When used in the FROM clause, a SQL subquery creates a temporary table that can be queried like any other table. These queries are sometimes called derived tables or table expressions because the outer query uses the results of the subquery as a data source.
In this article we look at three different examples of SQL subqueries that are placed into the FROM clause.
In SQL, indexing can be a great way to improve query performance across our database. We can create indexes to support the most commonly used queries, and these indexes can help our queries run much faster.
But it’s not always a matter of just “adding an index” and leaving it at that. If we really want the best bang for our buck, we usually need to look at how best to implement the index. There are many decisions that go into designing the best index for the job. And one such decision might be, should this be a filtered index?
But what exactly is a filtered index, and how can it improve our database efficiency?
Let’s find out.
Ever wanted to populate a table with data from another table? That’s where the INSERT statement with a subquery comes in handy. Using this method, we can insert the full contents of another table, or we can insert just a select number of rows or columns based on certain criteria.
Below are four examples of using the INSERT statement with a subquery, with each one slightly more complex than the previous.
A UNIQUE constraint is a rule we apply to one or more columns in a database table to ensure that the values in those columns are unique across all rows. This means that no two rows in the table can have the same value(s) in the column(s) where we’ve applied the UNIQUE constraint. It’s an essential tool for maintaining data integrity and preventing duplicate entries in our database.
When working with databases, performance can be very important. This is especially true in production environments where the end users expect their queries and reports to be generated within seconds (or even milliseconds).
While blistering fast queries may be the norm with smaller datasets, as our databases grow larger and more complex, it can become much more of a challenge to keep our queries nice and snappy. When working with smaller datasets, it’s often possible to get lightning speed results even when not optimizing for speed. But as the datasets grow larger, we need more efficient tools and techniques to retrieve and manipulate data. One such tool is the index.
SQL stands for Structured Query Language. It’s the go-to query language for most of us who develop relational databases. If you want to work with relational database management systems, learning SQL is pretty much non-negotiable. Especially if you want to become a SQL developer.
If you’re interested in becoming a SQL developer, here are ten essential SQL concepts that you need to learn.
The Enum (enumeration) data type in SQL is a specialised string object that allows us to define a list of predefined values for a column. This data type can be particularly useful when we want to restrict the possible values that can be stored in a specific column to a fixed set of options.
One of the things I love about SQL is just how easy it is to write a simple query that returns a meaningful result set, without having to a whole bunch of complex code. And I imagine most beginners are relieved when they discover this too.
However, while basic queries can go a long way, there’s a whole world of advanced techniques that can take our SQL development to another level. Somewhere in that world, would be the SQL subquery.
In this article, we look at the SQL subquery. We’ll explore what subqueries are, how they work, and when to use them. We’ll also look at some simple examples to demonstrate their use.
In SQL databases, the TRUNCATE statement can be used to quickly remove all data from a table. Unlike the DELETE statement, TRUNCATE is typically faster and uses fewer system resources, especially for large tables.
In this article, we’ll explore the TRUNCATE statement, its usage, and provide some examples.