Sql-Server

Deleting rows from a table can take quite a long time because there is a lot of work going on under the covers. One way that you can just completely empty a table is by executing a TRUNCATE TABLE statement.

While this is fast, there are two issues with it:

First, if the table had delete triggers, and I used a DELETE statement, those triggers would fire. But if I say TRUNCATE TABLE instead, it just quickly nukes the entire contents of the table, by deallocating all the storage used for table rows. But those triggers wouldn’t fire.

2025-04-05

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: Administration Level: Medium

Question:

You query the sys.databases view and notice that the value in the physical_database_name column for a database is different to the name column.

Can you give an example of why this might occur?

2025-04-04

In a previous post, I did an INSERT followed by a SELECT. When I do that, the table needs to already exist.

But what if I want to take the rows returned from a SELECT statement and use them to create a new table?

That’s what a SELECT INTO statement does. In the example above, I’ve taken a distinct list of OrderID and OrderContact values and used them to create a new dbo.OrderContacts table. If the table already exists, the statement will fail.

2025-04-03

Another way you can get rows to insert into a table is by executing a stored procedure or, in fact, even by executing dynamic SQL. Either way, these both use an EXEC statement.

In the example shown, I’m doing an INSERT into the dbo.CinemaGroups table. I’ve declared the two columns that are being inserted. The stored procedure returns the rows that I want to insert.

This is again fairly easy to troubleshoot because the EXEC clause can usually be executed by itself, and you can see the rows coming back.

2025-04-01

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 thing I always check for is the use of data types, including when they’ve been used in views. So we added a tool that can help to find these. It’s called ListViewColumnsAndDataTypes.

The procedure takes four parameters:

@DatabaseName is the database to process (not used in the Azure SQL DB version) @SchemasToList is a comma-delimited list of schema names or ALL @ViewsToList is a comma-delimited list of view names or ALL @ColumnsToList is a comma-delimited list of column names or ALL

2025-03-31

When you’re performing an INSERT, you can also get values from a query., rather than specifying the values in the INSERT statement.

In the example shown above, I’ve queried for a distinct list of order contacts and their phone number from the dbo.Orders table. I’m then inserting those values into the dbo.OrderContacts table.

These types of statements are easy to debug because I can just highlight the SELECT clause and run it to see what comes back.

2025-03-30

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:

You have a scalar T-SQL function in SQL Server and want to change it.

Apart from permissions, what would stop you from using ALTER FUNCTION to change it?

Answer:

You can’t change from an inline function to a multi-statement function.

2025-03-29

The DELETE statement is one of the most common SQL statements. We use it to remove rows from a table.

In the example, I’ve said to delete rows from the dbo.CinemaGroups table where the value in the CinemaGroupID column is greater than 3.

You need to make sure that your DELETE statements have WHERE clauses, unless what you’re intending to do is delete all the rows in the table. You don’t want to make the common mistake where people highlight a DELETE statement in SQL Server Management Studio and forget to include the WHERE clause as well.

2025-03-28

Foreign keys are mostly used where you store the primary key of another table, and want to check that it’s valid. This is used to ensure database integrity.

For example, I might have a CustomerID stored in an Orders table, and I want to make sure that the customer already exists. In the example above, I have a Courses table and I want to make sure that the CollegeID that’s stored for each course, actually matches a college that’s already in the Colleges table. I check that the CollegeID already exists by declaring a foreign key reference to that table.

2025-03-26

In the previous post, I talked about check constraints. Another type of constraint in SQL Server is a unique constraint.

What I want to specify is that a value in a column is unique i.e., no other row has that same value in that column. While a check of the values can be manually requested, the constraint is normally checked when SQL Server is inserting or updating the row.

It is also possible to specify a unique constraint at the table level, rather than at the column level. You might want a combination of columns to be unique. Single column unique constraints can be declared at either the column or the table level.

2025-03-24