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

Question

我想做一些与这些非常相似的事情：

When mysql WHERE clause is empty, return all rows

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

如果我作为子集的变量是NULL，我希望忽略 where 子句。但是，我正在使用 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p_id，则写入正确的where 语句行为以及代码：

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

这可以正常工作，但是，如果我希望在 where 子句中插入更多条件，例如如果我想在p_id = NULL 时添加条件and status = 'complete'。

有人对我如何在 R 中优雅地做到这一点有什么好的想法吗？

编辑 这是更多代码来演示我正在尝试做的事情，这是关于加入后的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

从你的功能开始，我提出了类似的建议：

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'"

根本没有争论会导致什么重要：

make_and_clauses(,,)
# [1] ""

所以可以写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 "

Answer 2

我认为没有一种简单/简单/清晰的方法可以实现您想要的。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 放入 SQL 语句中，然后为每个值添加 AND column = value。这样，如果你漏掉一个，语法仍然是正确的。

我看到@Marek 提供了带有代码的答案；也许您可以结合这些答案来产生一个可行的解决方案。