Question

我想做一些与这些非常类似的事情：

When mysql WHERE clause is empty, return all rows

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

如果我按子集化的变量是NULL，我希望忽略where子句。但是，我使用dbGetQuery从R访问我的MySQL数据库。到目前为止，我有这样的代码

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

如果未指定where，则会写入正确的p_id语句行为以及代码：

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

这是正常的，但是，如果我想在where子句中插入更多条件，我会很困难，例如如果我想在and status = 'complete'时添加条件p_id = NULL。

有没有人对如何在R中优雅地做到这一点有任何好的想法？

修改这里有一些代码来演示我想要做的事情，这是关于连接后的and子句（执行where有点棘手）

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="")

}

给出输出

> 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

我不认为有一种简单/简单/清晰的方法可以达到你想要的效果。如果您希望能够添加多个子句，每个子句都依赖于变量的内容，则必须构建每个子句。我能想到的唯一相关的SQL技巧是将WHERE 1=1放在SQL语句中，然后为每个值添加AND column = value。这样，如果你留下一个，语法仍然是正确的。

我看到@Marek提供了代码答案;也许你可以结合答案来产生一个可行的解决方案。

Answer 2

从你的功能开始我提出了类似的内容：

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 ")
}

返回：

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'"

没有任何论据可以带来什么重要：

make_and_clauses(,,)
# [1] ""

因此可以编写WHERE子句：

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 ""
}

并将其用作

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 "

清空mysql中的where子句（当我使用RMySql dbGetQuery时）

2 个答案: