Introduction In this article, we are going to see how the SQL EXISTS operator works and when you should use it. Although the EXISTS operator has been available since SQL:86, the very first edition of the SQL Standard, I found that there are still many application developers who don’t realize how powerful SQL subquery expressions really are when it comes to filtering a given table based on a condition evaluated on a different table. Database table model Let’s assume we have the following two tables in our database, that form a one-to-many... Read More
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Introduction In this article, we are going to see how a CROSS JOIN works, and we will also make use of this SQL join type to build a poker card game. Database table model For our poker card game application, we have created the ranks and suits database tables: The ranks table defines the ranking of cards, as well as the name and symbol used for each card rank: The suits table describes the four possible categories used by the French playing cards: Cartesian product In the set theory, the Cartesian product... Read More
I miss a proper database related newsletter for busy people. There’s so much happening in the space, from tech, to licensing, and even usage. Anyway, quick tab sweep.
Paul Vallée (of Pythian fame) has been working on Tehama for sometime, and now he gets to do it full time as a PE firm, bought control of Pythian’s services business. Pythian has more than 350 employees, and 250 customers, and raised capital before. More at Ottawa’s Pythian spins out software platform Tehama.
Percona has …[Read more]
Introduction In this article, we are going to see how a deadlock can occur in a relational database system, and how Oracle, SQL Server, PostgreSQL, or MySQL recover from a deadlock situation. Database locking Relational database systems use various locks to guarantee transaction ACID properties. For instance, no matter what relational database system you are using, locks will always be acquired when modifying (e.g., UPDATE or DELETE) a certain table record. Without locking a row that was modified by a currently running transaction, Atomicity would be compromised. Using locking for controlling access... Read More
Introduction Ever wanted to connect to a relational database using Java and didn’t know which JDBC Driver Maven dependency to use? If so, this article is surely going to help you from now on. Oracle Since September 2019, the Oracle JDBC Driver is available on Maven Central. For JDK 10 and 11, use the following Maven dependency: For JDK 8, use the ojdbc8 artifact instead: For more details about the proper version to use, check out the following Maven Central link. MySQL The MySQL Driver is available on Maven Central, so just... Read More
In MySQL and Oracle, you set a session variable quite differently. That means you should expect there differences between setting a session variable in Postgres. This blog post lets you see how to set them in all three databases. I’m always curious what people think but I’m willing to bet that MySQL is the simplest approach. Postgres is a bit more complex because you must use a function call, but Oracle is the most complex.
The difference between MySQL and Postgres is an “
symbol versus a
current_setting() function call.
Oracle is more complex because it involves the mechanics in
sqlplus shell, SQL dialect, and PL/SQL
language (required to assign a value to a variable).
MySQL lets you declare a session variable in one step and use it one way in a SQL statement or stored procedure.
- You set a session variable on a single line with the following …
Percona Live is always a great opportunity to learn from the best and brightest in the open-source database community. This time, Percona Live Europe is being held at the Hilton Amsterdam Airport Schiphol, The Netherlands from September 30 to October 2, 2019.
Pythian will be present, as has been the case for the past few years, with some of our technical experts speaking on a variety of subjects and technologies.
Feel free to ping any of us during the conference breaks or community events, as we’ll be happy to answer any questions you have about anything open source, cloud, or data-related.
Here are some sessions you won’t want to miss:
Wednesday, October 2
9:00 AM – …[Read more]
One of the biggest and undead myths in SQL is that COUNT(*) is faster than COUNT(1). Or was it that COUNT(1) is faster than COUNT(*)? Impossible to remember, because there’s really no reason at all why one should be faster than the other. But is the myth justified?
How does COUNT(…) work?
But first, let’s look into some theory. The two ways to count things are not exactly the same thing. Why?
COUNT(*)counts all the tuples in a group
COUNT(<expr>)counts all the tuples in a group for which
<expr>evaluates to something that
IS NOT NULL
This distinction can be quite useful. Most of the time, we’ll
COUNT(*) for convenience, but there are (at
least) two cases where we don’t want that, for example:
When outer joining…[Read more]
.NET Core 3 .NET Core was presented by Microsoft in 2016, but its 1.x versions had limited set of features comparing to Full .NET Framework. Since then .NET Core has been drastically improved. .NET Core 2.0 has a significant part of Full .NET Framework features and includes new functionality and significant performance optimizations. This year, […]
Introduction In this article, we are going to see how we can limit the SQL query result set to the Top-N rows only. Limiting the SQL result set is very important when the underlying query could end up fetching a very large number of records, which can have a significant impact on application performance. Why limit the number of rows of a SQL query? Fetching more data than necessary is the number one cause of data access performance issues. When a given business use case is developed, the amount of data available... Read More
The post How to limit the SQL query result set to Top-N rows only appeared first on Vlad Mihalcea.
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