Read the summary and the most important questions on BDS: Toolbox

• 1 tidyverse

• What are some shortcut codes to use in select to search for variables with similarities in their names?

  • starts_with("abc"): matches names that begin with “abc” 
    

  • ends_with("xyz"): matches names that end with “xyz”. 
  • contains("ijk"): matches names that contain “ijk”. 
  • matches("(.)\\1"): selects variables that match a regular expression. 
  • num_range("x", 1:3): matches x1, x2 and x3

• 2 SQL

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• How can you get two variables by name from a dataframe in sql, and select only items where the value for the first variable is larger than 5?

  
  SELECT var_1, var_2
  FROM dataframe
  WHERE var_1 > 5

• How can you select all variables from a dataframe, but only select observations where variable one has a value of 4, and variable 2 has a value of lower than 10?

  SELECT * 
  FROM data
  WHERE var_1 = 4 AND var_2 < 10

• How can you select two variables from the dataframe, rename them to 1 and 2, order the observations by values from var_1, and only show the first six results?

  
  SELECT var_1 AS 1, var_2 AS 2
  FROM data
  ORDER BY var_1 DESC (descending order)
  LIMIT 6

• What would be the equivalent of the following tidyverse code in sql?data %>%select(id) %>%summarise(mean)

  SELECT AVG(id)
  FROM data

• Why does the following code not work?SELECT application_id, COUNT(*)GROUP BY application_idFROM domains

  In sql, the order of the statements matter.
  the GROUP BY argument always comes after the FROM argument.

• What is the sql equivalent of the following tidyversecode:domains %>%  group_by(application_id) %>%  summarise(n())

  
  SELECT application_id, COUNT(*)
  FROM domains
  GROUP BY application_id

• How would you leftjoin data_1 on data_2 by var_2 from data_1 and var_5 from data_2?

  
  SELECT *
  FROM data_2
  LEFT JOIN data_1 ON var_2 = var_5

• How can you make nested queries in sql?

  For example:
  SELECT var_1
  FROM data_1
  WHERE var_1 < ( SELECT AVG(var_1) FROM data_2)
  
  This statement retrieves all instances of variable 1 in data_1, that have a lower value than the average of variable 1 in data_2

• How can you run a sql statement in r?

  
  con <- sqldatabase::connect()
  
  DBI::dbGetQuery(con, " *sql statement as string* ")