Empty where clauses in mysql (while I am using RMySql dbGetQuery)

Question

I want to do something really similar to these:

When mysql WHERE clause is empty, return all rows

Possible to have PHP MYSQL query ignore empty variable in WHERE clause?

I want the where clause to be ignored if the variable I am subsetting by is NULL. However, I am accessing my MySQL database from R using dbGetQuery. So far, I have code like this

write_pid_clause = function(p_id=NULL){
 if(is.null(p_id)){return(NULL)}
 else {return(paste0("where project_id = ",p_id)  )}
}

which writes the correct where statement behaviour if p_id is not specified, together with the code:

  dbGetQuery( con,paste0("select MIN(completion_date) from run ",write_pid_clause(p_id))))

This works properly, however, I struggle if I wish to insert more conditions in the where clause, e.g. if I wanted to add the condition and status = 'complete' when p_id = NULL.

Does anyone have any good ideas on how I can elegantly do this in R?

EDIT Here is some more code to demonstrate what I am trying to do, this is with respect to and clauses after a join (doing the where is a little bit trickier)

make_and_clauses = function(p_id = "",start_date="", end_date=""){

 conditions = list(
   list(" and r.project_id ='", p_id,"'"),
   list(" and r.completion_date >= '",start_date,"'"),
   list(" and r.completion_date <= '", end_date, "'"))
 condition_values = c(p_id,start_date, end_date)
 conditions[which(condition_values =="")] <- ""
 conditions = unlist(conditions,recursive=TRUE)
 paste0(conditions,collapse="")

}

which gives output

> make_and_clauses(2,3,4)
[1] " and r.project_id ='2' and r.completion_date >= '3' and r.completion_date <= '4'"
> make_and_clauses(2,,4)
[1] " and r.project_id ='2' and r.completion_date <= '4'"
> make_and_clauses(,3,2)
[1] " and r.completion_date >= '3' and r.completion_date <= '2'"

>

Answer 1

Start with your function I proposed something like:

make_and_clauses <- function(p_id, start_date, end_date) {
    conditions = c(
        if (!missing(p_id)) sprintf("r.project_id ='%i'", p_id),
        if (!missing(start_date)) sprintf("r.completion_date >= '%i'", start_date),
        if (!missing(end_date)) sprintf("r.completion_date <= '%i'", end_date)
    )
    paste(conditions, collapse=" and ")
}

which returns:

make_and_clauses(2,3,4)
# [1] "r.project_id ='2' and r.completion_date >= '3' and r.completion_date <= '4'"
make_and_clauses(2,,4)
# [1] "r.project_id ='2' and r.completion_date <= '4'"
make_and_clauses(,3,2)
# [1] "r.completion_date >= '3' and r.completion_date <= '2'"
make_and_clauses(,3,)
# [1] "r.completion_date >= '3'"

What important that no arguments at all leads to:

make_and_clauses(,,)
# [1] ""

So it's possible to write WHERE clause:

make_and_clauses <- function(p_id, start_date, end_date) {
    conditions = c(
        if (!missing(p_id)) sprintf("r.project_id ='%i'", p_id),
        if (!missing(start_date)) sprintf("r.completion_date >= '%i'", start_date),
        if (!missing(end_date)) sprintf("r.completion_date <= '%i'", end_date)
    )
    if (length(conditions)>0) paste("WHERE", paste(conditions, collapse=" and ")) else ""
}

and use it as

paste("select MIN(completion_date) from run", make_and_clauses(,3,2))
# [1] "select MIN(completion_date) from run WHERE r.completion_date >= '3' and r.completion_date <= '2'"
paste("select MIN(completion_date) from run", make_and_clauses())
# [1] "select MIN(completion_date) from run "

Answer 2

I don't think there is an easy/simple/clear way to achieve what you want. If you want to be able to add multiple clauses, each dependent upon the contents of a variable, you have to build up each one. The only relevant SQL trick I can think of is to put WHERE 1=1 in the SQL statement, then add AND column = value for each value. This way, if you leave one out, the syntax is still correct.

I see that @Marek has provided an answer with code; perhaps you can combine the answers to produce a workable solution.