Sql-Server

The first example of subqueries types in this series is what’s called a scalar subquery. In this case, I’m trying to work out which orders were on the last day that we delivered any orders. We deliver orders on various days, but on the last day that we did deliver orders, tell me which orders they were.

The problem with this is that we don’t know what the delivery date was. If I did, I could just use that date value in a WHERE clause. So what we can do instead is to use a subquery, where we work out the maximum delivery date from the orders table, and then that will return us back a date. Finally, we ask for all the orders for that particular day.

2025-02-18

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 the last time that SQL Server was restarted. So we added the LastSQLServerRestart function.

It takes no parameters.

I used to have a more complex way to work this out, but thanks to a great suggestion from our buddy Rob Wylie, it now just looks for the creation time of tempdb. That’s pretty good and simple to implement.

2025-02-17

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

In previous posts, we’ve seen how to query tables. Another type of object that you might need to query is a table-valued function.

Table-valued functions (TVFs) are predefined code i.e., somebody’s written the code, and they take parameters, but they return row set of data, much like a table.

So in the example above, I’m saying I want to declare a Cinema ID variable, and I’ve said let’s have Cinema 27. Then I’ve said I wanted to select from GetRecentOrders. It’s  a TVF that takes two parameters: the Cinema ID, and the maximum number of orders to return. In the case, I’ve said that I want the last two orders for cinema 27.

2025-02-12

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: Intro

Question:

SQL Server can use both logins and users.

What is the difference between them?

Answer:

A login provides access to the server, but not necessarily to any databases.

A user is used to provide access to a database. It might be mapped to a login, but it is possible for databases to authenticate users directly.

2025-02-11

Another interesting operator is INTERSECT. We saw how EXCEPT takes a set of rows and removes any duplicates, and removes any rows that are also contained in a second set of rows.

INTERSECT is similar in the way it works, but it only returns the rows that are common to both row sets.

You could replace an INTERSECT statement with a WHERE EXISTS clause but if you need to deal with NULLable columns, and as the number of columns increases, you can see that INTERSECT becomes quite an elegant solution.

2025-02-10

Our free SDU Tools for developers and DBAs, now includes a very large number of tools, with procedures, functions, and views. One request that we had some while back, was the ability to format phone numbers using Australian phone number format. To make that easy, we added the FormatAustralianPhoneNumber function.

The only parameter for this function is the phone number that needs to be formatted.

It starts by finding any digits. Then if it finds either 6, 7, 8, or (the standard) 10 digits, it formats the number appropriately.

2025-02-09