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Foundations of Querying Microsoft SQL Server 2012

Lesson 1: Understanding the Foundations of T-SQL

Many aspects of computing, like programming languages, evolve based on intuition and the current trend. Without strong foundations, their lifespan can be very short, and if they do survive, often the changes are very rapid due to changes in trends. T-SQL is different, mainly because it has strong foundations—mathematics. You don’t need to be a mathematician to write good SQL (though it certainly doesn’t hurt), but as long as you understand what those foundations are, and some of their key principles, you will better understand the language you are dealing with. Without those foundations, you can still write T-SQL code—even code that runs successfully—but it will be like eating soup with a fork!

Evolution of T-SQL

As mentioned, unlike many other aspects of computing, T-SQL is based on strong mathematical foundations. Understanding some of the key principals from those foundations can help you better understand the language you are dealing with. Then you will think in T-SQL terms when coding in T-SQL, as opposed to coding with T-SQL while thinking in procedural terms.

Figure 1-1 illustrates the evolution of T-SQL from its core mathematical foundations.

Figure 1-1

Figure 1-1 Evolution of T-SQL.

T-SQL is the main language used to manage and manipulate data in Microsoft’s main relational database management system (RDBMS), SQL Server—whether on premises or in the cloud (Microsoft Windows Azure SQL Database). SQL Server also supports other languages, like Microsoft Visual C# and Microsoft Visual Basic, but T-SQL is usually the preferred language for data management and manipulation.

T-SQL is a dialect of standard SQL. SQL is a standard of both the International Organization for Standards (ISO) and the American National Standards Institute (ANSI). The two standards for SQL are basically the same. The SQL standard keeps evolving with time. Following is a list of the major revisions of the standard so far:

  • SQL-86

  • SQL-89

  • SQL-92

  • SQL:1999

  • SQL:2003

  • SQL:2006

  • SQL:2008

  • SQL:2011

All leading database vendors, including Microsoft, implement a dialect of SQL as the main language to manage and manipulate data in their database platforms. Therefore, the core language elements look the same. However, each vendor decides which features to implement and which not to. Also, the standard sometimes leaves some aspects as an implementation choice. Each vendor also usually implements extensions to the standard in cases where the vendor feels that an important feature isn’t covered by the standard.

Writing in a standard way is considered a best practice. When you do so, your code is more portable. Your knowledge is more portable, too, because it is easy for you to start working with new platforms. When the dialect you’re working with supports both a standard and a nonstandard way to do something, you should always prefer the standard form as your default choice. You should consider a nonstandard option only when it has some important benefit to you that is not covered by the standard alternative.

As an example of when to choose the standard form, T-SQL supports two “not equal to” operators: <> and !=. The former is standard and the latter is not. This case should be a no-brainer: go for the standard one!

As an example of when the choice of standard or nonstandard depends on the circumstances, consider the following: T-SQL supports multiple functions that convert a source value to a target type. Among them are the CAST and CONVERT functions. The former is standard and the latter isn’t. The nonstandard CONVERT function has a style argument that CAST doesn’t support. Because CAST is standard, you should consider it your default choice for conversions. You should consider using CONVERT only when you need to rely on the style argument.

Yet another example of choosing the standard form is in the termination of T-SQL statements. According to standard SQL, you should terminate your statements with a semicolon. T-SQL currently doesn’t make this a requirement for all statements, only in cases where there would otherwise be ambiguity of code elements, such as in the WITH clause of a common table expression (CTE). You should still follow the standard and terminate all of your statements even where it is currently not required.

Standard SQL is based on the relational model, which is a mathematical model for data management and manipulation. The relational model was initially created and proposed by Edgar F. Codd in 1969. Since then, it has been explained and developed by Chris Date, Hugh Darwen, and others.

A common misconception is that the name “relational” has to do with relationships between tables (that is, foreign keys). Actually, the true source for the model’s name is the mathematical concept relation.

A relation in the relational model is what SQL calls a table. The two are not synonymous. You could say that a table is an attempt by SQL to represent a relation (in addition to a relation variable, but that’s not necessary to get into here). Some might say that it is not a very successful attempt. Even though SQL is based on the relational model, it deviates from it in a number of ways. But it’s important to note that as you understand the model’s principles, you can use SQL—or more precisely, the dialect you are using—in a relational way. More on this, including a further reading recommendation, is in the next section, Using T-SQL in a Relational Way

Getting back to a relation, which is what SQL attempts to represent with a table: a relation has a heading and a body. The heading is a set of attributes (what SQL attempts to represent with columns), each of a given type. An attribute is identified by name and type name. The body is a set of tuples (what SQL attempts to represent with rows). Each tuple’s heading is the heading of the relation. Each value of each tuple’s attribute is of its respective type.

Some of the most important principals to understand about T-SQL stem from the relational model’s core foundations—set theory and predicate logic.

Remember that the heading of a relation is a set of attributes, and the body a set of tuples. So what is a set? According to the creator of mathematical set theory, Georg Cantor, a set is described as follows:

By a “set” we mean any collection M into a whole of definite, distinct objects m (which are called the “elements” of M) of our perception or of our thought.


There are a number of very important principles in this definition that, if understood, should have direct implications on your T-SQL coding practices. For one, notice the term whole. A set should be considered as a whole. This means that you do not interact with the individual elements of the set, rather with the set as a whole.

Notice the term distinct—a set has no duplicates. Codd once remarked on the no duplicates aspect: “If something is true, then saying it twice won’t make it any truer.” For example, the set {a, b, c} is considered equal to the set {a, a, b, c, c, c}.

Another critical aspect of a set doesn’t explicitly appear in the aforementioned definition by Cantor, but rather is implied—there’s no relevance to the order of elements in a set. In contrast, a sequence (which is an ordered set), for example, does have an order to its elements. Combining the no duplicates and no relevance to order aspects means that the set {a, b, c} is equal to the set {b, a, c, c, a, c}.

The other branch of mathematics that the relational model is based on is called predicate logic. A predicate is an expression that when attributed to some object, makes a proposition either true or false. For example, “salary greater than $50,000” is a predicate. You can evaluate this predicate for a specific employee, in which case you have a proposition. For example, suppose that for a particular employee, the salary is $60,000. When you evaluate the proposition for that employee, you get a true proposition. In other words, a predicate is a parameterized proposition.

The relational model uses predicates as one of its core elements. You can enforce data integrity by using predicates. You can filter data by using predicates. You can even use predicates to define the data model itself. You first identify propositions that need to be stored in the database. Here’s an example proposition: an order with order ID 10248 was placed on February 12, 2012 by the customer with ID 7, and handled by the employee with ID 3. You then create predicates from the propositions by removing the data and keeping the heading. Remember, the heading is a set of attributes, each identified by name and type name. In this example, you have orderid INT, orderdate DATE, custid INT, and empid INT.

Using T-SQL in a Relational Way

As mentioned in the previous section, T-SQL is based on SQL, which in turn is based on the relational model. However, there are a number of ways in which SQL—and therefore, T-SQL—deviates from the relational model. But the language gives you enough tools so that if you understand the relational model, you can use the language in a relational manner, and thus write more-correct code.

Remember that a relation has a heading and a body. The heading is a set of attributes and the body is a set of tuples. Remember from the definition of a set that a set is supposed to be considered as a whole. What this translates to in T-SQL is that you’re supposed to write queries that interact with the tables as a whole. You should try to avoid using iterative constructs like cursors and loops that iterate through the rows one at a time. You should also try to avoid thinking in iterative terms because this kind of thinking is what leads to iterative solutions.

For people with a procedural programming background, the natural way to interact with data (in a file, record set, or data reader) is with iterations. So using cursors and other iterative constructs in T-SQL is, in a way, an extension to what they already know. However, the correct way from the relational model’s perspective is not to interact with the rows one at a time; rather, use relational operations and return a relational result. This, in T-SQL, translates to writing queries.

Remember also that a set has no duplicates. T-SQL doesn’t always enforce this rule. For example, you can create a table without a key. In such a case, you are allowed to have duplicate rows in the table. To follow relational theory, you need to enforce uniqueness in your tables—for example, by using a primary key or a unique constraint.

Even when the table doesn’t allow duplicate rows, a query against the table can still return duplicate rows in its result. You’ll find further discussion about duplicates in subsequent chapters, but here is an example for illustration purposes. Consider the following query.


SELECT country
FROM HR.Employees;

The query is issued against the TSQL2012 sample database. It returns the country attribute of the employees stored in the HR.Employees table. According to the relational model, a relational operation against a relation is supposed to return a relation. In this case, this should translate to returning the set of countries where there are employees, with an emphasis on set, as in no duplicates. However, T-SQL doesn’t attempt to remove duplicates by default.

Here’s the output of this query.


In fact, T-SQL is based more on multiset theory than on set theory. A multiset (also known as a bag or a superset) in many respects is similar to a set, but can have duplicates. As mentioned, the T-SQL language does give you enough tools so that if you want to follow relational theory, you can do so. For example, the language provides you with a DISTINCT clause to remove duplicates. Here’s the revised query.

FROM HR.Employees;

Here’s the revised query’s output.


Another fundamental aspect of a set is that there’s no relevance to the order of the elements. For this reason, rows in a table have no particular order, conceptually. So when you issue a query against a table and don’t indicate explicitly that you want to return the rows in particular presentation order, the result is supposed to be relational. Therefore, you shouldn’t assume any specific order to the rows in the result, never mind what you know about the physical representation of the data, for example, when the data is indexed.

As an example, consider the following query.

SELECT empid, lastname
FROM HR.Employees;

When this query was run on one system, it returned the following output, which looks like it is sorted by the column lastname.

empid  lastname
------ -------------
5      Buck
8      Cameron
1      Davis
9      Dolgopyatova
2      Funk
7      King
3      Lew
4      Peled
6      Suurs

Even if the rows were returned in a different order, the result would have still been considered correct. SQL Server can choose between different physical access methods to process the query, knowing that it doesn’t need to guarantee the order in the result. For example, SQL Server could decide to parallelize the query or scan the data in file order (as opposed to index order).

If you do need to guarantee a specific presentation order to the rows in the result, you need to add an ORDER BY clause to the query, as follows.

SELECT empid, lastname
FROM HR.Employees
ORDER BY empid;

This time, the result isn’t relational—it’s what standard SQL calls a cursor. The order of the rows in the output is guaranteed based on the empid attribute. Here’s the output of this query.

empid  lastname
------ -------------
1      Davis
2      Funk
3      Lew
4      Peled
5      Buck
6      Suurs
7      King
8      Cameron
9      Dolgopyatova

The heading of a relation is a set of attributes that are supposed to be identified by name and type name. There’s no order to the attributes. Conversely, T-SQL does keep track of ordinal positions of columns based on their order of appearance in the table definition. When you issue a query with SELECT *, you are guaranteed to get the columns in the result based on definition order. Also, T-SQL allows referring to ordinal positions of columns from the result in the ORDER BY clause, as follows.

SELECT empid, lastname
FROM HR.Employees

Beyond the fact that this practice is not relational, think about the potential for error if at some point you change the SELECT list and forget to change the ORDER BY list accordingly. Therefore, the recommendation is to always indicate the names of the attributes that you need to order by.

T-SQL has another deviation from the relational model in that it allows defining result columns based on an expression without assigning a name to the target column. For example, the following query is valid in T-SQL.

SELECT empid, firstname + ' ' + lastname
FROM HR.Employees;

This query generates the following output.

------ ------------------
1      Sara Davis
2      Don Funk
3      Judy Lew
4      Yael Peled
5      Sven Buck
6      Paul Suurs
7      Russell King
8      Maria Cameron
9      Zoya Dolgopyatova

But according to the relational model, all attributes must have names. In order for the query to be relational, you need to assign an alias to the target attribute. You can do so by using the AS clause, as follows.

SELECT empid, firstname + ' ' + lastname AS fullname
FROM HR.Employees;

Also, T-SQL allows a query to return multiple result columns with the same name. For example, consider a join between two tables, T1 and T2, both with a column called keycol. T-SQL allows a SELECT list that looks like the following.

SELECT T1.keycol, T2.keycol ...

For the result to be relational, all attributes must have unique names, so you would need to use different aliases for the result attributes, as in the following.

SELECT T1.keycol AS key1, T2.keycol AS key2 ...

As for predicates, following the law of excluded middle in mathematical logic, a predicate can evaluate to true or false. In other words, predicates are supposed to use two-valued logic. However, Codd wanted to reflect the possibility for values to be missing in his model. He referred to two kinds of missing values: missing but applicable and missing but inapplicable. Take a mobilephone attribute of an employee as an example. A missing but applicable value would be if an employee has a mobile phone but did not want to provide this information, for example, for privacy reasons. A missing but inapplicable value would be when the employee simply doesn’t have a mobile phone. According to Codd, a language based on his model should provide two different marks for the two cases. T-SQL—again, based on standard SQL—implements only one general purpose mark called NULL for any kind of missing value. This leads to three-valued predicate logic. Namely, when a predicate compares two values, for example, mobilephone = ‘(425) 555-0136’, if both are present, the result evaluates to either true or false. But if one of them is NULL, the result evaluates to a third logical value—unknown.

Note that some believe that a valid relational model should follow two-valued logic, and strongly object to the concept of NULLs in SQL. But as mentioned, the creator of the relational model believed in the idea of supporting missing values, and predicates that extend beyond two-valued logic. What’s important from a perspective of coding with T-SQL is to realize that if the database you are querying supports NULLs, their treatment is far from being trivial. That is, you need to carefully understand what happens when NULLs are involved in the data you’re manipulating with various query constructs, like filtering, sorting, grouping, joining, or intersecting. Hence, with every piece of code you write with T-SQL, you want to ask yourself whether NULLs are possible in the data you’re interacting with. If the answer is yes, you want to make sure that you understand the treatment of NULLs in your query, and ensure that your tests address treatment of NULLs specifically.

Using Correct Terminology

Your use of terminology reflects on your knowledge. Therefore, you should make an effort to understand and use correct terminology. When discussing T-SQL–related topics, people often use incorrect terms. And if that’s not enough, even when you do realize what the correct terms are, you also need to understand the differences between the terms in T-SQL and those in the relational model.

As an example of incorrect terms in T-SQL, people often use the terms “field” and “record” to refer to what T-SQL calls “column” and “row,” respectively. Fields and records are physical. Fields are what you have in user interfaces in client applications, and records are what you have in files and cursors. Tables are logical, and they have logical rows and columns.

Another example of an incorrect term is referring to “NULL values.” A NULL is a mark for a missing value—not a value itself. Hence, the correct usage of the term is either “NULL mark” or just “NULL.”

Besides using correct T-SQL terminology, it’s also important to understand the differences between T-SQL terms and their relational counterparts. Remember from the previous section that T-SQL attempts to represent a relation with a table, a tuple with a row, and an attribute with a column; but the T-SQL concepts and their relational counterparts differ in a number of ways. As long as you are conscious of those differences, you can, and should, strive to use T-SQL in a relational way.

Practice: Using T-SQL in a Relational Way

In this practice, you exercise your knowledge of using T-SQL in a relational way.

If you encounter a problem completing an exercise, you can install the completed projects from the companion content for this chapter and lesson.

EXERCISE 1 Identify Nonrelational Elements in a Query

In this exercise, you are given a query. Your task is to identify the nonrelational elements in the query.

  1. Open SQL Server management Studio (SSMS) and connect to the sample database TSQL2012. (See the book’s introduction for instructions on how to create the sample database and how to work with SSMS.)

  2. Type the following query in the query window and execute it.

    SELECT custid, YEAR(orderdate)
    FROM Sales.Orders
    ORDER BY 1, 2;

    You get the following output shown here in abbreviated form.

    ----------- -----------
    1           2007
    1           2007
    1           2007
    1           2008
    1           2008
    1           2008
    2           2006
    2           2007
    2           2007
    2           2008
  3. Review the code and its output. The query is supposed to return for each customer and order year the customer ID (custid) and order year (YEAR(orderdate)). Note that there’s no presentation ordering requirement from the query. Can you identify what the nonrelational aspects of the query are?

    Answer: The query doesn’t alias the expression YEAR(orderdate), so there’s no name for the result attribute. The query can return duplicates. The query forces certain presentation ordering to the result and uses ordinal positions in the ORDER BY clause.

EXERCISE 2 Make the Nonrelational Query Relational

In this exercise, you work with the query provided in Exercise 1 as your starting point. After you identify the nonrelational elements in the query, you need to apply the appropriate revisions to make it relational.

  • In step 3 of Exercise 1, you identified the nonrelational elements in the last query. Apply revisions to the query to make it relational.

    A number of revisions are required to make the query relational.

    • Define an attribute name by assigning an alias to the expression YEAR(orderdate).

    • Add a DISTINCT clause to remove duplicates.

    • Also, remove the ORDER BY clause to return a relational result.

    • Even if there was a presentation ordering requirement (not in this case), you should not use ordinal positions; instead, use attribute names. Your code should look like the following.

      SELECT DISTINCT custid, YEAR(orderdate) AS orderyear
      FROM Sales.Orders;

Lesson Summary

  • T-SQL is based on strong mathematical foundations. It is based on standard SQL, which in turn is based on the relational model, which in turn is based on set theory and predicate logic.

  • It is important to understand the relational model and apply its principals when writing T-SQL code.

  • When describing concepts in T-SQL, you should use correct terminology because it reflects on your knowledge.

Lesson Review

Answer the following questions to test your knowledge of the information in this lesson. You can find the answers to these questions and explanations of why each answer choice is correct or incorrect in the Answers section at the end of this chapter.

  1. Why is it important to use standard SQL code when possible and know what is standard and what isn’t? (Choose all that apply.)

    1. It is not important to code using standard SQL.

    2. Standard SQL code is more portable between platforms.

    3. Standard SQL code is more efficient.

    4. Knowing what standard SQL code is makes your knowledge more portable.

  2. Which of the following is not a violation of the relational model?

    1. Using ordinal positions for columns

    2. Returning duplicate rows

    3. Not defining a key in a table

    4. Ensuring that all attributes in the result of a query have names

  3. What is the relationship between SQL and T-SQL?

    1. T-SQL is the standard language and SQL is the dialect in Microsoft SQL Server.

    2. SQL is the standard language and T-SQL is the dialect in Microsoft SQL Server.

    3. Both SQL and T-SQL are standard languages.

    4. Both SQL and T-SQL are dialects in Microsoft SQL Server.