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 you are writing an INSERT statement in T-SQL, the column list is usually optional.
Why would including it be a good idea? When would it ever be required (i.e. not optional)?
Answer:
2021-03-30
I’ve been talking about the basic window functions in T-SQL and one that’s not well known but is surprisingly useful is NTILE.
I’m not sure on the name but it’s probably short for percentile. I don’t know why they didn’t call it a slightly more meaningful name, but what it says is take the output and break it up into bands or chunks of data.
So if I say NTILE(10), that’ll give me a tenth of the rows with a value of 1, another tenth of the rows with a value of 2, and so on.
2021-03-29
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: Query Performance Level: Medium
Question:
In SQL Server a column that is defined with a bit data type can only have the values 0, or 1, or it can be NULL if the column allows it.
Given such a small number of possible values, are bit columns ever useful in indexes? Can you give an example of when they might or might not be useful?
2021-03-26
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 you are writing a CREATE TABLE statement to define a new table, after each column, you can write NULL or NOT NULL, but doing that is optional.
Why is it a good practice to include it, and what happens if you leave it out? Are the columns defined as NULL or as NOT NULL? Are there any exceptions to this?
2021-03-25
Dynamic Data Masking was added to SQL Server back in 2016. I’ve almost never used it since. Why? There were two reasons:
#1: The first reason was that it didn’t work in conjunction with other security features. In particular, you couldn’t use dynamic data masking in conjunction with Always Encrypted. From the start, I’ve been telling the product group that this is a misstep. There’s very little data that I’d want to hide from my own staff, that I’m happy still being visible to a hosting provider.
2021-03-23
In my last T-SQL 101 post, I mentioned ROW_NUMBER. It let you put a row number or position beside each row that was returned. Sometimes though, you want a rank instead. A rank is similar but it’s like a position in a race.
In the example above, I’ve used RANK to produce an ordering, based on an alphabetical listing of city names. Notice there’s Abercorn Abercorn Abercorn and then Aberdeen. So, like in a race, if three people came first they all get the value 1. The next person is fourth. Three people came forth, so then the next one is 7th, and so on.
2021-03-22
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:
I have an application that connects to the database as a user called ReaderApp. The only role that ReaderApp has been added to is db_datareader. The application was previously only using SELECT statements to read data that was required.
2021-03-19
Many clients hire me to carry out job interviews on their behalf. If they’re hiring someone for a data-related role, they want to make sure they’re hiring the right person. That usually means two things:
I can’t help with the first one. If I’ve worked with the client for a long time, I might have a pretty good idea but ultimately, that’s something the client needs to decide.
2021-03-18
In SQL Server 2000 and earlier versions, I often heard people ask “How do I output a row number beside each row that’s output in my query?”
I remember some people arguing that it wasn’t a valid request, as it didn’t feel “set-based” but it was an appropriate request, and it could be dealt with in a set-based manner. Sometimes it’s very, very useful to be able to do that.
2021-03-15
In the last few weeks, I’ve seen a surprising number of database scripts that deploy procedures and functions by dropping the object if it exists, then recreating it. Please don’t write scripts like this.
When you drop a procedure, you not only drop the procedure, but you drop all permissions associated with it. If you then create the procedure again, you’ll have the procedure, but you won’t have the permissions.
2021-03-11
