R循环查询数据库中的所有表

Question

I am new to R and have a database, DatabaseX which contains multiple tables a,b,c,d,etc . I want to use R to count number of rows for a common attribute message_id among all these tables and store it separately.

I can count message_id for all tables using below code:

list<-dbListTables(con)
# print list of tables for testing
print(list)
for (i in 1:length(list)){
  query <- paste("SELECT COUNT(message_id) FROM ",list[i], sep = "")
  t <- dbGetQuery(con,query)
}
print(t)

This prints :

### COUNT(message_id)
## 1  21519

but I want to keep record of count(message_id) for each individual table . So for example table a = 200, b = 300, c = 500, and etc.

any suggestions how to do this?

Answer 1

As @kaiten65 suggested, one option would be to create a helper function which executes the COUNT query. Outside of your loop I have defined a numeric vector counts which will store the number of records for each table in your database. You can then perform descriptive stats on the tables along with this vector of record counts.

doCountQuery <- function(con, table) {
    query <- paste("SELECT COUNT(message_id) FROM ", table, sep = "")
    t     <- dbGetQuery(con, query)

    return(t)
}

list <- dbListTables(con)
counts <- numeric(0)       # this will store the counts for all tables

for (i in 1:length(list)) {
    count <- doCountQuery(con, list[i])
    counts[i] <- count[[1]]
}

Answer 2

You can use functions to achieve what you need. For instance

readDB <- function(db, i){
     query <- paste("SELECT COUNT(message_id) FROM ",db, sep = "")
     t <- dbGetQuery(con,query)
     return(print(paste("Table ", i, " count:", t)
     }

list<-dbListTables(con)
for (i in 1:length(list)){
    readDB(list[i]);
}

This should print your list recursively but the actual code is in a nice editable function. Your output will be

 "Table 1 count: 2519"
 "Table 2 count: ---- "

More information on R functions here: http://www.statmethods.net/management/userfunctions.html