DATE arithmetic (DATEDIFF, DATE_ADD) in SQL - Cheat Sheet & Quick Revision

Recall & Review

beginner

What does the SQL function DATEDIFF do?

DATEDIFF calculates the number of days between two dates. It tells you how many days separate one date from another.

beginner

How do you add 10 days to a date in SQL?

You use DATE_ADD with an interval of 10 days. For example: DATE_ADD('2024-01-01', INTERVAL 10 DAY) gives you the date 10 days after January 1, 2024.

beginner

What is the difference between DATEDIFF and DATE_ADD?

DATEDIFF finds the number of days between two dates. DATE_ADD changes a date by adding a certain amount of time (days, months, etc.) to it.

intermediate

Write a SQL query to find how many days have passed since '2024-01-01' until today.

SELECT DATEDIFF(CURRENT_DATE, '2024-01-01');

This returns the number of days from January 1, 2024, to today.

beginner

How can you add 3 months to a date using SQL?

Use DATE_ADD with INTERVAL 3 MONTH. Example: DATE_ADD('2024-01-01', INTERVAL 3 MONTH) gives April 1, 2024.

What does DATEDIFF('2024-01-10', '2024-01-01') return?

A 1

B 9

C 11

D 10

DATEDIFF counts the days between the two dates, excluding the start date, so it returns 9.

Which SQL function adds a time interval to a date?

A DATE_ADD

B DATEDIFF

C DATE_SUB

D NOW

DATE_ADD adds a specified interval (days, months, etc.) to a date.

How do you subtract 5 days from a date in SQL?

A DATE_ADD(date, INTERVAL 5 DAY)

B DATEDIFF(date, 5)

C DATE_SUB(date, INTERVAL 5 DAY)

D SUBDATE(date, 5)

DATE_SUB subtracts a time interval from a date.

What will DATE_ADD('2024-02-28', INTERVAL 1 DAY) return?

A '2024-02-29'

B '2024-03-01'

C '2024-02-28'

D Error

2024 is a leap year, so adding 1 day to Feb 28 gives Feb 29.

If you want to find the number of days between two dates, which function do you use?

A DATE_ADD

B NOW

C CURDATE

D DATEDIFF

DATEDIFF returns the number of days between two dates.

Explain how to calculate the number of days between two dates in SQL and give an example.

Think about how to find the difference in days between '2024-01-01' and '2024-01-10'.

Describe how to add a time interval (like days or months) to a date in SQL and provide a sample query.

Consider adding 10 days or 3 months to a specific date.

Practice

1. What does the SQL function DATEDIFF('2024-06-10', '2024-06-01') return?

easy

A. 1

B. 10

C. 11

D. 9

Solution

1. Step 1: Understand DATEDIFF parameters

   DATEDIFF calculates the number of days between the first date and the second date.

2. Step 2: Calculate the difference

   From June 1 to June 10, there are 9 days difference (10 - 1 = 9).

3. Final Answer:

   9 -> Option D

4. Quick Check:

   DATEDIFF('2024-06-10', '2024-06-01') = 9 [OK]

Hint: Subtract dates to get days difference [OK]

Common Mistakes:

• Counting both start and end dates
• Mixing order of dates in DATEDIFF
• Assuming difference is 10 instead of 9

2. Which of the following is the correct syntax to add 5 days to the date '2024-06-01' using DATE_ADD?

easy

A. DATE_ADD('2024-06-01', INTERVAL 5 DAY)

B. DATE_ADD('2024-06-01', 5 DAY)

C. DATE_ADD('2024-06-01', INTERVAL DAY 5)

D. DATE_ADD('2024-06-01', DAY 5 INTERVAL)

Solution

1. Step 1: Recall DATE_ADD syntax

   The correct syntax is DATE_ADD(date, INTERVAL number unit), e.g., INTERVAL 5 DAY.

2. Step 2: Check each option

   Only DATE_ADD('2024-06-01', INTERVAL 5 DAY) follows the correct syntax with INTERVAL 5 DAY.

3. Final Answer:

   DATE_ADD('2024-06-01', INTERVAL 5 DAY) -> Option A

4. Quick Check:

   Correct DATE_ADD syntax uses INTERVAL before number and unit [OK]

Hint: Use INTERVAL keyword before number and unit [OK]

Common Mistakes:

• Omitting INTERVAL keyword
• Swapping number and unit order
• Placing INTERVAL after unit

3. What is the result of this SQL query?

SELECT DATEDIFF('2024-07-01', DATE_ADD('2024-06-25', INTERVAL 3 DAY)) AS diff;

medium

A. 6

B. 3

C. 5

D. 4

Solution

1. Step 1: Calculate DATE_ADD('2024-06-25', INTERVAL 3 DAY)

   Adding 3 days to June 25 gives June 28, 2024.

2. Step 2: Calculate DATEDIFF('2024-07-01', '2024-06-28')

   The difference between July 1 and June 28 is 3 days (June 29, 30, July 1).

3. Final Answer:

   3 -> Option B

4. Quick Check:

   DATEDIFF('2024-07-01', '2024-06-28') = 3 [OK]

Hint: Calculate DATE_ADD first, then DATEDIFF [OK]

Common Mistakes:

• Counting days incorrectly between dates
• Forgetting to add days before DATEDIFF
• Assuming DATEDIFF counts inclusive days

4. Identify the error in this SQL statement:

SELECT DATE_ADD('2024-06-01', 5 DAY);

medium

A. DATE_ADD cannot add days

B. Wrong date format

C. Missing INTERVAL keyword before 5 DAY

D. Extra semicolon inside function

Solution

1. Step 1: Review DATE_ADD syntax

   DATE_ADD requires INTERVAL keyword before the number and unit.

2. Step 2: Check the given statement

   The statement misses INTERVAL before '5 DAY', causing syntax error.

3. Final Answer:

   Missing INTERVAL keyword before 5 DAY -> Option C

4. Quick Check:

   DATE_ADD needs INTERVAL before number and unit [OK]

Hint: Always use INTERVAL before number and unit [OK]

Common Mistakes:

• Omitting INTERVAL keyword
• Using wrong date format
• Adding semicolon inside function

5. You want to find how many days are left until a deadline 10 days after '2024-06-15'. Which SQL query correctly calculates the days left from '2024-06-20'?

hard

A. SELECT DATEDIFF(DATE_ADD('2024-06-15', INTERVAL 10 DAY), '2024-06-20') AS days_left;

B. SELECT DATEDIFF('2024-06-20', DATE_ADD('2024-06-15', INTERVAL 10 DAY)) AS days_left;

C. SELECT DATEDIFF('2024-06-15', DATE_ADD('2024-06-20', INTERVAL 10 DAY)) AS days_left;

D. SELECT DATEDIFF('2024-06-20', '2024-06-15' + INTERVAL 10 DAY) AS days_left;

Solution

1. Step 1: Calculate deadline date

   Deadline is 10 days after June 15, so DATE_ADD('2024-06-15', INTERVAL 10 DAY) = June 25.

2. Step 2: Calculate days left from June 20 to deadline

   DATEDIFF(deadline, current_date) = DATEDIFF('2024-06-25', '2024-06-20') = 5 days left.

3. Final Answer:

   SELECT DATEDIFF(DATE_ADD('2024-06-15', INTERVAL 10 DAY), '2024-06-20') AS days_left; -> Option A

4. Quick Check:

   Days left = deadline - current date = 5 [OK]

Hint: Subtract current date from deadline date [OK]

Common Mistakes:

• Swapping date order in DATEDIFF
• Adding interval to wrong date
• Using wrong syntax for INTERVAL