Use variable with regex in string::str_detect in dbplyr SQL query

Question

I would like to filter a SQL database based whether a regular expression appears within any column. I would like to specify the regex as a variable; however it is read as a literal string. I am having trouble getting the regex in as a variable. Thank you for your help!

Resources I've consulted:

• https://www.tidyverse.org/blog/2018/01/dbplyr-1-2/
• https://github.com/tidyverse/dbplyr/issues/168

Note: I had trouble making a reprex using the mtcars dataset, following https://www.tidyverse.org/blog/2018/01/dbplyr-1-2/. I get the error: "Error: str_detect() is not available in this SQL variant". I cannot share a reprex using my actual SQL database. As such, below is a pseudo-reprex.

library(dplyr)
library(stringr)

# Variable with regex (either lower- or uppercase "m")
my_string <- "(?i)m"

# WITHOUT SQL DATABASE ----------------------------------------------------

# This runs
mtcars %>% 
  tibble::rownames_to_column() %>% 
  filter(str_length(rowname) > 5)

# This runs with STRING
mtcars %>% 
  tibble::rownames_to_column() %>% 
  filter(str_detect(rowname, "(?i)m"))

# This runs with VARIABLE
mtcars %>% 
  tibble::rownames_to_column() %>% 
  filter(str_detect(rowname, my_string))

# WITH SQL DATABASE -------------------------------------------------------

con <- DBI::dbConnect(RSQLite::SQLite(), ":memory:")
mtcars_db <- copy_to(con, tibble::rownames_to_column(mtcars), "mtcars")

# This runs
tbl(con, "mtcars") %>% 
  filter(str_length(rowname) > 5)

# This *should* run with STRING -- pretend it does ;)
tbl(con, "mtcars") %>%
  filter(str_detect(rowname, "M"))

# This does NOT run with VARIABLE
tbl(con, "mtcars") %>%
  filter(str_detect(rowname, my_string))

Answer 1

This might depend a lot on the flavour of SQL you are using. This issue mentions a translation for str_detect and also provides an alternative.

Testing for SQL Server:

library(dbplyr)
library(dplyr)
library(stringr)

data(mtcars)
df_sim = tbl_lazy(mtcars, con = simulate_mssql())
my_string <- "(?i)m"

df_sim %>%
  filter(str_detect(my_string, gear)) %>%
  show_query()
# Error: str_detect() is not available in this SQL variant

df_sim %>%
  filter(gear %like% my_string) %>%
  show_query()
# <SQL>
# SELECT *
# FROM `df`
# WHERE (`gear` like '(?i)m')

So it appears str_detect can not translate to SQL Server. But you can use %like% as a work around.

Testing for MySQL:

library(dbplyr)
library(dplyr)
library(stringr)

data(mtcars)
df_sim = tbl_lazy(mtcars, con = simulate_mysql()) # changed to mysql
my_string <- "(?i)m"

df_sim %>%
  filter(str_detect(my_string, gear)) %>%
  show_query()
# <SQL>
# SELECT *
# FROM `df`
# WHERE ('(?i)m' REGEXP `gear`)

df_sim %>%
  filter(gear %like% my_string) %>%
  show_query()
# <SQL>
# SELECT *
# FROM `df`
# WHERE (`gear` like '(?i)m')

So it appears str_detect can be translated correctly for MySQL.

In every case my_string is translated into the query.

A couple of other things to check:

• Updating the version of dbplyr. Older versions do not have all the translations in newer versions.
• Try a column other than rownames. Data frames in R can have rownames but tables in SQL can only have columns.