All the Matches of the League

All the Matches of the League

What is this problem about?

The "All the Matches of the League interview question" is a SQL problem that requires generating a schedule. Given a table of teams in a sports league, you need to produce a list of all possible matches. Each pair of teams should play twice: once as the home team and once as the away team. The output should usually include the names of both teams.

Why is this asked in interviews?

Microsoft often uses the "All the Matches of the League coding problem" as a warm-up for database roles. It tests basic SQL operations like CROSS JOIN and the ability to filter out invalid pairings (like a team playing against itself). It's a fundamental test of set-based thinking in relational databases.

Algorithmic pattern used

The core pattern here is the Self-Cross Join.

1. Cartesian Product: You join the Teams table with itself. This creates every possible combination of (Team A, Team B).
2. Filter: You apply a WHERE clause to ensure that Team A is not the same as Team B.
3. Selection: You select the team names from both tables to represent the "Home Team" and the "Away Team."

Example explanation

Table Teams: {Arsenal, Chelsea, Liverpool}

1. Join Teams T1 with Teams T2.
2. Resulting pairs:
   • (Arsenal, Arsenal) -> Filter out
   • (Arsenal, Chelsea) -> Keep
   • (Arsenal, Liverpool) -> Keep
   • (Chelsea, Arsenal) -> Keep
   • (Chelsea, Chelsea) -> Filter out
   • (Chelsea, Liverpool) -> Keep
   • (Liverpool, Arsenal) -> Keep
   • (Liverpool, Chelsea) -> Keep
   • (Liverpool, Liverpool) -> Filter out

Common mistakes candidates make

• Missing the "Away" matches: Only providing (A, B) and forgetting (B, A). In many leagues, the order matters.
• Forgetting to filter: Allowing a team to play against itself, which is logically impossible in this context.
• Using INNER JOIN without a condition: An INNER JOIN requires an ON clause, while a CROSS JOIN or a comma-separated list is more appropriate for a Cartesian product.

Interview preparation tip

Understand the difference between INNER JOIN, CROSS JOIN, and SELF JOIN. Practice generating combinations and permutations in SQL, as these are common requirements for reporting and scheduling tasks.