because their length can vary (months can have 28 to 31 days).

Summary

The interval type stores durations like hours, days, or months.

You can use intervals to add or subtract time from dates.

Intervals make working with time periods simple and clear.

Practice

1. What does the interval type in PostgreSQL represent?

easy

A. A numeric value for counting rows

B. A specific date and time

C. A duration of time like hours, days, or months

D. A text string describing time

Solution

1. Step 1: Understand the interval type purpose

   The interval type is designed to store durations, not fixed points in time.

2. Step 2: Compare with other types

   Unlike timestamp or date types, interval holds a length of time like '3 days' or '2 hours'.

3. Final Answer:

   A duration of time like hours, days, or months -> Option C

4. Quick Check:

   Interval = Duration [OK]

Hint: Interval means duration, not a fixed date/time [OK]

Common Mistakes:

• Confusing interval with timestamp
• Thinking interval stores exact dates
• Assuming interval is a string

2. Which of the following is the correct way to declare an interval of 5 days in PostgreSQL?

easy

A. INTERVAL 5 days

B. INTERVAL '5 days'

C. '5 days' INTERVAL

D. INTERVAL 'days 5'

Solution

1. Step 1: Recall interval literal syntax

   PostgreSQL requires the interval value to be a string inside single quotes after the INTERVAL keyword.

2. Step 2: Check each option

   Only INTERVAL '5 days' matches the correct syntax; others are invalid or reversed.

3. Final Answer:

   INTERVAL '5 days' -> Option B

4. Quick Check:

   Interval syntax = INTERVAL 'value unit' [OK]

Hint: Use INTERVAL followed by quoted string for durations [OK]

Common Mistakes:

• Omitting quotes around the duration
• Placing INTERVAL after the string
• Swapping number and unit order

3. What is the result of this query?

SELECT CURRENT_DATE + INTERVAL '3 days';

medium

A. The current date minus 3 days

B. The current date as text

C. An error because you cannot add interval to date

D. The date 3 days after today

Solution

1. Step 1: Understand adding interval to date

   Adding an interval to a date shifts the date forward by that duration.

2. Step 2: Apply the interval '3 days'

   Adding '3 days' to CURRENT_DATE results in a date 3 days later than today.

3. Final Answer:

   The date 3 days after today -> Option D

4. Quick Check:

   Date + Interval = Date shifted forward [OK]

Hint: Adding interval to date moves date forward [OK]

Common Mistakes:

• Thinking it subtracts days
• Expecting a text output
• Believing it causes an error

4. Identify the error in this query:

SELECT CURRENT_TIMESTAMP + '2 hours';

medium

A. Missing INTERVAL keyword before '2 hours'

B. CURRENT_TIMESTAMP cannot be used with intervals

C. Quotes around '2 hours' should be double quotes

D. No error, query is correct

Solution

1. Step 1: Check interval usage with timestamp

   To add a duration, the value must be an interval type, declared with INTERVAL keyword.

2. Step 2: Analyze the query

   The string '2 hours' lacks INTERVAL keyword, so PostgreSQL will error.

3. Final Answer:

   Missing INTERVAL keyword before '2 hours' -> Option A

4. Quick Check:

   Interval keyword required for durations [OK]

Hint: Always use INTERVAL before quoted duration strings [OK]

Common Mistakes:

• Omitting INTERVAL keyword
• Using wrong quotes
• Assuming timestamp can't add intervals

5. You want to find the date 1 month and 15 days after January 10, 2024. Which query gives the correct result?

hard

A. SELECT DATE '2024-01-10' + INTERVAL '1 month' + INTERVAL '15 days';

B. SELECT DATE '2024-01-10' + INTERVAL '1 month + 15 days';

C. SELECT DATE '2024-01-10' + INTERVAL '45 days';

D. SELECT DATE '2024-01-10' + INTERVAL '1.5 months';

Solution

1. Step 1: Understand interval addition

   Adding intervals separately ensures correct handling of months and days.

2. Step 2: Evaluate each option

   SELECT DATE '2024-01-10' + INTERVAL '1 month + 15 days'; tries to combine units in one string, which is invalid syntax. SELECT DATE '2024-01-10' + INTERVAL '45 days'; adds 45 days, which is not exactly 1 month and 15 days. SELECT DATE '2024-01-10' + INTERVAL '1.5 months'; uses fractional months, which PostgreSQL does not support directly.

3. Final Answer:

   SELECT DATE '2024-01-10' + INTERVAL '1 month' + INTERVAL '15 days'; -> Option A

4. Quick Check:

   Separate intervals for months and days [OK]

Hint: Add months and days as separate intervals for accuracy [OK]

Common Mistakes:

• Combining units in one interval string
• Using fractional months
• Adding days as total instead of separate