Common table expressions (CTEs) are a feature that we can use in SQL to help simplify complex queries and enhance readability. They can help with things like code readability, performance, recursive queries, and more.
This article explores what CTEs are, their syntax, types, use cases, and best practices, along with examples to help illustrate their usage.
Continue readingIf you’re running a subquery that uses FOR XML to produce XML in SQL Server, but you’re getting error 258, which reads “Cannot call methods on nvarchar(max)” it might be that you’re trying to query the XML in the outer query, but your inner query isn’t outputting the XML as an actual xml type.
The FOR XML result must be an xml type in order to process the result on the server.
To fix this issue, make sure the subquery outputs the result using the xml data type.
Continue readingIf you’re getting an error that reads “The ORDER BY clause is invalid in views, inline functions, derived tables, subqueries, and common table expressions, unless TOP, OFFSET or FOR XML is also specified” in SQL Server, it appears that you’re using the ORDER BY clause in a place that it must also be accompanied by a TOP, OFFSET or FOR XML clause, but you’re not including one of those clauses.
To fix this issue, be sure to include one of those clauses in your query if you need to use the ORDER BY clause.
If you’re getting an error that reads something like “The correlation name ‘a’ is specified multiple times in a FROM clause’” in SQL Server, it could be that you’re doing a join on two tables, but you’re assigning them the same aliases.
Each table’s alias must be unique.
To fix this issue, be sure to use unique table aliases in your query.
Continue readingPrepared statements are a feature in PostgreSQL that can improve performance and security when executing repetitive SQL queries.
By creating a prepared statement, the server can parse and plan the query once and reuse it multiple times, leading to faster execution. Prepared statements also help in preventing SQL injection attacks by safely handling user input.
In this article, we’ll explore prepared statements in PostgreSQL, discuss how to create and use them, and cover other important considerations such as query planning and cleaning up statements.
Continue readingSQL subqueries are like little helpers that fetch data for our main query. They’re super handy when we need to filter our results based on some other data in our database.
Probably the most common placement of a subquery in SQL is in the WHERE clause. Placing a subquery in the WHERE clause allows us to compare a column against a set of rows that match a certain criteria.
Let’s look at three quick examples of subqueries in the WHERE clause.
“That query’s not sargable” says one developer to another.
“I know it’s not sargable… would you love it more if it had sargability?” replies the other developer.
“What does sargable mean?” you wonder in silence.
Continue readingIf you’re getting an error that reads something like “Cannot use an aggregate or a subquery in an expression used for the group by list of a GROUP BY clause.” in SQL Server, it looks like you’re trying to use either an aggregate function or a subquery in the GROUP BY clause.
We can’t use aggregates or subqueries in the GROUP BY clause.
To fix this issue, remove any aggregate functions or subqueries from your GROUP BY clause.
When looking through the execution plan in SQL Server, you may occasionally see “key lookup” glaring back at you.
If you find this happening a lot, and on frequently run queries, then you might want to do something about it.
Continue readingIf you’ve spent any time working with heaps in SQL Server, you may have seen the RID lookup in the query plan. And if you haven’t, chances are it’s because you either didn’t look at the query plan, or the queries used full table scans.
In any case, it pays to understand what a RID lookup is and how it affects query performance. If we ever see RID lookup in the query plan, it could be a good opportunity to see if we can make changes that will improve performance.
Continue reading