Draw entity-relationship models/diagrams in plain text

What is a entity-relationship diagram? Per Entity–relationship model - Wikipedia - en.wikipedia.org:

An entity–relationship model (or ER model) describes interrelated things of interest in a specific domain of knowledge. A basic ER model is composed of entity types (which classify the things of interest) and specifies relationships that can exist between entities (instances of those entity types).

If you are designing a database, or that you're taking over a new project, you may need ERD to better understand the system. You can do it with a pen and a piece of a paper, or you can use this ERD app to "draw" ER diagrams with plain text.

A relatively complex ERD

Interesting, you may wonder, so why not go to ERD to play around.

It has simple syntax that looks like ini, and it's about: - comments with # this is a comment - entities and relationships - labels - other global directives - fonts and colors

Entities

Entities are declared inside [ and ], it has this format:

[`Entity name`]
attribute1
attribute2

If the entity names and attributes contain spaces or other non-alphanumeric characters, quote them with backticks, simple quotes or double quotes.

Some examples:

A entity with no attributes, for example, [App]
A entity with 2 attributes:
```
[`App Table`]
Id
Name
Desc
```
Prefix attributes of the primary key with a *, and a foreign key with +
```
[`App DB Table`]
*Id
Name
Desc
+Creator
```

Relationships

Every relationship includes exactly two entities (the two entities may be the same, for self-relationships).

There 4 cardinalities:

Cardinality    Syntax
0 or 1         ?
exactly 1      1
0 or more      *
1 or more      +

If you're familiar with Regular Expressions, you may have noticed that the syntax is almost the same. For example, App *--1 Creator means one creator could create >=0 applications.

You can declare relationships anywhere in the content.

Global directives

Global special directives:

title for specifying the diagram title and formatting options, e.g. title {label: "ERD demo"}
entity for specifying options for all entities, e.g. entity {bgcolor: "#ececfc", size: "20"}
relationship for all relationships

Global options are overwritten by local options.

Formating options

label, label for a diagram/entity/attribute
color, foreground color
bgcolor, background color
size, font size
border-color, border color
border, border size

Colors are in hexadecimal notation, such as, #ff2233, and they can also be english names like red/black.

An complete example

This example comes from https://github.com/BurntSushi/erd

title {label: "nfldb Entity-Relationship diagram (condensed)", size: "20"}

# Entities

[player] {bgcolor: "#d0e0d0"}
  *player_id {label: "varchar, not null"}
  full_name {label: "varchar, null"}
  team {label: "varchar, not null"}
  position {label: "player_pos, not null"}
  status {label: "player_status, not null"}

[team] {bgcolor: "#d0e0d0"}
  *team_id {label: "varchar, not null"}
  city {label: "varchar, not null"}
  name {label: "varchar, not null"}

[game] {bgcolor: "#ececfc"}
  *gsis_id {label: "gameid, not null"}
  start_time {label: "utctime, not null"}
  week {label: "usmallint, not null"}
  season_year {label: "usmallint, not null"}
  season_type {label: "season_phase, not null"}
  finished {label: "boolean, not null"}
  home_team {label: "varchar, not null"}
  home_score {label: "usmallint, not null"}
  away_team {label: "varchar, not null"}
  away_score {label: "usmallint, not null"}

[drive] {bgcolor: "#ececfc"}
  *+gsis_id {label: "gameid, not null"}
  *drive_id {label: "usmallint, not null"}
  start_field {label: "field_pos, null"}
  start_time {label: "game_time, not null"}
  end_field {label: "field_pos, null"}
  end_time {label: "game_time, not null"}
  pos_team {label: "varchar, not null"}
  pos_time {label: "pos_period, null"}

[play] {bgcolor: "#ececfc"}
  *+gsis_id {label: "gameid, not null"}
  *+drive_id {label: "usmallint, not null"}
  *play_id {label: "usmallint, not null"}
  time {label: "game_time, not null"}
  pos_team {label: "varchar, not null"}
  yardline {label: "field_pos, null"}
  down {label: "smallint, null"}
  yards_to_go {label: "smallint, null"}

[play_player] {bgcolor: "#ececfc"}
  *+gsis_id {label: "gameid, not null"}
  *+drive_id {label: "usmallint, not null"}
  *+play_id {label: "usmallint, not null"}
  *+player_id {label: "varchar, not null"}
  team {label: "varchar, not null"}

[meta] {bgcolor: "#fcecec"}
  version {label: "smallint, null"}
  season_type {label: "season_phase, null"}
  season_year {label: "usmallint, null"}
  week {label: "usmallint, null"}

# Relationships

player      *--1 team
game        *--1 team {label: "home"}
game        *--1 team {label: "away"}
drive       *--1 team
play        *--1 team
play_player *--1 team

game        1--* drive
game        1--* play
game        1--* play_player

drive       1--* play
drive       1--* play_player

play        1--* play_player

player      1--* play_player

Go to play with ERD, and have fun!