Android 检查游标是否有结果
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Check if cursor has results
提问by Faas
I am using a database to store date and want a single column returned based on which id is given in the where claus.
我正在使用数据库来存储日期，并希望根据 where 子句中给出的 id 返回单个列。
public String getPmax(String serial) {
String[] columns = new String[]{KEY_SERIAL, KEY_PMAX};
Cursor c = ourDatabase.query(DATABASE_TABLE, columns, "serial = " + serial,
null, null, null, null);
String result = "nothing found";
if(c != null) {
int iPmax = c.getColumnIndex(KEY_PMAX);
c.moveToFirst();
result = c.getString(iPmax);
}
return result;
}
This code works if I use a serial that is in the database but it gives me a force close when i am using a serial that's not in the database!
如果我使用数据库中的序列号，则此代码有效，但是当我使用不在数据库中的序列号时，它会强制关闭！
it says:
它说：
04-11 16:31:19.423: E/AndroidRuntime(21226): java.lang.RuntimeException: Unable to start activity ComponentInfo{nl.janjanus.solardetect/nl.janjanus.solardetect.DatabaseView}: android.database.sqlite.SQLiteException: no such column: g: , while compiling: SELECT serial, pmax FROM SolareCel WHERE serial = g
So my question is: How do I check if there is result? or do I need to check if the serial given is a serial that is in the database?
所以我的问题是：我如何检查是否有结果？还是我需要检查给定的序列号是否是数据库中的序列号？
回答by Samir Mangroliya
if (!(mCursor.moveToFirst()) || mCursor.getCount() ==0){
//cursor is empty
}
回答by j0ntech
If you look closely, the error is that the SQL is faulty.
It reads the gas a column. You have to encase it in apostrophes like so 'g'.
如果仔细观察，错误是 SQL 有问题。它将 读g作一列。你必须像这样用撇号包裹它'g'。
In your case, the line where you get the cursor should be:
在您的情况下，您获得光标的行应该是：
Cursor c = ourDatabase.query(DATABASE_TABLE, columns, "serial = '" + serial + "'",
null, null, null, null);
As to checking whether the cursor has no results, look at Samir Mangroliya's answer.
至于检查光标是否没有结果，看Samir Mangroliya的回答。
回答by DGomez
You can check if the Cursor has elements:
您可以检查 Cursor 是否有元素：
boolean isEmpty = cursor.getCount() < 1;
AFAIK, the Cursor never returns null when there is no row which the requested conditions, just return a empty Cursor.
AFAIK，当没有请求条件的行时，Cursor 永远不会返回 null，只返回一个空的 Cursor。
回答by Joe
int x = cursor.getCount(); //this will return number of records in current cursor
if ( x == 0 ) {
// there are no records
} else {
// do your stuff
}
