将列添加到sqlite数据库 [英] Adding column to sqlite database

问题描述

我试图将在R中生成的向量添加到sqlite表中作为新列。为此，我想使用 dplyr （我根据此安装了最新的开发版本以及 dbplyr 软件包。在此处）。我尝试过的操作：

I am trying to add a vector which I generated in R to a sqlite table as a new column. For this I wanted to use dplyr (I installed the most recent dev. version along with the dbplyr package according to this post here). What I tried:

library(dplyr)
library(DBI) 

#creating initial database and table
dbcon      <- dbConnect(RSQLite::SQLite(), "cars.db") 
dbWriteTable(dbcon, name = "cars", value = cars)
cars_tbl <- dplyr::tbl(dbcon, "cars")

#new values which I want to add as a new column 
new_values <- sample(c("A","B","C"), nrow(cars), replace = TRUE) 

#attempt to add new values as column to the table in the database
cars_tbl %>% mutate(new_col = new_values) #not working

有什么简单的方法可以做到这一点（不一定是dplyr ）？

What is an easy way to achieve this (not necessarily with dplyr)?

推荐答案

不知道使用 dyplr 的方法，但是您可以直接使用 RSQLite 完成。问题实际上不是 RSQLite ，而是我不知道如何将列表传递给 mutate 的事实。 。请注意，在您的代码中，类似的方法将起作用：

Not aware of a way of doing this with dyplr, but you can do it with RSQLite directly. The problem is not actually with RSQLite, but the fact that I don't know how to pass a list to mutate. Note that, in your code, something like this would work:

cars_tbl %>% mutate(new_col = another_column / 3.14)

无论如何，我的选择。我创建了一个玩具汽车数据框。

Anyway, my alternative. I've created a toy cars dataframe.

cars <- data.frame(year=c(1999, 2007, 2009, 2017), model=c("Ford", "Toyota", "Toyota", "BMW"))

我打开连接并实际创建表，

I open connection and actually create the table,

dbcon <- dbConnect(RSQLite::SQLite(), "cars.db")
dbWriteTable(dbcon, name = "cars", value = cars)

添加新列并检查，

dbGetQuery(dbcon, "ALTER TABLE cars ADD COLUMN new_col TEXT")
dbGetQuery(dbcon, "SELECT * FROM cars")
  year  model new_col
1 1999   Ford    <NA>
2 2007 Toyota    <NA>
3 2009 Toyota    <NA>
4 2017    BMW    <NA>

然后您可以更新新列，但唯一棘手的事情是您必须提供一个 where 语句，在这种情况下，我使用年份。

And then you can update the new column, but the only tricky thing is that you have to provide a where statement, in this case I use the year.

new_values <- sample(c("A","B","C"), nrow(cars), replace = TRUE) 
new_values
[1] "C" "B" "B" "B"

dbGetPreparedQuery(dbcon, "UPDATE cars SET new_col = ? where year=?",
                   bind.data=data.frame(new_col=new_values,
                                        year=cars$year))

dbGetQuery(dbcon, "SELECT * FROM cars")
  year  model new_col
1 1999   Ford       C
2 2007 Toyota       B
3 2009 Toyota       B
4 2017    BMW       B

作为唯一索引，您可以随时使用行名（汽车），但您必须将其添加为数据框中的列，然后再添加至表中。

As a unique index, you could always use rownames(cars), but you would have to add it as a column in your dataframe and then in your table.

@krlmlr建议后进行编辑：使用起来确实好得多 dbExecute 而不是已弃用的 dbGetPreparedQuery ，

EDIT after suggestion by @krlmlr: indeed much better using dbExecute instead of deprecated dbGetPreparedQuery,

dbExecute(dbcon, "UPDATE cars SET new_col = :new_col where year = :year",
          params=data.frame(new_col=new_values,
                            year=cars$year))

评论后编辑：我几天前没想到，但即使它是 SQLite ，您也可以使用 rowid 。我已经对此进行了测试。

EDIT after comments: I did not think about this a few days ago, but even if it is a SQLite you can use the rowid. I've tested this and it works.

dbExecute(dbcon, "UPDATE cars SET new_col = :new_col where rowid = :id",
          params=data.frame(new_col=new_values,
                            id=rownames(cars)))

尽管您必须确保表中的rowid与您的行名相同。无论如何，您总是可以这样获得rowid：

Although you have to make sure that the rowid's in the table are the same as your rownames. Anyway you can always get your rowid's like this:

dbGetQuery(dbcon, "SELECT rowid, * FROM cars")
  rowid year  model new_col
1     1 1999   Ford       C
2     2 2007 Toyota       B
3     3 2009 Toyota       B
4     4 2017    BMW       B

这篇关于将列添加到sqlite数据库的文章就介绍到这了，希望我们推荐的答案对大家有所帮助，也希望大家多多支持IT屋！