在 sqlite (Swift) 中选择 count(col_name) 不起作用 [英] Select count(col_name) in sqlite (Swift) not working

问题描述

我正在使用 sqlite 包装器此处在 Swift 中访问我的 sqlite 数据库.

I am using the sqlite wrapper here to access my sqlite database in Swift.

我根据包装器编写了这段代码来计算桌子上的项目.

I wrote this code based on the wrapper to count the items on a table.

public func numberOfRows() -> Int {

        let cmd = """
        SELECT count(*) from  \(self.tableName)
        """

        do{

            let db = DatabaseUtils.getInstance().getConnection()


            let stmt = try db.prepare( cmd )


            if try stmt.step() == .row {
                return Int(from: stmt,index:  1) ?? -1
            }

            return -1

        }catch {
            Log.out("Error occurred while fetching count data in \(tableName)")
            return -1
        }

    }

但是当找不到计数时，它会按照逻辑指示不断返回 -1.

But it keeps returning -1 as the logic instructs when the count is not found.

到目前为止，其他一切都有效；创建表、插入、批量插入.

Every other thing works so far; create tables, inserts, batched inserts.

还有一个 select count(col_name) from table_name where some_col = "?" 也有效.

问题:

我如何补救它以使其工作

How do I remedy it to work for

  SELECT count(*) from  table_name?

推荐答案

返回值时的列号(例如 sqlite3_column_xxx) 从零开始.您正在尝试检索列 1，而您的意思是列 0.

The column numbers when returning values (e.g. the result of sqlite3_column_xxx) are zero-based. You’re attempting to retrieve column 1, when you meant column 0.

顺便说一句，如果你想检测无效索引，你可以删除那个 nil 合并运算符.例如，所以，而不是:

By the way, if you want to detect invalid index, you can remove that nil coalescing operator. E.g., so, instead of:

if try stmt.step() == .row {
    return Int(from: stmt,index:  1) ?? -1
}

您可以改为检查是否不仅 step 成功，而且 Int(from:index:) 返回非 nil代码>值:

You could instead check to see if not only if step succeeded, but also that Int(from:index:) returned a non-nil value:

if try stmt.step() == .row {
    if let count = Int(from: stmt, index: 1) {
        return count
    } else {
        // do whatever you want when it fails, e.g.

        fatalError("no value returned; invalid column index?")
    }
}

或者更简洁:

if try stmt.step() == .row, let count = Int(from: stmt, index: 1) {
    return count
} else {
    fatalError("no value returned; invalid column index?")
}

这会让您认识到您需要从 0 开始的索引:

That would lead you to recognize that you need the 0-based index:

if try stmt.step() == .row, let count = Int(from: stmt, index: 0) {
    return count
} else {
    fatalError("no value returned; invalid column index?")
}

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