How do I get the row count in JDBC?

问题

I've executed a JDBC query to obtain a resultset. Before iterating over it, I'd like to quickly find out how many rows were returned. How can I do this with high performance?

I'm using Java 6, Oracle 11g, and the latest Oracle JDBC drivers.

回答1:

You're going to have to do this as a separate query, for example:

SELECT COUNT(1) FROM table_name

Some JDBC drivers might tell you but this is optional behaviour and, more to the point, the driver may not know yet. This can be due to how the query is optimised eg two example execution strategies in Oracle are to get all rows as quickly as possible or to get the first row as quickly as possible.

If you do two separate queries (one a count and the other the query) then you'll need to do them within the same transaction. This will work well on Oracle but can be problematic on other databases (eg SQL Server will either show you uncommitted data or block on an external uncommitted update depending on your isolation level whereas Oracle supports an isolation level that gives you a transactionally consistent view of the data without blocking on external updates).

Normally though it doesn't really matter how many rows there are. Typically this sort of query is either batch processed or paged and either way you have progress information in the form of rows loaded/processed and you can detect the end of the result set (obviously).

回答2:

Short answer: you can't.

Long answer: you can't, partly because the database may be lazily evaluating the query, only returning rows as you ask for them.

EDIT: Using a scrollable ResultSet you can :)

Indeed, I asked this very question in the Java databases newsgroup a long time ago (back in 2001!) and had some helpful responses.

回答3:

ResultSet rs = stmt.executeQuery(sql);
int rowCount = rs.last() ? rs.getRow() : 0; // Number of rows in result set. Don't forget to set cyrsor to beforeFirst() row! :)

回答4:

To get the number of rows from JDBC:

ResultSet rs = st.executeQuery("select count(*) from TABLE_NAME");
rs.next();
int count = rs.getInt(1);

回答5:

If your driver supports it(!), you can call ResultSet.afterLast() ResultSet.getRow() ResultSet.beforeFirst(). Performance may or may not be good.

A better solution would be to rewrite your algorithm not to require the size up front.

回答6:

Without ternary operator

rs.last();  // Moves the cursor to the last row in this ResultSet object.
int rowCount = rs.getRow();  //Retrieves the current row number.
rs.beforeFirst(); //Moves the cursor to the front of this ResultSet object,just before the first row.

With ternary operator one line

int rowCount = rs.last() ? rs.getRow() : 0; 
rs.beforeFirst();

回答7:

Code:

//Create a Statement class to execute the SQL statement
Statement stmt = con.createStatement();

ResultSet rs = stmt.executeQuery("SELECT COUNT(*) AS COUNT FROM
TABLENAME");

 while(rs.next()) {
    System.out.println("The count is " + rs.getInt("COUNT"));
 }

 //Closing the connection
 con.close();

来源：https://stackoverflow.com/questions/162571/how-do-i-get-the-row-count-in-jdbc

标签

java

jdbc

resultset