Post Increment date field in mySQL query using R - mysql

Question

I am trying to query a table in our mySQL database using the DBI R package. However, I need to pull the fields from the table by changing the date field on a monthly basis and limiting it to 1.
I'm having trouble with the looping and sql query text. I would like to create a loop that changes the date (monthly) and then prints that to a database query that will then pull all the data that matches the monthly conditions.
This is my code so far:
for (i in seq(0,12,1)){
results <- dbGetQuery(myDB, paste("SELECT * FROM cost_and_price_period WHERE start_date <=", '01-[[i]]-2019'))
}
The main issue is that R doesn't acknowledge post-increment operators like ++, so I know I could just make 12 individual queries and then rbind them, but I would prefer to do one efficient query. Does anyone have any ideas?

Answer 1

This below solution could give you an idea how to proceed with your problem.
DummyTable
id names dob
1 1 aa 2018-01-01
2 2 bb 2018-02-01
3 3 cc 2018-03-01
4 4 dd 2018-04-01
5 5 ee 2018-05-01
6 6 ff 2018-06-01
7 7 gg 2018-07-01
8 8 hh 2018-08-01
9 9 ii 2018-09-01
10 10 jj 2018-10-01
11 11 kk 2018-11-01
12 12 ll 2018-12-01
13 13 ll 2018-12-01
Imagine we have the above table in MySQL. Then we need to access the data for 1st day of every month and store whole records as a data frame.
### Using for loop like from your question
n <- 12
df <- vector("list", n)
for (i in seq(1:12)){
df[[i]] <- data.frame(dbGetQuery(pool, paste0("SELECT * FROM dummyTable WHERE dob = '2018-",i,"-01';" ))) # in iteration `i` corresponds for month number
}
df <- do.call(rbind, df)
### Using lapply(preferred way)
n <- seq(1:12)
df <- lapply(n, function(x){
dbGetQuery(pool, paste0("SELECT * FROM dummyTable WHERE dob = '2018-",x,"-01';" ))
})
df <- do.call(rbind, df)
So output of df data frame will give the matched records from MySQL.