Introduction to relational data models

Katharina Brunner

2021-06-20

Multiple, linked tables are common within computer science. Because many R users have backgrounds in other disciplines, we present six important terms in relational data modeling to help you to jump-start working with {dm}. These terms are:

  1. Data Frames and Tables
  2. Data Model
  3. Primary Keys
  4. Foreign Keys
  5. Referential Integrity
  6. Normalization
  7. Relational Databases

1. Data Frames and Tables

A data frame is a fundamental data structure in R. Columns represent variables, rows represent observations. In more technical terms: a data frame is a list of variables of identical length and unique row names. If you imagine it visually, the result is a typical table structure. That is why working with data from spreadsheets is so convenient and the users of the popular {dplyr} package for data wrangling mainly rely on data frames.

The downside is that data frames and flat file systems like spreadsheets can result in bloated tables because they hold many repetitive values. In the worst case, a data frame can contain multiple columns with only a single value different in each row.

This calls for better data organization by utilizing the resemblance between data frames and database tables, which also consist of columns and rows. The elements are just named differently:

Data Frame Table
Column Attribute
Row Tuple

Relational databases, unlike data frames, do not keep all data in one large table but instead split it into multiple smaller tables. That separation into sub-tables has several advantages:

It is for these reasons that separation of data helps with data quality, and they explain the popularity of relational databases in production-level data management.

The downside of this approach is that it is harder to merge together information from different data sources and to identify which entities refer to the same object, a common task when modeling or plotting data.

Thus, to take full advantage of the relational database approach, an associated data model is needed to overcome the challenges that arise when working with multiple tables.

Let’s illustrate this challenge with the data from the nycflights13 dataset that contains detailed information about the 336776 flights that departed from New York City in 2013. The information is stored in five tables.

Details like the full name of an airport are not available immediately; these can only be obtained by joining or merging the constituent tables, which can result in long and inflated pipe chains full of left_join(), anti_join() and other forms of data merging.

In classical {dplyr} notation, you will need three left_join() calls to merge the flights table gradually to airlines, planes, airports and weather tables to create one wide data frame.

library(tidyverse)
library(dm)
library(nycflights13)
#> Error in library(nycflights13): there is no package called 'nycflights13'

flights %>%
  left_join(airlines, by = "carrier") %>%
  left_join(planes, by = "tailnum") %>%
  left_join(airports, by = c("origin" = "faa")) %>%
  left_join(weather, by = c("origin", "time_hour"))
#> Error in left_join(., airlines, by = "carrier"): object 'flights' not found

{dm} offers a more elegant and shorter way to combine tables while augmenting {dplyr}/{dbplyr} workflows.

It is possible to have the best of both worlds: manage your data with {dm} as linked tables, then flatten multiple tables into one for your analysis with {dplyr} on an as-needed basis.

The next step is to create a data model based on multiple tables:

2. Data Model

A data model shows the structure between multiple tables that can be linked together.

The nycflights13 relations can be transferred into the following graphical representation:

dm <- dm_nycflights13(cycle = TRUE)

dm %>%
  dm_draw()
#> Error in loadNamespace(x): there is no package called 'DiagrammeRsvg'

The flights table is linked to three other tables: airlines, planes and airports. By using directed arrows, the visualization shows explicitly the connection between different columns (they are called attributes in the relational data sphere).

For example: The column carrier in flights can be joined with the column carrier from the airlines table.

The links between the tables are established through primary keys and foreign keys.

Further Reading: The {dm} methods for visualizing data models.

3. Primary Keys

In a relational data model, each table should have one or several columns that uniquely identifies a row. These columns define the primary key (abbreviated with “pk”). If the key consists of a single column, it is called simple key. A key consisting of more than one column is called a compound key.

Example: In the airlines table of nycflights13 the column carrier is the primary key, a simple key. The weather table has the combination of origin and time_hour as primary key, a compound key.

You can get all primary keys in a dm by calling dm_get_all_pks():

dm %>%
  dm_get_all_pks()
#> # A tibble: 4 x 2
#>   table    pk_col           
#>   <chr>    <keys>           
#> 1 airlines carrier          
#> 2 airports faa              
#> 3 planes   tailnum          
#> 4 weather  origin, time_hour

dm_enum_pk_candidates() checks all columns if they are suitable as a simple primary key:

dm %>%
  dm_enum_pk_candidates(airports)
#> # A tibble: 8 x 3
#>   columns candidate why                                                    
#>   <keys>  <lgl>     <chr>                                                  
#> 1 faa     TRUE      ""                                                     
#> 2 name    TRUE      ""                                                     
#> 3 lat     TRUE      ""                                                     
#> 4 lon     TRUE      ""                                                     
#> 5 alt     FALSE     "has duplicate values: 30 (4), 13 (3), 9 (2), 19 (2), …
#> 6 tz      FALSE     "has duplicate values: -5 (48), -6 (21), -8 (12), -7 (…
#> 7 dst     FALSE     "has duplicate values: A (84), N (2)"                  
#> 8 tzone   FALSE     "has duplicate values: America/New_York (48), America/…

Further Reading: The {dm} package offers several functions for dealing with primary keys.

4. Foreign Keys

The counterpart of a primary key in one table is the foreign key in another table. In order to join two tables, the primary key of the first table needs to be referenced from the second table. This column or these columns are called the foreign key (abbreviated with “fk”).

For example, if you want to link the airlines table to the flights table, the primary key in airlines needs to match the foreign key in flights. This condition is satisfied because the column carrier is present as a primary key in the airlines table as well as a foreign key in the flights table. In the case of compound keys, the origin and time_hour columns (which form the primary key of the weather table) are also present in the flights table.

You can find foreign key candidates for simple keys with the function dm_enum_fk_candidates(), they are marked with TRUE in the candidate column.

dm %>%
  dm_enum_fk_candidates(flights, airlines)
#> # A tibble: 19 x 3
#>    columns      candidate why                                              
#>    <keys>       <lgl>     <chr>                                            
#>  1 carrier      TRUE      ""                                               
#>  2 year         FALSE     "Can't combine `value1` <integer> and `value1` <…
#>  3 month        FALSE     "Can't combine `value1` <integer> and `value1` <…
#>  4 day          FALSE     "Can't combine `value1` <integer> and `value1` <…
#>  5 dep_time     FALSE     "Can't combine `value1` <integer> and `value1` <…
#>  6 sched_dep_t… FALSE     "Can't combine `value1` <integer> and `value1` <…
#>  7 dep_delay    FALSE     "Can't combine `value1` <double> and `value1` <c…
#>  8 arr_time     FALSE     "Can't combine `value1` <integer> and `value1` <…
#>  9 sched_arr_t… FALSE     "Can't combine `value1` <integer> and `value1` <…
#> 10 arr_delay    FALSE     "Can't combine `value1` <double> and `value1` <c…
#> 11 flight       FALSE     "Can't combine `value1` <integer> and `value1` <…
#> 12 tailnum      FALSE     "values of `flights$tailnum` not in `airlines$ca…
#> 13 origin       FALSE     "values of `flights$origin` not in `airlines$car…
#> 14 dest         FALSE     "values of `flights$dest` not in `airlines$carri…
#> 15 air_time     FALSE     "Can't combine `value1` <double> and `value1` <c…
#> 16 distance     FALSE     "Can't combine `value1` <double> and `value1` <c…
#> 17 hour         FALSE     "Can't combine `value1` <double> and `value1` <c…
#> 18 minute       FALSE     "Can't combine `value1` <double> and `value1` <c…
#> 19 time_hour    FALSE     "Can't combine `value1` <datetime<America/New_Yo…

Further Reading: All {dm} functions for working with foreign keys.

5. Referential Integrity

A data set has referential integrity if all relations between tables are valid. That is, every foreign key holds a primary key that is present in the parent table. If a foreign key contains a reference where the corresponding row in the parent table is not availabe, that row is an orphan row and the database no longer has referential integrity.

{dm} allows checking referential integrity with the dm_examine_constraints() function. The following conditions are checked:

In the example data model, for a substantial share of the flights, detailed information for the corresponding airplane is not available:

dm %>%
  dm_examine_constraints()
#> ! Unsatisfied constraints:
#>  Table `flights`: foreign key dest into table `airports`: values of `flights$dest` not in `airports$faa`: SJU (30), BQN (6), STT (4), PSE (2)
#>  Table `flights`: foreign key tailnum into table `planes`: values of `flights$tailnum` not in `planes$tailnum`: N725MQ (6), N537MQ (5), N722MQ (5), N730MQ (5), N736MQ (5), …

Establishing referential integrity is important for providing clean data for analysis or downstream users. See vignette("howto-dm-rows") for more information on adding, deleting or updating individual rows, and vignette("tech-dm-zoom") for operations on the data in a data model.

6. Normalization

Normalization is a technical term that describes the central design principle of a relational data model: splitting data into multiple tables.

A normalized data schema consists of several relations (tables) that are linked with attributes (columns). The relations can be joined together by means of primary and foreign keys. The main goal of normalization is to keep data organization as clean and simple as possible by avoiding redundant data entries.

For example, if you want to change the name of one airport in the nycflights13 dataset, you will only need to update a single data value. This principle is sometimes called the single point of truth.

#  Update in one single location...
airlines[airlines$carrier == "UA", "name"] <- "United broke my guitar"
#> Error in airlines[airlines$carrier == "UA", "name"] <- "United broke my guitar": object 'airlines' not found

airlines %>%
  filter(carrier == "UA")
#> Error in filter(., carrier == "UA"): object 'airlines' not found

# ...propagates to all related records
flights %>%
  left_join(airlines) %>%
  select(flight, name)
#> Error in left_join(., airlines): object 'flights' not found

Another way to demonstrate normalization is splitting a table into two parts.

Let’s look at the planes table, which consists of 3322 individual tail numbers and corresponding information for the specific airplane, like the year it was manufactured or the average cruising speed.

The function decompose_table() extracts two new tables and creates a new key model_id, that links both tables.

This results in a parent_table and a child_table that differ massively in the number of rows:

planes %>%
  decompose_table(model_id, model, manufacturer, type, engines, seats, manufacturer, speed)
#> Error in is.data.frame(x): object 'planes' not found

While child_table contains 3322 unique tailnum rows and therefore consists of 3322 rows, just like the original planes table, the parent_table shrunk to just 147 rows, enough to store all relevant combinations and avoid storing redundant information.

Further Reading: See the Simple English Wikipedia article on database normalization for more details.

7. Relational Databases

{dm} is built upon relational data models but it is not a database itself. Databases are systems for data management and many of them are constructed as relational databases, e.g. SQLite, MySQL, MSSQL, Postgres. As you can guess from the names of the databases, SQL, short for Structured Querying Language, plays an important role: it was invented for the purpose of querying relational databases.

In production, the data is stored in a relational database and {dm} is used to work with the data.

Therefore, {dm} can copy data from and to databases, and works transparently with both in-memory data and with relational database systems.

For example, let’s create a local SQLite database and copy the dm object to it:

con_sqlite <- DBI::dbConnect(RSQLite::SQLite())
#> Error in (function (cond) : error in evaluating the argument 'drv' in selecting a method for function 'dbConnect': there is no package called 'RSQLite'
con_sqlite
#> Error in eval(expr, envir, enclos): object 'con_sqlite' not found
DBI::dbListTables(con_sqlite)
#> Error in h(simpleError(msg, call)): error in evaluating the argument 'conn' in selecting a method for function 'dbListTables': object 'con_sqlite' not found

copy_dm_to(con_sqlite, dm)
#> Error in is.src(dest): object 'con_sqlite' not found
DBI::dbListTables(con_sqlite)
#> Error in h(simpleError(msg, call)): error in evaluating the argument 'conn' in selecting a method for function 'dbListTables': object 'con_sqlite' not found

In the opposite direction, dm can also be populated with data from a DB. Unfortunately, keys currently can be learned only for MSSQL and Postgres, but not for SQLite. Therefore, the dm contains the tables but not the keys:

dm_from_src(con_sqlite)
#> Error in dm_from_src(con_sqlite): object 'con_sqlite' not found

Remember to terminate the database connection:

DBI::dbDisconnect(con_sqlite)
#> Error in h(simpleError(msg, call)): error in evaluating the argument 'conn' in selecting a method for function 'dbDisconnect': object 'con_sqlite' not found