A table (or relation) is an ordinary ASCII file, with some additional rules that make it possible to use it as a database table. The file has records (rows) and fields (columns). The relation, or table structure is achieved by separating the columns with ASCII TAB characters, and terminating the rows with ASCII NEWLINE characters. That is, each row of data in a file contains the data values (a data field) separated by TAB characters and terminated with a NEWLINE character. Therefore a fundamental rule is that data values must NOT contain TAB characters.
The first section of the file, called the header, contains the file structure information used by the operators. The rest of the file, called the body, contains the actual data values. A file of data, so structured, is said to be a 'table'.
The header consists of exactly two lines that contain the structure information: the column name row and the dashline. The fields in the column name row contain the names of each column, and are separated from each other by a single TAB character. The dashline is a set of dashed lines, one set for each column, separated by single TAB characters. The dashline signals the start of the actual data rows and its sole purpose is to make the header visually easy to find.
The column names are case sensitive, i.e. 'COUNT' is different from 'Count'. The guideline for characters that may be used in column names is that alphabetic, numeric, and the underscore (_) are good choices. Numeric-only column names are not allowed. No rows, except the dashline, should contain only dashes and TABs.
The TAB character must never be used in column names, nor should spaces or UNIX I/O redirection characters (<,>,|) be used. To be on the safe side, column names should always start with a letter and contain only upper and lower case letters, numbers and the underscore (_). The following names are reserved to the awk programming language, and should not be used to indicate column names:
BEGIN, END, break, continue, else, exit, exp, for, getline, gsub, if, in, index, int, length, log, next, print, printf, split, sprintf, sqrt, sub, substr, while, and possibly others. Refer to the mawk(1) man page. Furthermore, the '_nosql_' prefix is reserved for NoSQL internal use, and should never be used at the beginning of column names.
For instance, suppose you have a table that maps names to nicknames, then its two columns could be called Name and Nickname. Some NoSQL operators create new columns that have the same name as pre-existing table columns, with lower-case letters prepended to them. This is why you really should stick to these rules.
Not abiding by these naming rules may still work, but there may be unexpected results.
A sample table (named SAMPLE) that will be used in later examples is shown in Table 1. The picture in Table 1 is for illustrative purposes; what the file would actually look like is shown in Table 2, where a TAB character is represented by '<T>' and a NEWLINE character is represented by '<N>'.
Table 1
table (SAMPLE)
NAME COUNT TYP AMT
---- ----- --- ---
Bush 44 A 133
Hansen 44 A 23
Jones 77 X 77
Perry 77 B 244
Hart 77 D 1111
Holmes 65 D 1111
Table 2
table (SAMPLE) actual content
NAME<T>COUNT<T>TYP<T>AMT<N>
----<T>-----<T>---<T>---<N>
Bush<T>44<T>A<T>133<N>
Hansen<T>44<T>A<T>23<N>
Jones<T>77<T>X<T>77<N>
Perry<T>77<T>B<T>244<N>
Hart<T>77<T>D<T>1111<N>
Holmes<T>65<T>D<T>1111<N>
It is important to note that only actual data is stored in the data fields, with no leading or trailing space characters. This fact can (and usually does) have a major effect on the size of the resulting datafiles (tables) compared to data stored in "fixed field width" systems. The datafiles in NoSQL are almost always smaller, sometimes dramatically smaller.
A table can also be represented in a different format, called 'list format'. The list format of the above SAMPLE table is:
NAME Bush
COUNT 44
TYP A
AMT 133
NAME Hansen
COUNT 44
TYP A
AMT 23
NAME Jones
COUNT 77
TYP X
AMT 77
NAME Perry
COUNT 77
TYP B
AMT 244
NAME Hart
COUNT 77
TYP D
AMT 1111
NAME Holmes
COUNT 65
TYP D
AMT 1111
The actual contents of a table in 'list' format, showing newlines and TABs is:
<N>
NAME<T>Bush<N>
COUNT<T>44<N>
TYP<T>A<N>
AMT<T>133<N>
<N>
NAME<T>Hansen<N>
COUNT<T>44<N>
TYP<T>A<N>
AMT<T>23<N>
<N>
NAME<T>Jones<N>
COUNT<T>77<N>
TYP<T>X<N>
AMT<T>77<N>
<N>
NAME<T>Perry<N>
COUNT<T>77<N>
TYP<T>B<N>
AMT<T>244<N>
<N>
NAME<T>Hart<N>
COUNT<T>77<N>
TYP<T>D<N>
AMT<T>1111<N>
<N>
NAME<T>Holmes<N>
COUNT<T>65<N>
TYP<T>D<N>
AMT<T>1111<N>
<N>
Long lines, i.e. lines that are too long to fit in the width of the screen, may be folded over multiple rows in the 'list' format, provided that each continuation row starts with one or more spaces (blanks, not TABs). Field (column) names need to be separated by the associated data by exactly one TAB characters. The data part may contain physical TABs and newlines, which will be turned into '\t' and '\n' escapes respectively by the 'listtotable' operator when the list is turned into a table.
COMMENTS This is a very looong comment, that I want to fold over
multiple lines.
and the actual content is :
<N>
COMMENTS<T>This is a very looong comment, that I want to fold over<N>
<T>multiple lines.<N>
<N>
As we will see, there are NoSQL operators that convert back and forth between 'table' and 'list' formats.
It is suggested, though not required, that table file names be given the filename extension '.rdb', to make them recognizable right away.
Unlike most other database systems, NoSQL knows nothing about data-types. Everything is just a string, that occurs between one TAB character and the next one. This was done on purpose, of course, as NoSQL tables can be accessed in a number of ways, even directly with a text editor. NoSQL has no way of enforcing any data-typing that we may possibly establish, so why bother about types at all. This model goes well with the plethora of text utilities that come with most Unices, and with Linux in particular, and is a very natural way of representing data, more on the human-level than other conventions. The drawback is that it is up to the application to enforce datatypes if necessary.
As I have already pointed-out, NoSQL should be seen just as a simple data-dictionary toolkit. Its main purpose is to attach names to slices of an otherwise flat-data file. Having a dictionary means that you can reference individual pieces of data by name rather than by their physical position in the file, thus attaining a basic level of information abstraction.
A table column can contain anything except physical tabs and newlines. The data itself can be anything that is considered to be text according to the local character-set (mine is iso-8859-1, or Latin1). A field can even contain an entire text-encoded file (a BLOB). Common encodings are uuencode, base64 and quoted-printable. Large fields may of course break AWK or the other utilities, but that must be seen as a limitation in those programs or in the operating system, not something pertaining to the paradigm.
A valid NoSQL table needs always to contain the header. Keeping the latter in a separate file is possible but strongly deprecated.
Table editing/writing/locking/unlocking/versioning should not be seen as core NoSQL features, but simply add-on facilities. In real applications the locking policy may become quite complicated and should be provided by the application program itself, according to its needs. The same is true for modifying/versioning a table and ensuring overall database consistency.
The structure of a NoSQL table is record-oriented, so that it can easily be acted upon with the wealth of existing Unix utilities, which are mostly record-oriented. This does not mean that a table cannot map a more complicated structure, like an XML document or any other hierarchical tree-like structure. Such higher-order dictionaries will not pertain to the paradigm though, but rather to the application that uses the table.
Besides NoSQL and RDB there are other UNIX DBMS's, both commercial and free, that are based on ASCII tables. A commercial implementation is /rdb, by Revolutionary Software, while among the free ones there are Starbase, developed at the Harvard Smithsonian Astrophysical Observatory, and Gunnar Stefansson's reldb, a collection of interesting tools available at sites that carry archives of the comp.sources.unix Usenet newsgroup.
The ASCII table format of those database engines is very close to that of NoSQL, therefore data can easily be converted back and forth between them and NoSQL.