尝试将Java变量传递到sql字符串中

Question

I've had a look around on the web but can't seem to find a definite answer to my question.

Basically, I have a database and table that are successfully working. Now I want to read each line from my table one by one and store the result into a array and I am trying to use a for loop to be more professional rather then using repetition.

I have this code

for (int i=1; i<=8; i++)
{
    String query = "Select * FROM Table1 WHERE ID = i";
    Rs = St.executeQuery(query);
    COL1Title[i] = Rs.getString("CO1Name");
    COL2Age[i] = Rs.getString("CO2Rating");
}

The for loop is in a try catch statement and it's complaining with the error "Unknown column 'i' in 'where clause'"

Im guessing there's a certain way for how variable i is to be inserted in the the query.

I should point out ID is a column that has the auto increment feature added on and is primary key if that helps

Could anyone help me out here?

Answer 1

First, we can simplify the task be executing a single query. Note the addition of the range limit and the ORDER BY - without an ORDER BY the results have an unspecified order!

PreparedStatement stmt = "Select ID, CO1Name, CO2Rating"
    + " FROM Table1"
    + " WHERE ID >= ? AND ID <= ?"
    + " ORDER BY ID";

And bind in placeholders (unless there is good reason otherwise, always use placeholders when injecting data into a query). The values could have been hard-coded above in this case, just as they are hard-coded in the for-loop, but the binding is shown here for future reference:

stmt.setInt(1, 1);
stmt.setInt(2, 8);

Then execute the query:

ResultSet rs = stmt.executeQuery();

And iterate the results. Note that rs.next() must be invoke once before any column is read (the cursor starts before any records) and, in this case, it makes it easy to handle a bunch of results.

while (rs.next()) {
   int id = rs.getInt("ID");
   String title = rs.getString("CO1Name");
   String name = rs.getString("CO2Rating");
   // do stuff with this record
}

Note that even though the ORDER BY guarantees that the results are iterated in order of ID, assuming a database cardinality rule ensures each result has a unique ID, there may be 0 to 8 records returned - that is, non-existent records may need to be detected/handled separately.

Also (but not shown), make sure to cleanup ( close ) the ResultSet when done: use a try/finally or try-with-resources construct.

Answer 2

您需要以整数形式将i传递给字符串，并替换为：

String query = String.format("Select * FROM Table1 WHERE ID = %d",i);