Comma-separated values


A comma-separated values file is a delimited text file that uses a comma to separate values. Each line of the file is a data record. Each record consists of one or more fields, separated by commas. The use of the comma as a field separator is the source of the name for this file format. A CSV file typically stores tabular data in plain text, in which case each line will have the same number of fields.
The CSV file format is not fully standardized. The basic idea of separating fields with a comma is clear, but that idea gets complicated when the field data may also contain commas or even embedded line breaks. CSV implementations may not handle such field data, or they may use quotation marks to surround the field. Quotation does not solve everything: some fields may need embedded quotation marks, so a CSV implementation may include escape characters or escape sequences.
In addition, the term "CSV" also denotes some closely related delimiter-separated formats that use different field delimiters, for example, semicolons. These include tab-separated values and space-separated values. A delimiter that is not present in the field data keeps the format parsing simple. These alternate delimiter-separated files are often even given a.csv extension despite the use of a non-comma field separator. This loose terminology can cause problems in data exchange. Many applications that accept CSV files have options to select the delimiter character and the quotation character. Semicolons are often used in most European countries, such as Italy, France or Spain, instead of commas. Because of that, the term character-separated values is suggested as a wider definition of this file format.

Data exchange

CSV is a common data exchange format that is widely supported by consumer, business, and scientific applications. Among its most common uses is moving tabular data between programs that natively operate on incompatible formats. This works despite lack of adherence to RFC 4180, because so many programs support variations on the CSV format for data import.
For example, a user may need to transfer information from a database program that stores data in a proprietary format, to a spreadsheet that uses a completely different format. The database program most likely can export its data as "CSV"; the exported CSV file can then be imported by the spreadsheet program.

Specification

proposes a specification for the CSV format; however, actual practice often does not follow the RFC and the term "CSV" might refer to any file that:
  1. is plain text using a character set such as ASCII, various Unicode character sets, EBCDIC, or Shift JIS,
  2. consists of records,
  3. with the records divided into fields separated by delimiters,
  4. where every record has the same sequence of fields.
Within these general constraints, many variations are in use. Therefore, without additional information, a file claimed simply to be in "CSV" format is not fully specified. As a result, many applications supporting CSV files allow users to preview the first few lines of the file and then specify the delimiter character, quoting rules, etc. If a particular CSV file's variations fall outside what a particular receiving program supports, it is often feasible to examine and edit the file by hand or write a script or program to produce a conforming format.

History

Comma-separated values is a data format that pre-dates personal computers by more than a decade: the IBM Fortran compiler under OS/360 supported them in 1972. List-directed input/output was defined in FORTRAN 77, approved in 1978. List-directed input used commas or spaces for delimiters, so unquoted character strings could not contain commas or spaces.
The "comma-separated value" name and "CSV" abbreviation were in use by 1983. The manual for the Osborne Executive computer, which bundled the SuperCalc spreadsheet, documents the CSV quoting convention that allows strings to contain embedded commas, but the manual does not specify a convention for embedding quotation marks within quoted strings.
Comma-separated value lists are easier to type than fixed-column-aligned data, and were less prone to producing incorrect results if a value was punched one column off from its intended location.
Comma separated files are used for the interchange of database information between machines of two different architectures. The plain-text character of CSV files largely avoids incompatibilities such as byte-order and word size. The files are largely human-readable, so it is easier to deal with them in the absence of perfect documentation or communication.
The main standardization initiative—transforming "de facto fuzzy definition" into a more precise and de jure one—was in 2005, with RFC4180, defining CSV as a MIME Content Type. Later, in 2013, some of RFC4180's deficiencies were tackled by a W3C recommendation.
In 2014 IETF published RFC7111 describing application of URI fragments to CSV documents. RFC7111 specifies how row, column, and cell ranges can be selected from a CSV document using position indexes.
In 2015 W3C, in an attempt to enhance CSV with formal semantics, publicized the first drafts of recommendations for CSV-metadata standards, that began as recommendations in December of the same year.

General functionality

CSV formats are best used to represent sets or sequences of records in which each record has an identical list of fields. This corresponds to a single relation in a relational database, or to data in a typical spreadsheet.
The format dates back to the early days of business computing and is widely used to pass data between computers with different internal word sizes, data formatting needs, and so forth. For this reason, CSV files are common on all computer platforms.
CSV is a delimited text file that uses a comma to separate values. Simple CSV implementations may prohibit field values that contain a comma or other special characters such as newlines. More sophisticated CSV implementations permit them, often by requiring " characters around values that contain reserved characters. Embedded double quote characters may then be represented by a pair of consecutive double quotes, or by prefixing a double quote with an escape character such as a backslash.
CSV formats are not limited to a particular character set. They work just as well with Unicode character sets as with ASCII. CSV files normally will even survive naive translation from one character set to another. CSV does not, however, provide any way to indicate what character set is in use, so that must be communicated separately, or determined at the receiving end.
Databases that include multiple relations cannot be exported as a single CSV file. Similarly, CSV cannot naturally represent hierarchical or object-oriented data. This is because every CSV record is expected to have the same structure. CSV is therefore rarely appropriate for documents created with HTML, XML, or other markup or word-processing technologies.
Statistical databases in various fields often have a generally relation-like structure, but with some repeatable groups of fields. For example, health databases such as the Demographic and Health Survey typically repeat some questions for each child of a given parent. Statistical analysis systems often include utilities that can "rotate" such data; for example, a "parent" record that includes information about five children can be split into five separate records, each containing the information on one child, and a copy of all the non-child-specific information. CSV can represent either the "vertical" or "horizontal" form of such data.
In a relational database, similar issues are readily handled by creating a separate relation for each such group, and connecting "child" records to the related "parent" records using a foreign key. In markup languages such as XML, such groups are typically enclosed within a parent element and repeated as necessary. With CSV there is no widely accepted single-file solution.

Standardization

The name "CSV" indicates the use of the comma to separate data fields. Nevertheless, the term "CSV" is widely used to refer a large family of formats, which differ in many ways. Some implementations allow or require single or double quotation marks around some or all fields; and some reserve the very first record as a header containing a list of field names. The character set being used is undefined: some applications require a Unicode byte order mark to enforce Unicode interpretation. Files that use the tab character instead of comma can be more precisely referred to as "TSV" for tab-separated values.
Other implementation differences include handling of more commonplace field separators and newline characters inside text fields. One more subtlety is the interpretation of a blank line: it can equally be the result of writing a record of zero fields, or a record of one field of zero length; thus decoding it is ambiguous.

OKI frictionless tabular data package

In 2011 Open Knowledge International and various partners created a data protocols working group, which later evolved into the Frictionless Data initiative. One of the main formats they released was the Tabular Data Package. Tabular Data package was heavily based on CSV, using it as the main data transport format and adding basic type and schema metadata.
The Frictionless Data Initiative has also provided a standard CSV Dialect Description Format for describing different dialects of CSV, for example specifying the field separator or quoting rules.

Internet W3C tabular data standard

In 2013 the W3C "CSV on the Web" working group began to specify technologies providing a higher interoperability for web applications using CSV or similar formats. The working group completed its work in February 2016, and is officially closed in March 2016 with the release of a set documents and W3C recommendations
for modeling "Tabular Data", and enhancing CSV with metadata and semantics.

RFC 4180 standard

The 2005 technical standard RFC 4180 formalizes the CSV file format and defines the MIME type "text/csv" for handling of text-based fields. However, interpretation of the text of each field is still application-specific. Files that follow the RFC 4180 standard can simplify CSV exchange and should be widely portable. Among its requirements:
The format can be processed by most programs that claim to read CSV files. The exceptions are ' programs may not support line-breaks within quoted fields, ' programs may confuse the optional header with data or interpret the first data line as an optional header and double quotes in a field may not be parsed correctly automatically.

Basic rules

Many informal documents exist that describe "CSV" formats.
IETF RFC 4180 defines the format for the "text/csv" MIME type registered with the IANA.
Rules typical of these and other "CSV" specifications and implementations are as follows:

Example

The above table of data may be represented in CSV format as follows:
Year,Make,Model,Description,Price
1997,Ford,E350,"ac, abs, moon",3000.00
1999,Chevy,"Venture ""Extended Edition""","",4900.00
1999,Chevy,"Venture ""Extended Edition, Very Large""",,5000.00
1996,Jeep,Grand Cherokee,"MUST SELL!
air, moon roof, loaded",4799.00
Example of a USA/UK CSV file :
Year,Make,Model,Length
1997,Ford,E350,2.35
2000,Mercury,Cougar,2.38
Example of an analogous European CSV/DSV file :
Year;Make;Model;Length
1997;Ford;E350;2,35
2000;Mercury;Cougar;2,38
The latter format is not RFC 4180 compliant. Compliance could be achieved by the use of a comma instead of a semicolon as a separator and either the international notation for the representation of the decimal mark or the practice of quoting all numbers that have a decimal mark.

Application support

The CSV file format is supported by almost all spreadsheets and database management systems, including Apple Numbers, LibreOffice Calc, and Apache OpenOffice Calc. Microsoft Excel also supports CSV, but with restrictions in comparison to other spreadsheet software.
CSV format is supported by libraries available for many programming languages. Most provide some way to specify the field delimiter, decimal separator, character encoding, quoting conventions, date format, etc.
The emacs editor can operate on CSV files using csv-nav mode.
Many utility programs on Unix-style systems can split files on a comma delimiter, and can therefore process simple CSV files. However, this method does not correctly handle commas within quoted strings.