It is very difficult in today's job market to find well-qualified database developers. As a colleague of mine once said, "SQL development is a language that is easy to learn, but very difficult to master."
When I interview SQL Server database developer candidates, I use a standard set of baseline technical questions. The following questions are ones that I find can really help weed out less-qualified candidates. They are listed in order from easiest to more difficult. When you get to the question about primary and foreign keys, it starts to get challenging because the answers can be more difficult to explain and articulate, especially in an interview setting.
Can you give me an overview of some of the database objects available for use in SQL Server 2000?
You are looking for objects such as: tables, views, user-defined functions, and stored procedures; it's even better if they mention additional objects such as triggers. It's not a good sign if an applicant cannot answer this basic question.
What is an index? What types of indexes are available in SQL Server 2000?
Any experienced database developer should be able to answer this question with ease. Some of the less-experienced developers will be able to answer it, but with a little less clarity.
In its most simple terms, an index is a data structure used to provide quick access to data in a database table or view. In SQL Server, they come in two flavors: clustered and non-clustered. Clustered indexes store the data at the leaf level of the index. This means that whichever field(s) in your table are included in the clustered index, they will be stored in an orderly fashion in the table. Because of this sorting, you can only have one clustered index per table. Non-clustered indexes contain a row identifier at the leaf level of the index. This row identifier is a pointer to a location of the data on the disk. This allows you to have more than one non-clustered index per table.
What does NULL mean?
The value NULL is a very tricky subject in the database world, so don't be surprised if several applicants trip up on this question.
The value NULL means UNKNOWN; it does not mean '' (empty string). Assuming ANSI_NULLS are on in your SQL Server database, which they are by default, any comparison to the value NULL will yield the value NULL. You cannot compare any value with an UNKNOWN value and logically expect to get an answer. You must use the IS NULL operator instead.
What is a primary key? What is a foreign key?
A primary key is the field(s) in a table that uniquely defines the row in the table; the values in the primary key are always unique. A foreign key is a constraint that establishes a relationship between two tables. This relationship typically involves the primary key field(s) from one table with an adjoining set of field(s) in another table (although it could be the same table). The adjoining field(s) is the foreign key.
What are triggers? What are the different types of triggers in SQL Server 2000?
It's very beneficial for a potential database developer to know the types of triggers available, and how to implement them.
A trigger is a specialized type of stored procedure that is bound to a table or view in SQL Server 2000. In SQL Server 2000, there are INSTEAD-OF triggers and AFTER triggers. INSTEAD-OF triggers are procedures that execute in place of a Data Manipulation Language (DML) statement on a table. For example, if I have an INSTEAD-OF-UPDATE trigger on TableA, and I execute an update statement on that table, the code in the INSTEAD-OF-UPDATE trigger will execute instead of the update statement that I executed.
An AFTER trigger executes after a DML statement has taken place in the database. These types of triggers are very handy for auditing data changes that have occurred in your database tables.
How can you ensure that a table named TableB with a field named Fld1 will only have those values in the Fld1 field that are also in the table named TableA with a field named Fld1?
This relationship related question has two potential answers. The first answer (and the one that you want to hear) is the use of foreign key constraints. A foreign key constraint is used to maintain referential integrity. It is used to ensure that a field in a table will only hold values that are already defined in another field in a different (or the same) table. That field is the candidate key (usually a primary key of the other table).
The other option is the use of triggers. Triggers can be used to ensure the same effect of constraints in a roundabout way, but it is much more difficult to set up and maintain, and the performance is typically worse. Because of this, Microsoft recommends that developers use foreign key constraints instead of triggers for maintaining referential integrity.
What is a performance consideration of having too many indexes on a production online transaction processing (OLTP) table?
You are looking for the applicant to make some reference regarding data manipulations. The more indexes on a table, the more time it takes for the database engine to update, insert, or delete data, as the indexes all have to be maintained as the data manipulation occurs.
What can be used to ensure that a field in a table only accepts a certain range of values?
This question can be answered a couple of different ways, but only one answer is a "good" one. The answer you want to hear is a Check constraint, which is defined on a database table that limits the values entered into that column. These constraints are relatively easy to create, and they are the recommended type for enforcing domain integrity in SQL Server.
Triggers can also be used to restrict the values accepted in a field in a database table, but this solution requires the trigger to be defined on the table, which can hinder performance in certain situations. For this reason, Microsoft recommends Check constraints over all other methods for restricting domain integrity.
What is the difference between a return parameter and an OUTPUT parameter?
If the applicant is able to answer this question correctly, the odds are good that they have some experience working with stored procedures.
A return parameter is always returned by a stored procedure, and it is meant to indicate the success or failure of the stored procedure. The return parameter is always an INT data type.
An OUTPUT parameter is designated specifically by the developer, and it can return other types of data, such as characters and numeric values. (There are some limitations on the data types that can be used as output parameters.) You can use multiple OUTPUT parameters in a stored procedure, whereas you can only use one return parameter.
What is a correlated sub-query? How can these queries be useful?
The more seasoned developer will be able to accurately describe this type of query.
A correlated sub-query is a special type of query containing a sub-query. The sub-query contained in the query actually requests values from the outside query, creating a situation similar to a loop. You can find a more detailed description as to how these special types of queries work in this article.
Closing thoughts on the interview process
These questions are just a starting point to determining a SQL Server database developer's qualifications. Based on how well they perform in the discussion, I may ask the candidate to take my TSQL programming test, which is usually a set of 10-12 database queries based on different scenarios.
You will need to decide for yourself what the skill set should be of the developer you wish to hire. Then, you will need to use your own experience, judgment, and feel for applicants as you interview them.
What questions do you routinely ask when you interview database developer candidates? Let me know by posting to the article discussion.
Tim Chapman is a SQL Server database administrator who works for a bank in Louisville, KY, and has more than 7 years of IT experience. He is also Microsoft certified in SQL Server 2000 and SQL Server 2005. If you would like to contact Tim, please e-mail him at email@example.com.
Tim Chapman is a SQL Server MVP, a database architect, and an administrator who works as an independent consultant in Raleigh, NC, and has more than nine years of IT experience.