PostgreSQL is commonly used for querying relational databases, and JOINs and subqueries are two fundamental ways to combine or filter data in PostgreSQL.
While solving a query using PostgreSQL, users often face a choice between using a subquery or a JOIN. While both can often produce the same result, their performance and readability can differ significantly depending on the context.
What is a JOIN in PostgreSQL?
By using JOIN, we can combine records from two or more tables. A join will be performed whenever multiple tables appear in the FROM clause of the query. The select list of the query can select any columns from any of these tables.
Most join queries contain at least one join condition, either in the FROM clause or in the WHERE clause.
The main advantage of the use of 'join' is that by using this, we can minimize the calculation burden on the database
The disadvantage of using joins is that we must carefully choose the correct join type, minimize the number of join queries for better performance, and be aware that join queries are not always easy to read.
It is important that joins are performed correctly, as incorrect joins can result in serious performance degradation and inaccurate query results.
Syntax
SELECT a.column1, b.column2
FROM table_a a
JOIN table_b b ON a.common_column = b.common_column;
What is a Subquery in PostgreSQL?
A subquery, or nested query, is a query within an SQL query and embedded within the WHERE clause. A subquery is a SELECT statement that is embedded in a clause of another SQL statement.
They can be very useful for selecting rows from a table with a condition that depends on the data in the same or another table. subqueries can be used within the WHERE, HAVING, and FROM clauses, depending on the requirements of the query
The advantage of using subqueries is that they are easy to understand and allow the results of one query to be used in another (outer) query. They also help break down complex queries into smaller, more manageable parts.
We can't modify a table and select from the same table within a subquery in the same statement.
Syntax
SELECT column
FROM table
WHERE column IN (SELECT column FROM other_table WHERE condition);
⚖️ Subquery vs Join – Key Differences
| Feature | Join | Subquery |
|---|
| Performance | Usually faster, especially with indexes | May be slower for large data sets |
| Usage | Combine data from multiple tables | Filter or calculate data before main query |
| Flexibility | Good for returning columns from multiple tables | Good for filtering or aggregation |
| Readability | Can get complex with multiple joins | Cleaner for simple filtering logic |
Conclusion
In PostgreSQL, subqueries and joins are both useful and can often achieve the same results — but the best choice depends on what you're trying to do, the volume of data, and how easy you want the query to be to understand and maintain.
Use JOIN when you need to bring together rows from multiple related tables and want to retrieve several columns from those tables in a single result.
Use SUBQUERIES when you're filtering results based on specific conditions or when you're working with aggregated data (like totals, counts, or averages) before using it in your main query.