问题描述
我确实使用 dbplyr 远程连接到 sql 表。
其中一个表由一个 ID 列和其他几个存储 0 和 1 值的列组成(sql bit
- 解释为来自 R 端的布尔值 TRUE/FALSE 值),并且从 RI 中只想获得 1 的总数对于每一行。
在 R 中使用例如 rowSums()
的常用表很简单,不幸的是它不能通过 dbplyr(没有 sql 等效项)工作。
由于基础表的大小,显而易见的原因我不想collect()
数据。
在这样的背景下如何实现这一目标?
library(dplyr)
# Local case
DF <- tibble(ID = LETTERS[1:3],col1 = c(1,1,1),col2 = c(1,0),col3 = c(1,0))
DF %>%
summarise(sum = rowSums(select(.,-1)))
# sum
# 1 3
# 2 2
# 3 1
# If DF is a remote sql table,therefore one would get the following error message:
# Error: nanodbc/nanodbc.cpp:1655: 42000: [Microsoft][ODBC sql Server Driver][sql Server]'rowSums' is not a recognized built-in function name. [Microsoft][ODBC sql Server Driver][sql Server]
编辑 - 添加最小的可重现示例
关注@Simon.S.A.回复,在 MRE 下方:
# Table creation
DF <- tibble(ID = LETTERS[1:3],0))
colnames(DF) <- c("col 1","col 2","col 3","col 4")
# sql simulation
con <- DBI::dbConnect(Rsqlite::sqlite(),":memory:")
copy_to(con,DF)
con %>% tbl("DF") # just checking
#preparing formula
cols <- colnames(DF)[-1]
all_equations <- paste0("`",cols,"` = sum(`","`)")
# actual query
con %>%
tbl("DF") %>%
summarise(!!!rlang::parse_exprs(all_equations))
# Error: near "=": Syntax error
# %>% show_query() shows a strange query,but I am no sql expert as you understood.
# also tried:
# all_equations <- paste(cols,"= sum(",")")
# all_equations <- paste0("`[","]` = sum(`[","]`)")
解决方法
这里的部分挑战是 dbplyr 将 dplyr 命令翻译成 SQL,但翻译只为某些 R 命令定义。由于标准 dplyr 命令存在翻译,我们可以使用 summarise
。
总结一下,我们可以做到以下几点:
library(dplyr)
library(rlang)
cols = colnames(DF)
cols = cols[2:length(cols)]
all_equations = paste(cols,"= sum(",cols,")")
DF %>%
summarise(!!!parse_exprs(all_equations))
想法是构建每个和的文本字符串,然后使用 !!!parse_exprs(.)
将此文本转换为 R 代码。
编辑 - 相同的方法,但用于行总和
# Table creation
DF <- tibble(ID = LETTERS[1:3],col1 = c(1,1,1),col2 = c(1,0),col3 = c(1,0))
colnames(DF) <- c("col 1","col 2","col 3","col 4")
# SQL simulation
con <- DBI::dbConnect(RSQLite::SQLite(),":memory:")
copy_to(con,DF)
con %>% tbl("DF") # just checking
#preparing formula
cols <- colnames(DF)[-1]
eq <- paste0("`",paste0(cols,collapse = "` + `"),"`")
# actual query
con %>%
tbl("DF") %>%
mutate(new = !!parse_expr(eq))
但仍取决于 dbplyr 翻译,因此可能无法正确处理反引号。
,我发现一种可能的解决方法是写下实际查询,例如使用 DBI 包。但我仍然对使用 dbplyr 的更优雅的方式感兴趣。
DF <- tibble(ID = LETTERS[1:3],"col 4") # having spaces in column names increase handling complexity
# SQL simulation
con <- DBI::dbConnect(RSQLite::SQLite(),DF)
con %>% tbl("DF") # just checking
cols <- colnames(DF)[-1]
col2select <- colnames(DF) # column to select in the result
query <- paste0("SELECT ",paste0("[",col2select,"]",collapse =","),",# paste0("CAST([","] AS INT)",collapse = " + ")," AS sum FROM DF")
rs <- DBI::dbSendQuery(con,query)
DBI::dbFetch(rs)
DBI::dbClearResult(rs)
DBI::dbDisconnect(con)
,
使用 tidyr
使得 dplyr
代码可读性很强。这在大表上的表现如何还不太清楚。
library(dplyr,warn.conflicts = FALSE)
library(tidyr)
DF <- tibble(ID = LETTERS[1:3],0))
# SQL simulation
con <- DBI::dbConnect(RSQLite::SQLite(),":memory:")
DF <- copy_to(con,DF,overwrite = TRUE)
result <-
DF %>%
pivot_longer(cols = -ID) %>%
group_by(ID) %>%
summarize(sum = sum(value,na.rm = TRUE))
result
#> # Source: lazy query [?? x 2]
#> # Database: sqlite 3.35.5 [:memory:]
#> ID sum
#> <chr> <dbl>
#> 1 A 3
#> 2 B 2
#> 3 C 1
result %>% show_query()
#> <SQL>
#> SELECT `ID`,SUM(`value`) AS `sum`
#> FROM (SELECT `ID`,'col1' AS `name`,`col1` AS `value`
#> FROM `DF`
#> UNION ALL
#> SELECT `ID`,'col2' AS `name`,`col2` AS `value`
#> FROM `DF`
#> UNION ALL
#> SELECT `ID`,'col3' AS `name`,`col3` AS `value`
#> FROM `DF`)
#> GROUP BY `ID`
由 reprex package (v2.0.0) 于 2021 年 6 月 18 日创建
library(dplyr,warn.conflicts = FALSE)
library(DBI)
n <- 26e3
df <- tibble(ID = rep(LETTERS,n/26))
for (i in 1:100) df[[paste0("col",i)]] <- rbinom(prob = 0.5,n = n,size = 1)
# SQL simulation
con <- DBI::dbConnect(RSQLite::SQLite(),":memory:")
df_sql <- copy_to(con,df,overwrite = TRUE)
row_sum_1 <- function(df,con) {
sum_cols <- setdiff(colnames(df),"ID")
names <- paste(DBI::dbQuoteIdentifier(con,colnames(df)),collapse = ",")
sum_sql <- paste(DBI::dbQuoteIdentifier(con,sum_cols),collapse = " + ")
query <- paste0("SELECT ",names,sum_sql," AS sum FROM df")
tbl(con,sql(query))
}
row_sum_1(df_sql,con) %>% select(ID,sum)
#> # Source: lazy query [?? x 2]
#> # Database: sqlite 3.35.5 [:memory:]
#> ID sum
#> <chr> <int>
#> 1 A 49
#> 2 B 53
#> 3 C 54
#> 4 D 49
#> 5 E 51
#> 6 F 46
#> 7 G 55
#> 8 H 48
#> 9 I 44
#> 10 J 50
#> # … with more rows
system.time(compute(row_sum_1(df_sql,con)))
#> user system elapsed
#> 0.307 0.007 0.315
由 reprex package (v2.0.0) 于 2021 年 6 月 21 日创建