Join operations in Laravel - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ component_behavior

intermediate

What is the output of this Laravel join query?

Consider two tables: users and posts. Each post has a user_id linking to users.id. What will this query return?

Laravel

DB::table('users')
  ->join('posts', 'users.id', '=', 'posts.user_id')
  ->select('users.name', 'posts.title')
  ->get();

A All users with their post titles, including users without posts showing null for titles.

B A collection of user names with their post titles, only for users who have posts.

C All posts with user names, including posts without users showing null for names.

D An error because join requires an explicit on clause.

💡 Hint

Think about what a SQL INNER JOIN does in terms of matching rows.

The join method in Laravel performs an INNER JOIN by default, so only users with matching posts are returned.

📝 Syntax

intermediate

Which option correctly performs a left join in Laravel's query builder?

You want to get all users and their posts if any. Which code snippet correctly uses a left join?

A DB::table('users')->left_join('posts', 'users.id', '=', 'posts.user_id')->get();

B DB::table('users')->joinLeft('posts', 'users.id', '=', 'posts.user_id')->get();

C DB::table('users')->leftJoin('posts', 'users.id', '=', 'posts.user_id')->get();

D DB::table('users')->leftjoin('posts', 'users.id', '=', 'posts.user_id')->get();

💡 Hint

Check the exact method name Laravel uses for left joins.

The correct method is leftJoin with camelCase 'J'. Other options use incorrect method names or casing.

🔧 Debug

advanced

Why does this Laravel join query cause an error?

Examine this code and identify the cause of the error:

Laravel

DB::table('users')
  ->join('posts', 'users.id', 'posts.user_id')
  ->get();

A Missing the '=' operator in the join condition causes a syntax error.

B The table 'posts' does not exist, causing a runtime error.

C The join method requires a closure, so this syntax is invalid.

D The select method is missing, causing an error.

💡 Hint

Check the parameters passed to the join method carefully.

The join method expects four parameters: table, first column, operator, second column. Missing the operator causes an error.

❓ state_output

advanced

What is the count of results returned by this Laravel join query?

Given these tables:
- users: 5 rows
- posts: 3 rows, all linked to users
What does this query return?

Laravel

DB::table('users')
  ->join('posts', 'users.id', '=', 'posts.user_id')
  ->count();

A 8, sum of users and posts rows.

B 5, because all users are counted regardless of posts.

C 0, because count() requires a column name.

D 3, because only posts with matching users are counted.

💡 Hint

Remember that join filters to matching rows only.

The join returns rows where users and posts match, so count() returns the number of posts linked to users, which is 3.

🧠 Conceptual

expert

Which join type should you use to include all users even if they have no posts?

You want a list of all users and their posts if any. Posts should be null if none exist. Which join type is correct?

A Left join

B Inner join

C Right join

D Cross join

💡 Hint

Think about which join keeps all rows from the first table.

A left join returns all rows from the left table (users) and matches from the right (posts), filling nulls when no match.