A table is a collection of related data held in a
table format within a
database
In computing, a database is an organized collection of data stored and accessed electronically. Small databases can be stored on a file system, while large databases are hosted on computer clusters or cloud storage. The design of databases spa ...
. It consists of
columns
A column or pillar in architecture and structural engineering is a structural element that transmits, through compression, the weight of the structure above to other structural elements below. In other words, a column is a compression membe ...
and
rows.
In
relational databases, and
flat file databases, a ''table'' is a set of data elements (values) using a model of vertical
columns
A column or pillar in architecture and structural engineering is a structural element that transmits, through compression, the weight of the structure above to other structural elements below. In other words, a column is a compression membe ...
(identifiable by name) and horizontal
rows, the
cell being the unit where a row and column intersect. A table has a specified number of columns, but can have any number of rows. Each row is identified by one or more values appearing in a particular column subset. A specific choice of columns which uniquely identify rows is called the
primary key.
"Table" is another term for
"relation"; although there is the difference in that a table is usually a
multiset
In mathematics, a multiset (or bag, or mset) is a modification of the concept of a set that, unlike a set, allows for multiple instances for each of its elements. The number of instances given for each element is called the multiplicity of that ...
(bag) of rows where a relation is a
set and does not allow duplicates. Besides the actual data rows, tables generally have associated with them some
metadata, such as
constraints on the table or on the values within particular columns.
The data in a table does not have to be physically stored in the database.
Views also function as relational tables, but their data are calculated at query time. External tables (in
Informix
or
Oracle,
[
]
for example) can also be thought of as views.
In many systems for computational statistics, such as
R and
Python's
pandas, a data frame or data table is a
data type supporting the table abstraction. Conceptually, it is a list of records or
observations all containing the same fields or columns. The implementation consists of a list of
arrays or vectors, each with a name.
Tables versus relations
In terms of the
relational model of databases, a table can be considered a convenient representation of a
relation, but the two are not strictly equivalent. For instance, a
SQL table can potentially contain duplicate rows, whereas a true relation cannot contain duplicate rows that we call
tuples. Similarly, representation as a table implies a particular ordering to the rows and columns, whereas a relation is explicitly unordered. However, the database system does not guarantee any ordering of the rows unless an
ORDER BY clause is specified in the
SELECT statement that queries the table.
An equally valid representation of a relation is as an ''n''-dimensional
chart, where ''n'' is the number of attributes (a table's columns). For example, a relation with two attributes and three values can be represented as a table with two columns and three rows, or as a two-dimensional graph with three points. The table and graph representations are only equivalent if the ordering of rows is not significant, and the table has no duplicate rows.
Comparisons
Hierarchical databases
In non-relational systems,
hierarchical databases, the distant counterpart of a table is a structured
file, representing the rows of a table in each row of the file and each column in a row. This structure implies that a row can have repeating information, generally in the child data segments. Data are stored in sequence of physical records.
Spreadsheets
Unlike a
spreadsheet
A spreadsheet is a computer application for computation, organization, analysis and storage of data in tabular form. Spreadsheets were developed as computerized analogs of paper accounting worksheets. The program operates on data entered in ce ...
, the
datatype of a column is ordinarily defined by the
schema describing the table. Some
SQL systems, such as
SQLite, are less strict about column datatype definitions.
See also
*
Relation (database)
*
Row (database)
*
Column (database)
*
Virtual column
*
Table (information)
A table is an arrangement of information or data, typically in rows and columns, or possibly in a more complex structure. Tables are widely used in communication, research, and data analysis. Tables appear in print media, handwritten notes, co ...
References
{{DEFAULTSORT:Table (Database)
zh-yue:關係數據庫
SQL
Data modeling
Relational database management systems