Old code from an RSQLite tutorial

rsqlite-tutorial.R

# R and SQLite tutorial
# Steph Locke, Mango Solutions
# 2015-11-30

library("RSQLite")
library("DBI")

dbListTables(datasetsDb())[1:2]

myDB <- dbConnect(SQLite(), "local.db")
dbListTables(myDB)


# Data Definition Language (DDL)
# CREATE TABLE
# ALTER TABLE
# DROP TABLE
# CREATE INDEX

# Data Manipulation Language (DML)
# CRUD operations
# ACID operations (not present in SQLite)



#+------------+#
# --- CRUD --- #
#+------------+#




# Create = INSERT
# Read = SELECT
# Update = UPDATE
# Delete = DELETE
# Upsert = MERGE ## conditional insertion; insert if absent & update if present



## INSERT OPERATIONS ##

# INSERT INTO airquality (ozone, month, temp)
# VALUES (6, 7, 8)
## SQLite tables have types and will error if wrong types supplied
## INSERT operations insert NUL unless column default is set


# write a table
qResult <- dbWriteTable(myDB, "airquality", airquality)

# insert some data ()
newdata <- airquality[is.na(airquality$Ozone), ]
qResult <- dbWriteTable(myDB, "airquality", newdata, append = TRUE)

sql <- 
paste0("INSERT INTO airquality VALUES(",
       paste(rep("?", ncol(newdata)),
             collapse = ","),
       ")")
dbGetPreparedQuery(myDB, sql, bind.data = newdata)

# add a new table
dbWriteTable(myDB, "iris", iris)


# SELECT/read
# can also have JOIN, WHERE, GROUP BY, HAVING, ORDER BY

a <- dbReadTable(myDB, "airquality", select.cols = "ozone, wind")
b <- dbGetQuery(myDB, "SELECT ozone, wind FROM airquality")
identical(a,b)

dbGetQuery(myDB, "SELECT * FROM iris")
# all of these are equivalent:
dbGetQuery(myDB, "SELECT `Sepal.Length`, `Sepal.Width` FROM iris")
dbGetQuery(myDB, "SELECT `sepal.length`, `sepal.width` FROM iris")
dbGetQuery(myDB, 'SELECT "Sepal.Length", "Sepal.Width" FROM iris')
dbGetQuery(myDB, 'SELECT "sepal.length", "sepal.width" FROM iris')


# UPDATE (to modify records)
## typically need to use a WHERE modifier
dbGetQuery(myDB, 
"UPDATE airquality 
 SET ozone = -99 
 WHERE ozone IS NULL")

dbGetQuery(myDB, 
'UPDATE iris 
 SET Species = "versicolour" 
 WHERE Species = "versicolor"')


# dbSendQuery() # rarely needed; doesn't pull back all data when executing query

# DELETE
## use a `where` clause, otherwise everything gets deleted
## DELETE does not reset row identifiers


dbGetQuery(myDB, "DELETE FROM airquality WHERE month <= 4")


iris_deleted <- dbGetQuery(myDB, "SELECT * FROM iris WHERE `Sepal.Length` <= 4.5")
dbGetQuery(myDB, "DELETE FROM iris WHERE `Sepal.Length` <= 4.5")


#+-----------+#
# -- JOINS -- #
#+-----------+#


# INNER
# LEFT 
## RIGHT (can rewrite any right join as a left join)
# FULL
# CROSS (cartesian product)

set.seed(1)
monthTbl <- data.frame(month = 1:10)
monthTbl$value <- monthTbl$month * rnorm(10)
dbWriteTable(myDB, "months", monthTbl[-5,])

# INNER join
dbGetQuery(myDB,
"SELECT *
 FROM airquality a
 INNER JOIN months m
   ON m.month=a.month")

# LEFT join
dbGetQuery(myDB,
"SELECT *
 FROM airquality a
 LEFT JOIN months m
   ON m.month=a.month")

# FULL join
dbGetQuery(myDB,
"SELECT *
 FROM airquality a
 FULL JOIN months m
   ON m.month=a.month")

# CROSS join
dbGetQuery(myDB,
"SELECT *
 FROM (SELECT DISTINCT month
       FROM airquality) a
 CROSS JOIN months m")


# example INNER join on `iris` data

tmp <- aggregate(. ~ Species, data = iris, FUN = length)
tmp$Species <- as.character(tmp$Species)
tmp[nrow(tmp) + 1, "Species"] <- "unknown"
tmp
dbWriteTable(myDB, "irisagg", tmp)


## ??? THIS IS WRONG
dbGetQuery(myDB,
"SELECT i.*, j.`Sepal.Length`
 FROM irisagg i
 LEFT JOIN iris j
 ON i.Species=j.Species")

## WHERE statement


## retrieve rows matching two conditions
dbGetQuery(myDB,
"SELECT * FROM iris
 WHERE `Petal.Length`*`Petal.Width` > 3
       AND `Sepal.Length`*`Sepal.Width` <= 10")


## GROUP BY
dbGetQuery(myDB,
"SELECT AVG(`Sepal.Width`), AVG(`Sepal.Length`) FROM iris
 GROUP BY `Species`")
# equivalent to:
aggregate(cbind(Sepal.Width, Sepal.Length) ~ Species, data = iris, FUN = mean)




# DROP (deletes a table, not just its contents)
## dbSendQuery(myDB, "DROP TABLE airquality")
## dbRemoveTable(myDB, "airquality")





# disconnect
dbDisconnect(myDB)