In SQL, adding an index to a table can provide us with a significant performance boost, especially if it’s a covering index for commonly run queries.
But what exactly is a covering index?
Let’s find out.
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Aggregating Data in SQL with the GROUP BY and HAVING Clauses
When writing queries in SQL, we sometimes need to summarize data based on specific criteria. Fortunately SQL makes this easy to achieve with the GROUP BY and HAVING clauses. These two clauses allow us to aggregate data and filter the results based on group conditions.
More specifically, GROUP BY allows us to group the results, and HAVING is an optional clause that we can use to filter the results based on certain conditions.
Below is an example of where we can use these two clauses to aggregate data and filter it based on certain criteria.
Continue readingUnderstanding PostgreSQL’s FILTER Clause
When using aggregation functions in PostgreSQL, we have the option of including a FILTER clause. This clause allows us to narrow down – or filter – the rows that are fed to the aggregate function.
This can be be a handy way to provide functionality that we might otherwise need to use a CASE statement or perhaps a CTE.
In this article, we’ll take a look at PostgreSQL’s FILTER clause, and see how it can simplify our SQL queries by replacing CASE statements with more concise code.
Using a SQL Subquery in the SELECT List
Let’s dive into subqueries for a moment. Specifically, how to use a SQL subquery in the SELECT list. It’s a handy little trick that can help us fetch related data without resorting to complex joins.
How to Extract Fields from Composite Values in PostgreSQL
In PostgreSQL, a composite value is stored as a comma separated list of values, surrounded by parentheses.
When we have a table that contains composite values, we can extract individual fields by using dot notation to refer to the specific field that we want to extract.
Continue readingHow to Update Multiple Columns in SQL with a Subquery
In SQL, it’s not unusual to see subqueries in SELECT statements, where they can help narrow down the results based on a complex condition.
But the SELECT statement isn’t the only place we can put a subquery.
We can use subqueries in an UPDATE statement, and we can update multiple rows too, if required.
In this article, we look at how to update multiple columns by using a SQL subquery in the UPDATE statement.
Understanding the Correlated Subquery in SQL
When writing SQL queries, we sometimes find ourselves including a subquery in the query in order to get the results we want. Sometimes the subquery we include could be taken out of that query and run by itself to return its own independent result set. But not the correlated subquery.
A correlated subquery relies on the outer query for its data. We can’t simply move it outside of the outer query and expect it to work.
Let’s take a look the correlated subquery in SQL and observe it in action with a few examples.
Continue readingExamples of SQL Subqueries in the FROM Clause
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.
Using UPDATE with a Subquery in SQL
Ever found yourself needing to update a bunch of rows in your database, but the condition for the update depends on data from another table? That’s where UPDATE with a subquery comes in handy. Let’s break it down.
Understanding the SQL Subquery
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.
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