YEAR, MONTH, DAY extraction in SQL - Time & Space Complexity

Time Complexity: YEAR, MONTH, DAY extraction

O(n)

Understanding Time Complexity

We want to understand how the time to extract parts of a date grows as we have more data.

How does the work change when we extract year, month, or day from many rows?

Scenario Under Consideration

Analyze the time complexity of the following code snippet.

SELECT
  YEAR(order_date) AS order_year,
  MONTH(order_date) AS order_month,
  DAY(order_date) AS order_day
FROM orders;

This query extracts the year, month, and day parts from the order_date column for every row in the orders table.

Identify Repeating Operations

Identify the loops, recursion, array traversals that repeat.

• Primary operation: Extracting year, month, and day from each date value.
• How many times: Once for each row in the orders table.

How Execution Grows With Input

Each row requires the same fixed amount of work to extract the date parts.

Input Size (n)	Approx. Operations
10	10 extractions
100	100 extractions
1000	1000 extractions

Pattern observation: The work grows directly with the number of rows.

Final Time Complexity

Time Complexity: O(n)

This means the time to extract year, month, and day grows linearly with the number of rows.

Common Mistake

[X] Wrong: "Extracting year, month, and day is a constant time operation regardless of rows, so the query runs instantly no matter the data size."

[OK] Correct: While each extraction is quick, the database must do it for every row, so more rows mean more total work and longer time.

Interview Connect

Understanding how simple operations scale with data size helps you explain query performance clearly and confidently.

Self-Check

"What if we added a WHERE clause to filter rows before extracting year, month, and day? How would the time complexity change?"

Practice

1. What does the SQL function EXTRACT(YEAR FROM order_date) return when used on a date column?

easy

A. The full date in order_date

B. The year part of the date in order_date

C. The day part of the date in order_date

D. The month part of the date in order_date

Solution

1. Step 1: Understand the EXTRACT function

   The EXTRACT function is used to get a specific part of a date, such as year, month, or day.

2. Step 2: Identify the part requested

   Here, EXTRACT(YEAR FROM order_date) extracts the year part from the date in the order_date column.

3. Final Answer:

   The year part of the date in order_date -> Option B

4. Quick Check:

   EXTRACT(YEAR FROM date) = year number [OK]

Hint: EXTRACT(YEAR FROM date) gets the year number fast [OK]

Common Mistakes:

• Confusing YEAR with MONTH or DAY
• Expecting full date instead of part
• Using wrong syntax like EXTRACT(order_date YEAR)

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

easy

A. EXTRACT(MONTH FROM sale_date)

B. EXTRACT(MONTH sale_date)

C. EXTRACT(FROM sale_date MONTH)

D. EXTRACT(sale_date, MONTH)

Solution

1. Step 1: Recall the correct EXTRACT syntax

   The correct syntax is EXTRACT(part FROM source), where part is YEAR, MONTH, or DAY.

2. Step 2: Match syntax with options

   EXTRACT(MONTH FROM sale_date) uses EXTRACT(MONTH FROM sale_date), which matches the correct syntax.

3. Final Answer:

   EXTRACT(MONTH FROM sale_date) -> Option A

4. Quick Check:

   EXTRACT(part FROM date) is correct syntax [OK]

Hint: Remember EXTRACT(part FROM date) syntax [OK]

Common Mistakes:

• Omitting FROM keyword
• Swapping order of arguments
• Using commas inside EXTRACT

3. Given a table events with a column event_date storing dates, what will this query return?

SELECT EXTRACT(DAY FROM event_date) FROM events WHERE EXTRACT(MONTH FROM event_date) = 12;

medium

A. All month numbers of events that happened in December

B. Syntax error due to wrong EXTRACT usage

C. All full dates of events in December

D. All day numbers of events that happened in December

Solution

1. Step 1: Analyze the WHERE clause

   The WHERE clause filters rows where the month part of event_date is 12 (December).

2. Step 2: Analyze the SELECT clause

   The SELECT clause extracts the day part from event_date for those filtered rows.

3. Final Answer:

   All day numbers of events that happened in December -> Option D

4. Quick Check:

   EXTRACT filters month=12, selects day numbers [OK]

Hint: Filter by month, select day with EXTRACT [OK]

Common Mistakes:

• Confusing day with month in SELECT
• Expecting full date instead of day number
• Thinking query has syntax error

4. Identify the error in this SQL query:

SELECT EXTRACT(YEAR sale_date) FROM sales;

medium

A. Missing FROM keyword inside EXTRACT

B. sale_date should be in quotes

C. EXTRACT cannot be used on date columns

D. No error, query is correct

Solution

1. Step 1: Check EXTRACT syntax

   EXTRACT requires the syntax EXTRACT(part FROM source). The keyword FROM is missing here.

2. Step 2: Confirm correct usage

   The correct query should be SELECT EXTRACT(YEAR FROM sale_date) FROM sales;

3. Final Answer:

   Missing FROM keyword inside EXTRACT -> Option A

4. Quick Check:

   EXTRACT(part FROM date) needs FROM [OK]

Hint: Always include FROM in EXTRACT syntax [OK]

Common Mistakes:

• Omitting FROM keyword
• Putting column name in quotes
• Thinking EXTRACT can't use dates

5. You want to find all orders placed on the 15th day of any month in 2023 from the orders table with a order_date column. Which query correctly filters these orders?

hard

A. SELECT * FROM orders WHERE EXTRACT(MONTH FROM order_date) = 15 AND EXTRACT(YEAR FROM order_date) = 2023;

B. SELECT * FROM orders WHERE DAY(order_date) = 15 AND YEAR(order_date) = 2023;

C. SELECT * FROM orders WHERE EXTRACT(DAY FROM order_date) = 15 AND EXTRACT(YEAR FROM order_date) = 2023;

D. SELECT * FROM orders WHERE EXTRACT(DAY FROM order_date) = 2023 AND EXTRACT(YEAR FROM order_date) = 15;

Solution

1. Step 1: Identify correct EXTRACT usage for day and year

   We want day = 15 and year = 2023, so EXTRACT(DAY FROM order_date) = 15 and EXTRACT(YEAR FROM order_date) = 2023.

2. Step 2: Check each option

   SELECT * FROM orders WHERE EXTRACT(DAY FROM order_date) = 15 AND EXTRACT(YEAR FROM order_date) = 2023; matches the correct conditions. SELECT * FROM orders WHERE DAY(order_date) = 15 AND YEAR(order_date) = 2023; uses non-standard functions that may not work in all SQL dialects. SELECT * FROM orders WHERE EXTRACT(MONTH FROM order_date) = 15 AND EXTRACT(YEAR FROM order_date) = 2023; incorrectly uses MONTH = 15. SELECT * FROM orders WHERE EXTRACT(DAY FROM order_date) = 2023 AND EXTRACT(YEAR FROM order_date) = 15; swaps day and year values incorrectly.

3. Final Answer:

   SELECT * FROM orders WHERE EXTRACT(DAY FROM order_date) = 15 AND EXTRACT(YEAR FROM order_date) = 2023; -> Option C

4. Quick Check:

   Filter day=15 and year=2023 with EXTRACT [OK]

Hint: Use EXTRACT(DAY FROM date) and EXTRACT(YEAR FROM date) together [OK]

Common Mistakes:

• Mixing up day and month
• Using non-standard functions without checking dialect
• Swapping day and year values