10 Common Mistakes Java Developers Make When Writing SQL

Java developers often blend object‑oriented and imperative programming, but when they write SQL they face a different paradigm that can lead to many pitfalls.

The article lists ten typical mistakes and offers concrete remedies:

Forgetting NULL semantics – Misinterpreting NULL as a regular value leads to logical errors; developers should consistently consider NULL’s impact on query results and constraints.

Processing data in Java memory – Loading large result sets into Java and performing operations there wastes resources; instead, leverage database‑side OLAP features such as MODEL or window functions.

Using UNION instead of UNION ALL – UNION removes duplicates unnecessarily and incurs costly sorting; prefer UNION ALL when duplicates are acceptable.

Relying on JDBC pagination in application code – Native SQL pagination (LIMIT/OFFSET, FETCH, ROW_NUMBER, etc.) is far more efficient than manual paging in Java.

Inserting rows one by one – Single‑row INSERTs create many PreparedStatement objects; use batch inserts to reduce overhead and improve throughput.

Applying DISTINCT or UNION to eliminate duplicates from Cartesian products – This masks underlying join logic problems and degrades performance; correct the JOIN conditions instead.

Not using MERGE (UPSERT) statements – MERGE provides a concise, atomic way to insert or update rows and avoids race conditions.

Replacing window functions with aggregate functions – Window functions offer clearer syntax and better optimizer support for analytical queries.

Performing indirect sorting in Java memory – Whenever possible let the database handle ORDER BY to avoid unnecessary data transfer and memory usage.

Inserting large volumes of records row‑by‑row – Always employ batch processing for bulk data loads.

By reviewing each mistake and applying the suggested solutions, developers can write more efficient, reliable, and maintainable SQL code within Java applications.

Source: Translated from the original article on jOOQ’s blog.