Many languages have few enough characters to be represented in a single-byte character set. In such a character set, each character is represented by a single byte: a two-digit hexadecimal number.

At most, 256 characters can be represented in a single byte. No single-byte character set can hold all of the characters used internationally, including accented characters. This problem was addressed by the development of a set of code pages, each of which describes a set of characters appropriate for one or more national languages. For example, code page 1253 contains the Greek character set, and code page 1252 contains Western European languages. There are many code pages, and many names for code pages. The above examples are code pages for Windows.

Upper and lower pages

With few exceptions, characters 0 to 127 are the same for all of the code pages. The mapping for this range of characters is called the ASCII character set. It includes the English language alphabet in upper and lowercase, as well as common punctuation symbols and the digits. This range is often called the seven-bit range (because only seven bits are needed to represent the numbers up to 127) or the lower page. The characters from 128 to 255 are called extended characters, or upper code page characters, and vary from one code page to another.

Problems with code page compatibility are rare if the only characters used are from the English alphabet, as these are represented in the ASCII portion of each code page (0 to 127). However, if other characters are used, as is generally the case in any non-English environment, there can be problems if the database and the application use different code pages.

For example, suppose a database using the UTF-8 character set loads a table from a file containing cp1252 data, and the encoding is not specified as cp1252 on the LOAD TABLE statement. Because the encoding is not specified, the data is assumed to be encoded in UTF-8, so no character conversion takes place; the cp1252 encoding is stored directly in the database. This means that characters such as the euro symbol, represented in cp1252 as hex 80, are not converted into UTF-8. The euro symbol in UTF-8 is represented by the three-byte sequence E2 82 AC, but, in this case, will be stored in the database as 80. Subsequently, when an application requests data, the database server attempts to convert the data from UTF-8 to the client character set. The conversion will produce corrupted characters.