Azure-Sql-Db

In the previous T-SQL 101 post, I mentioned that some subqueries depend upon the outer query and others don’t. That’s the distinction between what are called nested and correlated subqueries.

Nested Subquery

I mentioned that you can also use a subquery to return a table of values. In the first example above, I’ve selected some columns from order lines and then said inner join to a derived table of products where their outer quantity is greater than 24.

2025-02-24

This is a post in the SQL Interview series. These aren’t trick or gotcha questions, they’re just questions designed to scope out a candidate’s knowledge around SQL Server and Azure SQL Database.

Section: Development Level: Medium

Question:

A query included within a stored procedure, has a predicate written like this:

WHERE ISNULL(ProductSize, '') = ISNULL(@ProductSize, '')

A problem has been identified with this predicate. The system needs to be able to differentiate between NULL values for product size and blank strings. Currently both are treated as though they are the same.

2025-02-23

Sometimes what we’re interested in is just whether or not a row exists.

In the example shown, I’m saying I want to select product ID and description from the products table where there is at least one order line that has the picked quantity less than the ordered quantity. That means that someone ordered a certain number, but for some reason, we didn’t pick (i.e. got ready for shipment) the quantity that they wanted.

2025-02-22

This is a post in the SQL Interview series. These aren’t trick or gotcha questions, they’re just questions designed to scope out a candidate’s knowledge around SQL Server and Azure SQL Database.

Section: Security Level: Medium

Question:

With SQL Server on-premises, you can create a CREDENTIAL and they are stored in the master database.

What option do you have with Azure SQL Database?

Answer:

You can create a DATABASE SCOPED CREDENTIAL in both SQL Server and in Azure SQL Database.

2025-02-21

Another way that we can use a subquery is in place of a list of values. I’ve previously shown how the IN operator allows us to supply a set of values, and we check if something or some value is contained in that list.

In this example query, I’m saying that I want a list of all the cinemas that have a credit rating that isn’t OK. Perhaps it’s something to do with not paying their bills, or defaulted in some way.

2025-02-20

Our free SDU Tools for developers and DBAs, now includes a very large number of tools, with procedures, functions, and views. A request that I had a while back was to have a function that returned what type of SQL Server the code was being executed on. So we added the SQLServerType function.

It takes no parameters.

As this is an enumerated value, it’s just a CASE statement.

Find out more

You can see it in action in the main image above, and in the video here. The full current version of the code is also shown below:

2025-02-19

Subqueries allow us to reuse data from one query in another query. There are three basic ways that this can work:

Row sets (tables) - a subquery can return an entire row set with columns. The basic structure of that is shown in the example above.

Lists - a type of row set that has a single column. It’s often used with an IN clause.

Scalar values - a single value returned from a query i.e., one row with one column. These are often used in place of values in expressions or predicates.

2025-02-16

This is a post in the SQL Interview series. These aren’t trick or gotcha questions, they’re just questions designed to scope out a candidate’s knowledge around SQL Server and Azure SQL Database.

Section: Development Level: Medium

Question:

When writing T-SQL code, you often need to replace NULL values. The core two functions provided are ISNULL and COALESCE.

What is the difference between them?

Answer:

ISNULL only takes a single value to check and replaces it when NULL.

2025-02-15

We saw in previous post that we had a function called GetRecentOrders that gets the most recent orders for a particular cinema.

But what if we want to do that for all the cinemas?

If I’d like the two most recent orders for every cinema, that becomes messy. I can’t just do that with a join, like an inner join or outer join. The function provided is fine. We can use that, but we want to call it for every row in the table.

2025-02-14

Our free SDU Tools for developers and DBAs, now includes a very large number of tools, with procedures, functions, and views. I do a lot of reviewing of database designs and one of my least favourite things to see is the use of default system names for constraints. So we added a tool that can help to find when this has happened. It’s called ListConstraintsWithSystemNames.

The procedure takes three parameters.

2025-02-13