How to use custom SQL function in dbplyr?

问题

I would like to calculate the Jaro-Winkler string distance in a database. If I bring the data into R (with collect) I can easily use the stringdist function from the stringdist package.

But my data is very large and I'd like to filter on Jaro-Winkler distances before pulling the data into R.

There is SQL code for Jaro-Winkler (https://androidaddicted.wordpress.com/2010/06/01/jaro-winkler-sql-code/ and a version for T-SQL) but I guess I'm not sure how best to get that SQL code to work with dbplyr. I'm happy to try and map the stringdist function to the Jaro-Winkler sql code but I don't know where to start on that. But even something simpler like executing the SQL code directly from R on the remote data would be great.

I had hoped that SQL translation in the dbplyr documentation might help, but I don't think so.

回答1:

You can build your own SQL functions in R. They just have to produce a string that is a valid SQL query. I don't know the Jaro-Winkler distance, but I can provide an example for you to build from:

union_all = function(table_a,table_b, list_of_columns){
  # extract database connection
  connection = table_a$src$con

  sql_query = build_sql(con = connection,
                      sql_render(table_a),
                      "\nUNION ALL\n",
                      sql_render(table_b)
  )

  return(tbl(connection, sql(sql_query)))
}

unioned_table = union_all(table_1, table_2, c("who", "where", "when"))

Two key commands here are:

sql_render, which takes a dbplyr table and returns the SQL code that produces it
build_sql, which assembles a query from strings.

You have choices for your execution command:

tbl(connection, sql(sql_query)) will return the resulting table
dbExecute(db_connection, as.character(sql_query)) will execute a query without returning the result (useful for for dropping tables, creating indexes, etc.)

来源：https://stackoverflow.com/questions/50661862/how-to-use-custom-sql-function-in-dbplyr

标签

stringdist

dbplyr