The smallest data item in a computer can assume the value 0 or the value 1. It’s called a bit (short for “binary digit”—a digit that can assume one of two values). Remarkably, the impressive functions performed by computers involve only the simplest manipulations of 0s and 1s—examining a bit’s value, setting a bit’s value and reversing a bit’s value (from 1 to 0 or from 0 to 1).

It’s tedious for people to work with data in the low-level form of bits. Instead, they prefer to work with decimal digits (0–9), letters (A–Z and a–z), and special symbols (e.g., $, @, %, &, *, (, ), –, +, ″, :, ? and /). Digits, letters and special symbols are known as characters. The computer’s character set is the set of all the characters used to write programs and represent data items. Computers process only 1s and 0s, so a computer’s character set represents every character as a pattern of 1s and 0s. C# supports various character sets (including Unicode^®), with some requiring more than one byte per character. Unicode supports many of the world’s languages, as well as emojis. See Appendix B for more information on the ASCII (American Standard Code for Information Interchange) character set—the popular subset of Unicode that represents uppercase and lowercase letters of the English alphabet, digits and some common special characters. We also provide an online appendix describing Unicode.

Just as characters are composed of bits, fields are composed of characters or bytes. A field is a group of characters or bytes that conveys meaning. For example, a field consisting of uppercase and lowercase letters can be used to represent a person’s name, and a field consisting of decimal digits could represent a person’s age.

Several related fields can be used to compose a record. In a payroll system, for example, the record for an employee might consist of the following fields (possible types for these fields are shown in parentheses):

• Employee or student identification number (a whole number).

• Name (a string of characters).

• Address (a string of characters).

• Hourly pay rate (a number with a decimal point).

• Year-to-date earnings (a number with a decimal point).

• Amount of taxes withheld (a number with a decimal point).

Thus, a record is a group of related fields. In the preceding example, all the fields belong to the same employee. A company might have many employees and a payroll record for each.

To facilitate the retrieval of specific records from a file, at least one field in each record is chosen as a record key, which identifies a record as belonging to a particular person or entity and distinguishes that record from all others. For example, in a payroll record, the employee identification number normally would be the record key.

A file is a group of related records. More generally, a file contains arbitrary data in arbitrary formats. In some operating systems, a file is viewed simply as a sequence of bytes—any organization of the bytes in a file, such as organizing the data into records, is a view created by the application programmer. It’s not unusual for an organization to have many files, some containing billions, or even trillions, of characters of information.

A database is a collection of data organized for easy access and manipulation. The most popular model is the relational database, in which data is stored in simple tables. A table includes records and fields. For example, a table of students might include first name, last name, major, year, student ID number and grade-point-average fields. The data for each student is a record, and the individual pieces of information in each record are the fields. You can search, sort and otherwise manipulate the data based on its relationship to multiple tables or databases. For example, a university might use data from the student database in combination with data from databases of courses, on-campus housing, meal plans, etc.

The amount of data being produced worldwide is enormous and growing quickly. According to IBM, approximately 2.5 quintillion bytes (2.5 exabytes) of data are created daily,³ and according to Salesforce.com, as of October 2015 90% of the world’s data was created in just the prior 12 months!⁴ According to an IDC study, the global data supply will reach 40 zettabytes (equal to 40 trillion gigabytes) annually by 2020.⁵ Figure 1.4 shows some common byte measurements. Big data applications deal with massive amounts of data and this field is growing quickly, creating lots of opportunity for software developers. According to a study by Gartner Group, over four million IT jobs globally were expected to support big data in 2015.⁶