Aggregates (count, sum, avg) in Laravel - Cheat Sheet & Quick Revision

Recall & Review

beginner

What does the count() aggregate function do in Laravel?

It counts the number of records in a database table or query result.

beginner

How do you calculate the sum of a column using Laravel's query builder?

Use the sum('column_name') method to add all values in that column.

beginner

What is the purpose of the avg() method in Laravel?

It calculates the average (mean) value of a specified column in the database.

beginner

Show a simple example of using count() in Laravel's query builder.

Example:
$totalUsers = DB::table('users')->count();
This counts all users in the 'users' table.

intermediate

Can you chain aggregate functions with other query builder methods in Laravel?

Yes, you can add conditions before calling aggregates, like:
$sum = DB::table('orders')->where('status', 'paid')->sum('amount');

Which Laravel method counts the number of rows in a table?

A total()

B count()

C avg()

D sum()

The count() method returns the number of rows.

How do you get the total sum of a column named 'price' in Laravel?

A DB::table('items')->total('price')

B DB::table('items')->count('price')

C DB::table('items')->avg('price')

D DB::table('items')->sum('price')

Use sum('price') to add all values in the 'price' column.

What does avg('score') return in Laravel?

A The average score

B The highest score

C The total score

D The number of scores

avg('score') calculates the average value of the 'score' column.

Can you add a where condition before using sum()?

A No, aggregates don't support conditions

B Only with <code>count()</code>

C Yes, you can chain <code>where</code> before <code>sum()</code>

D Only with <code>avg()</code>

You can chain where to filter records before summing.

Which aggregate method would you use to find how many orders are in the database?

A count()

B avg()

C sum()

D max()

count() returns the number of orders.

Explain how to use Laravel's aggregate functions to get the total number of users, the sum of their balances, and the average age.

Think about chaining methods on the DB facade with the right aggregate functions.

Describe a real-life scenario where you would use the sum() and avg() aggregates in a Laravel application.

Imagine a store or review system needing totals and averages.

Practice

1. What does the count() method do in Laravel Eloquent?

easy

A. Returns the number of records matching the query

B. Calculates the sum of a column

C. Finds the average value of a column

D. Deletes all records in the table

Solution

1. Step 1: Understand the purpose of count()

   The count() method counts how many records match the query conditions.

2. Step 2: Compare with other aggregate methods

   Unlike sum() or avg(), count() returns a number of rows, not a calculation on column values.

3. Final Answer:

   Returns the number of records matching the query -> Option A

4. Quick Check:

   count() = number of records [OK]

Hint: count() counts rows, sum() and avg() work on columns [OK]

Common Mistakes:

• Confusing count() with sum() or avg()
• Thinking count() returns column values
• Assuming count() deletes records

2. Which of the following is the correct syntax to get the sum of the price column using Eloquent?

easy

A. Product::sum(['price']);

B. Product::sum(price);

C. Product->sum('price');

D. Product::sum('price');

Solution

1. Step 1: Identify correct method call syntax

   In Laravel Eloquent, static calls use double colon :: and column names must be strings in quotes.

2. Step 2: Check each option

   Product::sum('price'); uses Product::sum('price'); which is correct. Product::sum(price); misses quotes, C uses arrow on class, D passes array incorrectly.

3. Final Answer:

   Product::sum('price'); -> Option D

4. Quick Check:

   sum('column') needs quotes and :: [OK]

Hint: Use quotes around column name and :: for static call [OK]

Common Mistakes:

• Omitting quotes around column name
• Using -> instead of :: on model class
• Passing column name as array

3. Given the code:

$total = Order::where('status', 'completed')->sum('amount');

What does $total contain?

medium

A. The count of orders with status 'completed'

B. An error because sum() cannot be used with where()

C. The sum of 'amount' for orders with status 'completed'

D. The average 'amount' for all orders

Solution

1. Step 1: Understand the query filter

   The where('status', 'completed') filters orders to only those completed.

2. Step 2: Apply the sum aggregate

   The sum('amount') adds up the 'amount' values of the filtered orders.

3. Final Answer:

   The sum of 'amount' for orders with status 'completed' -> Option C

4. Quick Check:

   where() filters, sum() adds column values [OK]

Hint: where() filters rows, sum() adds column values of filtered rows [OK]

Common Mistakes:

• Thinking sum() returns count of rows
• Believing sum() cannot be chained with where()
• Confusing sum() with avg()

4. What is wrong with this code snippet?

$average = User::avg(age);

medium

A. avg() requires a second parameter

B. Missing quotes around the column name 'age'

C. avg() cannot be used on User model

D. Using avg() without where() causes error

Solution

1. Step 1: Check method parameter syntax

   The avg() method requires the column name as a string, so it must be quoted.

2. Step 2: Identify the error cause

   Without quotes, PHP treats age as a constant or variable, causing an error or unexpected behavior.

3. Final Answer:

   Missing quotes around the column name 'age' -> Option B

4. Quick Check:

   Column names must be strings in quotes [OK]

Hint: Always quote column names in aggregate methods [OK]

Common Mistakes:

• Omitting quotes around column names
• Thinking avg() needs multiple parameters
• Assuming avg() requires where()

5. You want to get the average price of products that are in stock (stock > 0). Which Eloquent query is correct?

hard

A. Product::where('stock', '>', 0)->avg('price');

B. Product::avg('price')->where('stock', '>', 0);

C. Product::where('stock', '>', 0)->sum('price') / Product::count();

D. Product::sum('price')->where('stock', '>', 0) / Product::count();

Solution

1. Step 1: Filter products with stock > 0

   The where('stock', '>', 0) filters only products that are in stock.

2. Step 2: Calculate average price of filtered products

   The avg('price') calculates the average price of those filtered products.

3. Step 3: Check other options for correctness

   Product::avg('price')->where('stock', '>', 0); calls avg() before where(), which is invalid. Manual calculations with sum divided by count do not filter correctly or mix sum and count improperly.

4. Final Answer:

   Product::where('stock', '>', 0)->avg('price'); -> Option A

5. Quick Check:

   Filter first, then aggregate [OK]

Hint: Chain where() before avg() to filter then calculate [OK]

Common Mistakes:

• Calling aggregate before where()
• Manually dividing sum by count without filtering
• Mixing sum() and avg() incorrectly