Finding gaps in sequences in SQL - Cheat Sheet & Quick Revision

Recall & Review

beginner

What does 'finding gaps in sequences' mean in a database context?

It means identifying missing values or breaks in a series of numbers or dates that should be continuous, like missing invoice numbers or dates in a log.

intermediate

Which SQL function can help detect gaps by comparing current and next row values?

The LEAD() function lets you look at the next row's value to compare and find where the sequence breaks.

beginner

Why is it useful to find gaps in sequences in real life?

Finding gaps helps spot missing data, errors in numbering, or skipped steps, which is important for accurate reports and audits.

intermediate

What is a simple SQL approach to find missing numbers in a sequence of IDs?

Use a query that compares each ID with the next one using LEAD(), then find where the difference is more than 1 to spot missing IDs.

advanced

How can you generate a list of missing sequence numbers once gaps are found?

You can use a numbers or tally table to generate all possible values, then LEFT JOIN with your data to find which numbers are missing.

Which SQL function helps you look at the next row's value in a sequence?

A ROW_NUMBER()

B LAG()

C LEAD()

D COUNT()

LEAD() returns the next row's value, useful to compare current and next values to find gaps.

If your sequence is 1, 2, 4, 5, which number is missing?

A 6

B 2

C 5

D 3

3 is missing because the sequence jumps from 2 to 4.

What does a gap in a sequence usually indicate?

A Missing or skipped data

B Correct continuous data

C Duplicate values

D Sorted data

A gap means some expected values are missing or skipped.

Which SQL clause is often used with LEAD() to order the sequence?

A HAVING

B OVER (ORDER BY ...)

C WHERE

D GROUP BY

LEAD() requires an OVER clause with ORDER BY to define the sequence order.

To find missing sequence numbers, you can compare each number with:

A The next number in the sequence

B The first number only

C The maximum number

D The average number

Comparing each number with the next helps detect where numbers jump, indicating gaps.

Explain how you would find gaps in a sequence of numbers stored in a database table.

Think about comparing each number with the next one.

Why is it important to detect gaps in sequences in real-world data?

Consider the impact of missing data on reports.

Practice

1. What is the main purpose of using the LEAD() function when finding gaps in a sequence of numbers in SQL?

easy

A. To find the maximum number in the sequence

B. To compare each number with the next number in the sequence

C. To sum all numbers in the sequence

D. To sort the numbers in descending order

Solution

1. Step 1: Understand the role of LEAD()

   The LEAD() function allows you to look at the next row's value in the current row's context.

2. Step 2: Identify how LEAD() helps find gaps

   By comparing the current number with the next number, you can detect if there is a gap (difference greater than 1).

3. Final Answer:

   To compare each number with the next number in the sequence -> Option B

4. Quick Check:

   LEAD() compares current and next values [OK]

Hint: Use LEAD() to peek at the next number in sequence [OK]

Common Mistakes:

• Thinking LEAD() sums values
• Confusing LEAD() with aggregate functions
• Using LEAD() to sort data

2. Which of the following SQL snippets correctly uses LEAD() to find gaps in a sequence column named num in table numbers?

easy

A. SELECT num, LEAD(num) AS next_num FROM numbers;

B. SELECT num, LEAD(num) FROM numbers ORDER BY num;

C. SELECT num, LEAD(num) OVER (ORDER BY num) AS next_num FROM numbers;

D. SELECT num, LEAD(num) OVER () AS next_num FROM numbers;

Solution

1. Step 1: Check LEAD() syntax

   The correct syntax requires LEAD(column) OVER (ORDER BY column) to define the order for the window function.

2. Step 2: Identify the correct option

   SELECT num, LEAD(num) OVER (ORDER BY num) AS next_num FROM numbers; uses LEAD(num) OVER (ORDER BY num), which is the correct form to get the next number in order.

3. Final Answer:

   SELECT num, LEAD(num) OVER (ORDER BY num) AS next_num FROM numbers; -> Option C

4. Quick Check:

   LEAD() needs OVER with ORDER BY [OK]

Hint: Always include OVER(ORDER BY column) with LEAD() [OK]

Common Mistakes:

• Omitting OVER clause
• Not specifying ORDER BY inside OVER
• Using LEAD() without alias

3. Given the table numbers with column num containing values (1, 2, 4, 5, 7), what will the following query return?

SELECT num, LEAD(num) OVER (ORDER BY num) AS next_num FROM numbers;

medium

A. [{num:1, next_num:2}, {num:2, next_num:4}, {num:4, next_num:5}, {num:5, next_num:7}, {num:7, next_num:null}]

B. [{num:1, next_num:1}, {num:2, next_num:2}, {num:4, next_num:4}, {num:5, next_num:5}, {num:7, next_num:7}]

C. [{num:1, next_num:3}, {num:2, next_num:4}, {num:4, next_num:6}, {num:5, next_num:7}, {num:7, next_num:null}]

D. [{num:1, next_num:null}, {num:2, next_num:null}, {num:4, next_num:null}, {num:5, next_num:null}, {num:7, next_num:null}]

Solution

1. Step 1: Understand LEAD() output

   LEAD(num) returns the next row's num value ordered by num. So each row shows its own num and the next number.

2. Step 2: Apply to given data

   For values 1,2,4,5,7:
   1's next is 2
   2's next is 4
   4's next is 5
   5's next is 7
   7 has no next, so null

3. Final Answer:

   [{num:1, next_num:2}, {num:2, next_num:4}, {num:4, next_num:5}, {num:5, next_num:7}, {num:7, next_num:null}] -> Option A

4. Quick Check:

   LEAD() shifts values down by one [OK]

Hint: LEAD() shows next row's value or null if none [OK]

Common Mistakes:

• Assuming LEAD() repeats current value
• Expecting gaps filled automatically
• Confusing LEAD() with LAG()

4. The following query is intended to find gaps in the num sequence but returns no rows:

SELECT num FROM numbers WHERE LEAD(num) OVER (ORDER BY num) - num > 1;

What is the error?

medium

A. You cannot use window functions directly in WHERE clause

B. The ORDER BY inside LEAD() is missing

C. The subtraction operator is invalid here

D. LEAD() requires a GROUP BY clause

Solution

1. Step 1: Understand window function restrictions

   Window functions like LEAD() cannot be used directly in the WHERE clause because WHERE filters rows before window functions are applied.

2. Step 2: Correct approach

   Use a subquery or CTE to compute LEAD() first, then filter in an outer query.

3. Final Answer:

   You cannot use window functions directly in WHERE clause -> Option A

4. Quick Check:

   Window functions not allowed in WHERE [OK]

Hint: Use subquery or CTE to filter on window function results [OK]

Common Mistakes:

• Trying to filter window function in WHERE
• Missing OVER clause
• Expecting LEAD() to work like aggregate

5. You have a table orders with an integer column order_id that should be sequential but may have missing numbers. Which query correctly finds all missing order_id values between the minimum and maximum present?

hard

A. SELECT order_id FROM orders WHERE LEAD(order_id) - order_id > 1;

B. SELECT DISTINCT order_id + 1 FROM orders WHERE order_id + 1 NOT IN (SELECT order_id FROM orders);

C. SELECT order_id FROM orders WHERE order_id NOT IN (SELECT order_id FROM orders);

D. WITH seq AS (SELECT generate_series(MIN(order_id), MAX(order_id)) AS id FROM orders) SELECT id FROM seq LEFT JOIN orders ON seq.id = orders.order_id WHERE orders.order_id IS NULL;

Solution

1. Step 1: Generate full sequence range

   Use generate_series from minimum to maximum order_id to create all possible IDs.

2. Step 2: Find missing IDs by left join

   Left join this sequence to existing orders and select IDs where no matching order exists (NULL in orders).

3. Final Answer:

   WITH seq AS (SELECT generate_series(MIN(order_id), MAX(order_id)) AS id FROM orders) SELECT id FROM seq LEFT JOIN orders ON seq.id = orders.order_id WHERE orders.order_id IS NULL; -> Option D

4. Quick Check:

   Generate full range, then find missing with LEFT JOIN [OK]

Hint: Generate full range then find missing with LEFT JOIN [OK]

Common Mistakes:

• Using LEAD() alone misses multiple gaps
• Incorrect NOT IN subquery logic
• Assuming order_id + 1 always missing