4 Introduction to DBI

4.1 Local database basics

Connecting and adding data to a database

  1. Load the DBI package

  2. Use dbConnect to open a database connection

  3. Use dbListTables() to view existing tables, there should be 0 tables

    ## character(0)

  4. Use dbWriteTable() to create a new table using mtcars data. Name it db_mtcars

  5. Use dbListTables() to view existing tables, it should return db_mtcars

    ## [1] "db_mtcars"

  6. Use dbGetQuery() to pass a SQL query to the database

    ##     mpg cyl  disp  hp drat    wt  qsec vs am gear carb
## 1  21.0   6 160.0 110 3.90 2.620 16.46  0  1    4    4
## 2  21.0   6 160.0 110 3.90 2.875 17.02  0  1    4    4
## 3  22.8   4 108.0  93 3.85 2.320 18.61  1  1    4    1
## 4  21.4   6 258.0 110 3.08 3.215 19.44  1  0    3    1
## 5  18.7   8 360.0 175 3.15 3.440 17.02  0  0    3    2
## 6  18.1   6 225.0 105 2.76 3.460 20.22  1  0    3    1
## 7  14.3   8 360.0 245 3.21 3.570 15.84  0  0    3    4
## 8  24.4   4 146.7  62 3.69 3.190 20.00  1  0    4    2
## 9  22.8   4 140.8  95 3.92 3.150 22.90  1  0    4    2
## 10 19.2   6 167.6 123 3.92 3.440 18.30  1  0    4    4
## 11 17.8   6 167.6 123 3.92 3.440 18.90  1  0    4    4
## 12 16.4   8 275.8 180 3.07 4.070 17.40  0  0    3    3
## 13 17.3   8 275.8 180 3.07 3.730 17.60  0  0    3    3
## 14 15.2   8 275.8 180 3.07 3.780 18.00  0  0    3    3
## 15 10.4   8 472.0 205 2.93 5.250 17.98  0  0    3    4
## 16 10.4   8 460.0 215 3.00 5.424 17.82  0  0    3    4
## 17 14.7   8 440.0 230 3.23 5.345 17.42  0  0    3    4
## 18 32.4   4  78.7  66 4.08 2.200 19.47  1  1    4    1
## 19 30.4   4  75.7  52 4.93 1.615 18.52  1  1    4    2
## 20 33.9   4  71.1  65 4.22 1.835 19.90  1  1    4    1
## 21 21.5   4 120.1  97 3.70 2.465 20.01  1  0    3    1
## 22 15.5   8 318.0 150 2.76 3.520 16.87  0  0    3    2
## 23 15.2   8 304.0 150 3.15 3.435 17.30  0  0    3    2
## 24 13.3   8 350.0 245 3.73 3.840 15.41  0  0    3    4
## 25 19.2   8 400.0 175 3.08 3.845 17.05  0  0    3    2
## 26 27.3   4  79.0  66 4.08 1.935 18.90  1  1    4    1
## 27 26.0   4 120.3  91 4.43 2.140 16.70  0  1    5    2
## 28 30.4   4  95.1 113 3.77 1.513 16.90  1  1    5    2
## 29 15.8   8 351.0 264 4.22 3.170 14.50  0  1    5    4
## 30 19.7   6 145.0 175 3.62 2.770 15.50  0  1    5    6
## 31 15.0   8 301.0 335 3.54 3.570 14.60  0  1    5    8
## 32 21.4   4 121.0 109 4.11 2.780 18.60  1  1    4    2

  7. Close the database connection using dbDisconnect()

4.2 Options for writing tables

Understand how certain arguments in dbWriteTable() work

  1. Use dbConnect() to open a Database connection again

  2. Use dbWriteTable() to re-create the db_mtcars table using mtcars data

     Error: Table db_mtcars exists in database, and both overwrite and append are FALSE

  3. Use the append argument in dbWriteTable() to add to the data in the db_mtcars table

  4. Using dbGetQuery(), check the current record count of db_mtcars with the following query: “select count() from db_mtcars”

    ##   count()
## 1      64

  5. Use the overwrite argument to dbWriteTable() to replace the data in the db_mtcars table

  6. Check the record count of db_mtcars again

    ##   count()
## 1      32

4.3 Database operations

Understand how to use dbSendStatement() and dbExecute() to modify the database

  1. Use dbSendStatement() to pass a SQL commands that deletes any automatic car from db_mtcars: “delete from db_mtcars where am = 1”. Load the command to a variable called rs

  2. Call the rs variable to view information about the results of the requested change

    ## <SQLiteResult>
##   SQL  delete from db_mtcars where am = 1
##   ROWS Fetched: 0 [complete]
##        Changed: 13

  3. Use dbHasCompleted() to confirm that the job is complete

    ## [1] TRUE

  4. Use dbGetRowsAffected() to see the number of rows that were affected by the request

    ## [1] 13

  5. Clear the results using dbClearResult()

  6. Confirm that the result set has been removed by calling the rs variable once more

    ## <SQLiteResult>
## EXPIRED

  7. Check the record count of db_mtcars again, the new count should be 19 (32 original records - 13 deleted records)

    ##   count()
## 1      19

  8. Use dbWriteTable() to overwrite db_mtcars with the value of mtcars

  9. Use dbExeceute() to delete rows where am = 1 using the same query as before. Load the results in a variable called rs

  10. rs contains the number of rows affected by the satement that was executed

    ## [1] 13

  11. Check the record count of db_mtcars again.

    ##   count()
## 1      19

4.4 knitr SQL engine

See how to run SQL queries as code chunks

  1. Start a new code chunk, but using sql instead of r as the first argument of the chunk. Also add connection=con as another argument of the chunk.

    {sql, connection=con} select * from db_mtcars

Table 4.1: Displaying records 1 - 5
mpg cyl disp hp drat wt qsec vs am gear carb
21.4 6 258.0 110 3.08 3.215 19.44 1 0 3 1
18.7 8 360.0 175 3.15 3.440 17.02 0 0 3 2
18.1 6 225.0 105 2.76 3.460 20.22 1 0 3 1
14.3 8 360.0 245 3.21 3.570 15.84 0 0 3 4
24.4 4 146.7 62 3.69 3.190 20.00 1 0 4 2

  1. Add the max.print options to the chunk, and set it to 5

    {sql, connection=con, max.print = 5} select * from db_mtcars

Table 4.2: Displaying records 1 - 5
mpg cyl disp hp drat wt qsec vs am gear carb
21.4 6 258.0 110 3.08 3.215 19.44 1 0 3 1
18.7 8 360.0 175 3.15 3.440 17.02 0 0 3 2
18.1 6 225.0 105 2.76 3.460 20.22 1 0 3 1
14.3 8 360.0 245 3.21 3.570 15.84 0 0 3 4
24.4 4 146.7 62 3.69 3.190 20.00 1 0 4 2

  1. Set defaults for the sql chunks by using the knitr::opts_chunk$set() command in the setup at the beginning of the document. {r setup} knitr::opts_chunk$set(connection = "con", max.print = 5)

  2. Run the same query in a new sql chunk, but without any other argument

    select * from db_mtcars
Table 4.3: Displaying records 1 - 5
mpg cyl disp hp drat wt qsec vs am gear carb
21.4 6 258.0 110 3.08 3.215 19.44 1 0 3 1
18.7 8 360.0 175 3.15 3.440 17.02 0 0 3 2
18.1 6 225.0 105 2.76 3.460 20.22 1 0 3 1
14.3 8 360.0 245 3.21 3.570 15.84 0 0 3 4
24.4 4 146.7 62 3.69 3.190 20.00 1 0 4 2
  1. Store the results of the query into an R object called local_mtcars using the output.var option.
##     mpg cyl  disp  hp drat    wt  qsec vs am gear carb
## 1  21.4   6 258.0 110 3.08 3.215 19.44  1  0    3    1
## 2  18.7   8 360.0 175 3.15 3.440 17.02  0  0    3    2
## 3  18.1   6 225.0 105 2.76 3.460 20.22  1  0    3    1
## 4  14.3   8 360.0 245 3.21 3.570 15.84  0  0    3    4
## 5  24.4   4 146.7  62 3.69 3.190 20.00  1  0    4    2
## 6  22.8   4 140.8  95 3.92 3.150 22.90  1  0    4    2
## 7  19.2   6 167.6 123 3.92 3.440 18.30  1  0    4    4
## 8  17.8   6 167.6 123 3.92 3.440 18.90  1  0    4    4
## 9  16.4   8 275.8 180 3.07 4.070 17.40  0  0    3    3
## 10 17.3   8 275.8 180 3.07 3.730 17.60  0  0    3    3
## 11 15.2   8 275.8 180 3.07 3.780 18.00  0  0    3    3
## 12 10.4   8 472.0 205 2.93 5.250 17.98  0  0    3    4
## 13 10.4   8 460.0 215 3.00 5.424 17.82  0  0    3    4
## 14 14.7   8 440.0 230 3.23 5.345 17.42  0  0    3    4
## 15 21.5   4 120.1  97 3.70 2.465 20.01  1  0    3    1
## 16 15.5   8 318.0 150 2.76 3.520 16.87  0  0    3    2
## 17 15.2   8 304.0 150 3.15 3.435 17.30  0  0    3    2
## 18 13.3   8 350.0 245 3.73 3.840 15.41  0  0    3    4
## 19 19.2   8 400.0 175 3.08 3.845 17.05  0  0    3    2
  1. Close the database connection using dbDisconnect()