我倾向于使用存储过程来解决这个问题,但这通常需要为每个特定实例编写存储过程.在某些时候,我可能会想出一种在存储过程中更通用地执行此操作的方法,但我发现在存储过程中构造查询的过程是乏味且令人沮丧的.

出于这个原因,我只花了一些时间构建一个函数,该函数将执行涉及VARCHAR(MAX)变量的有限查询.这是一种蛮力的方法,对于一个17000个字符的变量将它导出为三个变量并将它们粘贴在一起.它很粗糙,可能不是很有效,但是我提出的最好的解决方案.

另一个限制是它不允许您重命名查询中的变量.你将被困在变量中,因为它们在数据库中被命名.如果您只涉及几张表,那可能不是问题.在非常复杂的数据库中,这可能会有问题.但是,至少有了这个,您可以使用一些必要的ID来查询VARCHAR(MAX)变量,在R中执行合并.

正如GitHub问题中所讨论的那样,最好尽量避免使用VARCHAR(MAX).如果确实需要未知长度,则VARBINARY(MAX)更容易查询.

例

源( “https://gist.githubusercontent.com/nutterb/d2e050dada608bb6213e61d0f8471b65/raw/be8717f318b3e3087e7c26c9a8f9d0a582a5daef/query_varchar_max”

channel <- odbcDriverConnect(...)

query_varchar_max(channel = channel,id = c("idvar"),varchar_max = c("varchar_max_var","varchar_max_var2"),from = "FROM dbo.table_name WHERE group = ?",data = list(group = "A"))

功能代码

#' @name query_varchar_max
#' @title Query a VARCHAR(MAX) Variable from sql Server
#' 
#' @description The RODBC driver to sql Server (sql Server Native Client 11.0)
#'   reports the lenght of a VARCHAR(MAX) variable to be zero.  This presents 
#'   difficulties in extracting long text values from the database. Often,the
#'   ODBC will assume a length of 255 characters and truncate the text to that
#'   many characters.  The approach taken here searches the VARCHAR(MAX) variables 
#'   for the longest length,and extracts the data in segments to be pasted 
#'   together in R.  
#'   
#' @param channel A valid ODBC channel to a sql Server database.
#' @param id A character vector of ID variables that may be used to merge the 
#'   data from this query into another dataset.
#' @param varchar_max a character vector of variable names that are to be 
#'   treated as if they are VARCHAR(MAX) variables. 
#' @param from A single character string providing the remainder of the query 
#'   to be run,beginning with the \code{FROM} statement.
#' @param stringsAsFactors \code{logical(1)}. Should character strings returned 
#'   from the database be converted to factors?
#' @param ... Additional arguments to \code{sqlExecute} when running the full 
#'   query.
#'   
#' @details \code{query_varchar_max} operates by determining how many columns of up to
#'   8000 characters each are required to export a complete VARCHAR(MAX) variable.
#'   It then creates the necessary number of intermediate variables and queries the 
#'   data using the sql Server \code{SUBSTRING} command,extracting the VARCHAR(MAX)
#'   variable in increments of 8000 characters. After completing the query,#'   the intemediary variables are concatenated and removed from the data.
#'   
#'   The function makes accommodation for multi-part queries as far as [TABLE].[VARIABLE]
#'   formats are concerned. It is not intended for use in [SCHEMA].[TABLE].[VARIABLE]
#'   formats. This at least allows \code{from} to include joins for more complex 
#'   queries.  Parameterized queries are also supported through \code{sqlExecute}.
#'
#' @export

query_varchar_max <- function(channel,id,varchar_max,from,stringsAsFactors = FALSE,...)
{
  coll <- checkmate::makeAssertCollection()

  checkmate::assert_class(x = channel,classes = "RODBC",add = coll)

  checkmate::assert_character(x = id,add = coll)

  checkmate::assert_character(x = varchar_max,add = coll)

  checkmate::assert_character(x = from,len = 1,add = coll)

  checkmate::assert_logical(x = stringsAsFactors,add = coll)

  checkmate::reportAssertions(coll)

  varchar_max_len <-
    paste0(
      sprintf("MAX(LEN(%s)) AS len_%s",sub("[.]","_",varchar_max)),collapse = ","
    )

  varchar_len <- 
    unlist(
      RODBCext::sqlExecute(
        channel = channel,query = sprintf("SELECT %s %s",varchar_max_len,from),fetch = TRUE
      )
    )

  varchar_max_cols <- 
    unlist(
      mapply(expand_varchar_max,varchar_len,SIMPLIFY = FALSE)
    )

  Prelim <- 
    RODBCext::sqlExecute(
      channel = channel,query = sprintf("SELECT %s,%s %s",paste0(id,"),paste0(varchar_max_cols,fetch = TRUE,stringsAsFactors = stringsAsFactors,...
    )

  var_stub_to_combine <-
    unique(
      sub(
        "(part)(\\d{1,3})","\\1",sub(".+AS ","",varchar_max_cols)
      )
    )

  col_to_combine <- 
    lapply(var_stub_to_combine,grep,names(Prelim))

  Prelim[sub(".+[.]",varchar_max)] <-
    lapply(col_to_combine,function(col) apply(Prelim[col],1,paste0,collapse = ""))

  Prelim[-unlist(col_to_combine)]

}


expand_varchar_max <- function(varchar_max,varchar_len)
{
  nvar <- varchar_len %/% 8000 + 1

  var_list <- vector("character",length = nvar)

  for (i in seq_along(var_list))
  {
    var_list[i] <- 
      sprintf("SUBSTRING(%s,%s,%s) AS %s_part%s",1 + (i - 1) * 8000,8000,paste0(sub("[.]",i)
  }
  var_list
}