SQL ORDER BY RANDOM
Introduction In this article, we are going to see how we can sort an SQL query result set using an ORDER BY clause that takes a RANDOM function provided by a database-specific function. This is a very handy trick, especially when you want to shuffle a given result set. Note that sorting a large result set using a RANDOM function might turn out to be very slow, so make sure you do that on small result sets. If you have to shuffle a large result set and limit it afterward, then it’s… Read More
How do UPSERT and MERGE work in Oracle, SQL Server, PostgreSQL and MySQL
Introduction Last week, Burkhard Graves asked me to answer the following StackOverflow question: And, since he wasn’t convinced about my answer: I decided to turn it into a dedicated article and explain how UPSERT and MERGE work in the top 4 most common relational database systems: Oracle, SQL Server, PostgreSQL, and MySQL.
Why you should definitely learn SQL Window Functions
Introduction I found this question on the Hibernate forum, and it’s a very good opportunity to show why mastering Windows Functions is a very important skill for any backend software developer.
How to map Java and SQL arrays with JPA and Hibernate
Introduction In this article, we are going to see how you can map SQL arrays to JPA entity attributes when using Hibernate. Hibernate custom Types allow you to map all sorts of database-specific column types, like IP address, JSON columns, bit sets, or SQL arrays. However, while you can create your own custom Hibernate Types for mapping PostgreSQL arrays, you don’t need to implement your own Hibernate Type. All you need to do is use the Hypersistence Utils open-source project.
How to install DB2 Express-C on Docker and set up the JDBC connection properties
Introduction While developing Hibernate, I need to test the code base against a plethora of relational database systems: Oracle, SQL Server, PostgreSQL, MySQL, MariaDB, Informix, and of course DB2. However, having all these databases installed on my system is far from ideal, so I rely a lot on Docker for this task. In this article, I’m going to show how easily you can install DB2 on Docker and set up the JDBC connection so that you can run Hibernate tests on DB2.
How does a relational database work
Introduction While doing my High-Performance Java Persistence training, I came to realize that it’s worth explaining how a relational database works, as otherwise, it is very difficult to grasp many transaction-related concepts like atomicity, durability, and checkpoints. In this post, I’m going to give a high-level explanation of how a relational database works internally while also hinting some database-specific implementation details.
How does database pessimistic locking interact with INSERT, UPDATE, and DELETE SQL statements
Introduction Relational database systems employ various Concurrency Control mechanisms to provide transactions with ACID property guarantees. While isolation levels are one way of choosing a given Concurrency Control mechanism, you can also use explicit locking whenever you want a finer-grained control to prevent data integrity issues. As previously explained, there are two types of explicit locking mechanisms: pessimistic (physical) and optimistic (logical). In this post, I’m going to explain how explicit pessimistic locking interacts with non-query DML statements (e.g. insert, update, and delete).
A beginner’s guide to the Write Skew anomaly, and how it differs between 2PL and MVCC
Introduction Unlike SQL Server which, by default, relies on the 2PL (Two-Phase Locking) to implement the SQL standard isolation levels, Oracle, PostgreSQL, and MySQL InnoDB engine use MVCC (Multi-Version Concurrency Control), so handling the Write Skew anomaly can differ from one database to the other. However, providing a truly Serializable isolation level on top of MVCC is really difficult, and, in this post, I’ll demonstrate that it’s very difficult to prevent the Write Skew anomaly without resorting to pessimistic locking.
How to map table rows to columns using SQL PIVOT or CASE expressions
Introduction While reading the wonderful SQL Antipatterns book by Bill Karwin, which is a great reference for any developer that needs to interact with a Relational Database System, I found an example where the SQL PIVOT clause would work like a charm. In this post, I’m going to explain how to transpose a ResultSet using PIVOT so that rows become columns.
A beginner’s guide to SQL injection and how you should prevent it
Introduction One of my readers asked me to answer the following StackOverflow question. Right from the start, I noticed that the entity query was constructed by concatenating strings, which can expose your application to SQL Injection attacks. Unfortunately, I’ve been seeing this problem over and over throughout my career, and not all developers are aware of the serious risk associated with SQL Injection. For this reason, this post is going to demonstrate what damage can SQL Injection do to your system.
