Of course, you might also need to join more than two tables. The example above shows how to do that.
I started with the Orders table, then joined to the OrderLines table, and finally, I’ve also joined to Products.
It’s another example of why I prefer the modern join syntax. Instead of just listing three tables and a bunch of WHERE clause predicates, for each table involved, I specify how it’s joined.
2025-02-04
Our free SDU Tools for developers and DBAs, now includes a very large number of tools, with procedures, functions, and views. A while back, we added the ability to retrieve the version of the toolset programmatically, but an interesting request that we had, asked for the ability to query the toolset for the tools themselves. The Tools view now does that.
The view returns the following columns:
ToolName ToolTypeCode ToolType Category Description VideoURL
2025-02-03
After discussing LEFT OUTER JOIN, you might wonder if there is also a RIGHT OUTER JOIN. There is, and it’s the reverse of the left one.
What a RIGHT OUTER JOIN from Products to ProductGroups says is that I want at least one row for every row in ProductGroups. You can see that in the query example above. The product group Hot Food currently has no products in it. The RIGHT OUTER JOIN causes it to be returned, as similar to what happens with a LEFT OUTER JOIN, the columns from the non-matching table are returned as NULL. In this example, the product description for that group is returned as NULL.
2025-02-03
Another request that I received a while back, for a new function in our free SDU Tools for developers and DBAs, was to be able to find someone’s age in months. The same would apply to anything where the distance between two dates needs to be measured in months. In response, we added a new function CalculateAgeInMonths.
It takes two parameters:
@StartingDate date - the date to calculate from (could be a birth date if it’s an age) @CalculationDate date - the date to calculate the age to
2025-02-02
The next type of join that I want to discuss in these posts is a left outer join. We use this mostly, when we know that not all rows are matching.
For example, in the query shown above, I wanted to add up the ordered quantity values for all products. The problem is that if I used an INNER JOIN, I would only get products that match at least one order line.
2025-02-02
It was great to catch up with Joey D’Antoni today and to have him on a SQL Down Under podcast.
Joey D’Antoni is an Architect and SQL Server MVP with over two decades of experience, working in both Fortune 500 and smaller firms. He holds a BS in Computer Information Systems from Louisiana Tech University and an MBA from North Carolina State University. He is a Microsoft Data Platform MVP and VMware vExpert.
2025-02-01
There are two ways you can write common join syntax. There’s an older way to write the query and a new way to write the query.
In the image above, you can see the new way written first, and then the old way written below. The newer syntax was introduced as part of ANSI SQL, back in 1992. It has been recommended for over 30 years now.
When you look at the older syntax, you’ll notice that I’m doing two things in the WHERE clause. I’m both determining how the tables are joined, and I’m also filtering the rows that I want.
2025-02-01
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:
User1 has CONNECT permission to a SQL Server and is a member of the db_datawriter role in the database Database1.
User2 has CONNECT permission to the same SQL Server and is a member of the db_datareader role in the database Database1.
2025-01-31
The most common form of join between two tables is what’s called an inner join.
In the example shown above, I’ve selected some columns from different tables. I’ve got description and product ID from the product table and I’ve got the product group name from the product group. What I wanted was a list of products but I want to know what group they’re part of. So to do that I had to use the products table, but I also had to join to the product groups table so I could get the name of the product group.
2025-01-31
A foreign key is a situation where we store the primary key of one table in another table. It’s very useful. For example, we might need to know which sales transactions were for which customers.
Again, keep in mind that like primary keys, there could be more than one column in a key. While a foreign key can reference a unique key, it’s almost always the primary key.
In the example shown above, the column OrderID is the primary key of the Orders table. It’s highlighted in red. The column CinemaID is the primary key of the Cinemas table, but it has been stored in the Orders table. It’s highlighted in green. We refer to that as a foreign key. Clearly in a system that handles orders for cinemas, we’d need to know which cinema each order is for.
2025-01-30
