Foreign keys and referential constraints allow you to set relationships between tables and modify some of the database engine’s actions. This beginner’s guide explains referential integrity and foreign key use in MySQL. One of the most important aspects of database usage is being able to trust the information you store. Database engines provide several features that help you maintain the quality of your data, like defining required columns as NOT NULL and setting an exact data type for each column.
-- -- Previously, we've discussed the use of SQL aggregate functions with the GROUP BY statement. Regular readers of the Vertabelo Academy blog will also remember our recent tutorial about JOINs. If you're a bit rusty on either subject, I encourage you to review them before continuing this article. That's because we will dig further into aggregate functions by pairing them with JOINs.
Aggregate functions are powerful SQL tools that compute numerical calculations on data, allowing the query to return summarized information about a given column or result set. These functions can be used in conjunction with the GROUP BY statement. Let’s see how they work using some easy examples. SQL Aggregate Functions Suppose we have users residing in a city, and we store their information in two tables. These tables and their relationship are shown below:
The JOIN statement lets you work with data stored in multiple tables. This article is a practical introduction to the SQL JOIN. Imagine if you could only work with one database table at a time. Fortunately, this isn’t anything we have to worry about. Once you learn the JOIN statement, you can start linking data together. This article will give you examples that illustrate how we use JOINs, how each type of JOIN works, and when to use each type.
Regardless of the engine you are using (SQL Server, mySQL, Oracle, etc), you can prevent common errors and simplify the debugging process. This article will discuss some of the issues you’ll face and will help you tackle them successfully. One of the best ways to prevent SQL errors is to keep your queries clear and readable. It’s very easy to forget the ideas behind your code! If you have to revisit it (and you will), messy code can be hard to understand.