Inner Joins

Query Execution
Updated on:
July 11, 2024

What is an inner join?

An inner join returns only the subset of rows that match between two tables based on the join predicate. For each row in the first table, it finds matching rows in the second table and combines corresponding columns into output rows.

The join predicate typically compares the values of a common column across the two tables, such as a foreign key relationship. Only rows satisfying the join condition are included in the result.

Inner joins filter out unmatched rows, unlike outer joins which include them. Join order, type, and predicate impacts query plan efficiency.

Database query optimizers like Apache Arrow DataFusion analyze join relationships and statistics to pick optimal join sequences, algorithms and execution via cost-based query optimization. The query execution engine then performs the optimized join.

Inner joins allow efficiently combining relational data based on matching criteria. They provide filtered join output unlike outer joins and cross joins.

What does it do/how does it work?

An inner join creates a Cartesian product of two tables and filters rows that don't satisfy the join predicate. It compares values of the join key column(s) from the first table to values from the second table to find matches.

For matching rows, it combines columns from both tables and adds the result to the output. The join key columns are included only once in the output.

Why is it important? Where is it used?

Inner joins are used to connect related data across tables. This is very common in relational databases for queries that need to combine attributes from multiple tables.

For example, joining a customer table to an orders table to get order details for each customer. Joins also aid in normalizing data by separating entities and relationships across tables.

FAQ

What is the difference between inner and outer joins?

  • Inner join outputs only rows with matches between tables.
  • Outer joins output rows from one table regardless of matches in the other table.

Can inner joins be used to filter data?

Yes, inner joins filter out non-matching rows, so they can focus queries on a subset of related data.

What types of join algorithms are used?

  • Nested loop join checks each pair of rows
  • Hash join builds hash table from first table
  • Merge join simultaneously scans sorted tables

What are some challenges with inner joins?

  • Joining large tables can get expensive
  • Missing join keys lead to missing rows
  • Multiple complex joins can slow performance
  • Denormalized data can avoid joins but has tradeoffs

References:

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