NTILE for distribution in PostgreSQL - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to divide rows into 4 groups using NTILE.

PostgreSQL

SELECT name, salary, NTILE([1]) OVER (ORDER BY salary) AS quartile FROM employees;

Drag options to blanks, or click blank then click option'

A4

B3

C5

D2

Attempts:

3 left

💡 Hint

Common Mistakes

Using 3 or 5 instead of 4 for quartiles.

Forgetting to order rows inside OVER clause.

The NTILE function divides rows into the specified number of groups. Here, 4 groups (quartiles) are needed.

2fill in blank

medium

Complete the code to assign 3 groups to rows ordered by score.

PostgreSQL

SELECT player, score, NTILE([1]) OVER (ORDER BY score DESC) AS group_num FROM leaderboard;

Drag options to blanks, or click blank then click option'

A3

B2

C4

D5

Attempts:

3 left

💡 Hint

Common Mistakes

Using wrong number of groups.

Not ordering scores descending to get top scores first.

NTILE(3) divides the rows into 3 groups ordered by score descending.

3fill in blank

hard

Complete the code to divide rows into 5 groups.

PostgreSQL

SELECT id, value, NTILE([1]) OVER (ORDER BY value) AS group_id FROM data;

Drag options to blanks, or click blank then click option'

A4

B6

C3

D5

Attempts:

3 left

💡 Hint

Common Mistakes

Missing ORDER BY inside OVER clause.

Using wrong number of groups.

The NTILE function needs the number of groups as argument. Here, 5 groups are required.

4fill in blank

hard

Fill both blanks to divide employees into 3 groups ordered by age.

PostgreSQL

SELECT name, age, NTILE([1]) OVER (ORDER BY [2]) AS age_group FROM employees;

Drag options to blanks, or click blank then click option'

A3

Bage

Csalary

D4

Attempts:

3 left

💡 Hint

Common Mistakes

Using wrong column in ORDER BY.

Using wrong number of groups.

NTILE(3) divides into 3 groups. Ordering by age sorts rows by age for grouping.

5fill in blank

hard

Fill all three blanks to create quartiles of sales and show group and sales.

PostgreSQL

SELECT salesperson, sales, NTILE([1]) OVER (ORDER BY [2] [3]) AS sales_quartile FROM sales_data;

Drag options to blanks, or click blank then click option'

A4

Bsales

CDESC

DASC

Attempts:

3 left

💡 Hint

Common Mistakes

Using wrong number of groups.

Ordering by wrong column or wrong direction.

NTILE(4) creates 4 groups (quartiles). Ordering by sales ascending groups from lowest to highest sales.

Practice

1. What does the NTILE(n) function do in PostgreSQL?

easy

A. It returns the maximum value of a column.

B. It counts the total number of rows in a table.

C. It divides rows into n groups with roughly equal size, numbering each group.

D. It sorts rows alphabetically by a specified column.

Solution

1. Step 1: Understand NTILE purpose

   NTILE(n) splits rows into n groups based on order.

2. Step 2: Recognize group numbering

   Each group is numbered starting from 1 to n, distributing rows evenly.

3. Final Answer:

   It divides rows into n groups with roughly equal size, numbering each group. -> Option C

4. Quick Check:

   NTILE splits rows into groups [OK]

Hint: NTILE splits rows into equal groups numbered 1 to n [OK]

Common Mistakes:

• Confusing NTILE with COUNT or MAX functions
• Thinking NTILE sorts rows alphabetically
• Assuming NTILE returns row counts

2. Which of the following is the correct syntax to assign quartiles using NTILE in PostgreSQL?

easy

A. SELECT column, NTILE(4) PARTITION BY column FROM table;

B. SELECT column, NTILE OVER (4 ORDER BY column) AS quartile FROM table;

C. SELECT column, NTILE(4) AS quartile FROM table ORDER BY column;

D. SELECT column, NTILE(4) OVER (ORDER BY column) AS quartile FROM table;

Solution

1. Step 1: Recall NTILE syntax

   The correct syntax is NTILE(n) OVER (ORDER BY column) to assign groups.

2. Step 2: Check each option

   SELECT column, NTILE(4) OVER (ORDER BY column) AS quartile FROM table; uses correct syntax with NTILE(4) OVER (ORDER BY column). Others misuse OVER clause or omit it.

3. Final Answer:

   SELECT column, NTILE(4) OVER (ORDER BY column) AS quartile FROM table; -> Option D

4. Quick Check:

   NTILE needs OVER with ORDER BY [OK]

Hint: NTILE requires OVER and ORDER BY clauses [OK]

Common Mistakes:

• Omitting OVER clause
• Placing number inside OVER incorrectly
• Using PARTITION BY without ORDER BY

3. Given the table scores with column score values: 10, 20, 30, 40, 50, what is the output of this query?

SELECT score, NTILE(3) OVER (ORDER BY score) AS tile FROM scores ORDER BY score;

medium

A. [{10,1}, {20,1}, {30,2}, {40,2}, {50,3}]

B. [{10,1}, {20,2}, {30,2}, {40,3}, {50,3}]

C. [{10,1}, {20,1}, {30,1}, {40,2}, {50,3}]

D. [{10,1}, {20,2}, {30,3}, {40,3}, {50,3}]

Solution

1. Step 1: Understand NTILE(3) distribution

   NTILE(3) divides 5 rows into 3 groups as evenly as possible: 2 rows in first group, 2 in second, 1 in third.

2. Step 2: Assign tiles by order

   Ordered scores: 10,20 (tile 1), 30,40 (tile 2), 50 (tile 3).

3. Final Answer:

   [{10,1}, {20,1}, {30,2}, {40,2}, {50,3}] -> Option A

4. Quick Check:

   5 rows split into 3 groups: 2,2,1 [OK]

Hint: Divide rows evenly; extra rows go to earlier tiles [OK]

Common Mistakes:

• Assigning one row per tile ignoring total rows
• Not ordering rows before applying NTILE
• Assuming equal rows per tile always

4. Identify the error in this query:

SELECT id, NTILE(4) ORDER BY score FROM results;

medium

A. Missing OVER() clause after NTILE(4).

B. ORDER BY cannot be used inside NTILE function.

C. NTILE requires a PARTITION BY clause.

D. The query is correct as is.

Solution

1. Step 1: Check NTILE syntax

   NTILE requires an OVER() clause with ORDER BY inside it, like NTILE(4) OVER (ORDER BY score).

2. Step 2: Identify missing OVER()

   The query misses the OVER() clause, causing syntax error.

3. Final Answer:

   Missing OVER() clause after NTILE(4). -> Option A

4. Quick Check:

   NTILE needs OVER() with ORDER BY [OK]

Hint: NTILE always needs OVER() with ORDER BY [OK]

Common Mistakes:

• Placing ORDER BY outside OVER()
• Forgetting OVER() clause
• Assuming PARTITION BY is mandatory

5. You want to divide 7 employees into 3 salary groups using NTILE, but the salaries are:
5000, 7000, 7000, 9000, 11000, 11000, 13000.
Which NTILE(3) group assignment is correct when ordered by salary?

hard

A. [{5000,1}, {7000,1}, {7000,1}, {9000,2}, {11000,2}, {11000,3}, {13000,3}]

B. [{5000,1}, {7000,1}, {7000,2}, {9000,3}, {11000,3}, {11000,3}, {13000,3}]

C. [{5000,1}, {7000,2}, {7000,2}, {9000,2}, {11000,3}, {11000,3}, {13000,3}]

D. [{5000,1}, {7000,1}, {7000,2}, {9000,2}, {11000,3}, {11000,3}, {13000,3}]

Solution

1. Step 1: Calculate group sizes for 7 rows and 3 groups

   7 rows divided into 3 groups: first group gets 3 rows, next two get 2 rows each.

2. Step 2: Assign groups by ordered salary

   Order: 5000(1), 7000(1), 7000(2), 9000(3), 11000(3), 11000(3), 13000(3).

3. Final Answer:

   [{5000,1}, {7000,1}, {7000,2}, {9000,3}, {11000,3}, {11000,3}, {13000,3}] -> Option B

4. Quick Check:

   7 rows split into 3 groups: 3,2,2 [OK]

Hint: Extra rows go to earlier groups; order matters [OK]

Common Mistakes:

• Ignoring duplicate salaries affecting group boundaries
• Assigning equal rows per group ignoring total rows
• Misplacing group numbers for duplicates