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As with the other SQL APIs and embedded SQL, query processing requires an additional mechanism beyond those used for database updates to handle the returned query results. In JDBC, the ResultSet object provides that additional mechanism. To execute a simple query, a Java program invokes the executeQuery() method of a Statement object, passing the text of the query in the method call. The executeQuery() method returns a ResultSet object that embodies the query results. The Java program then invokes the methods of this ResultSet object to access the query results, row by row and column by column. Table 19-17 shows the methods provided by the ResultSet object.
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Moves cursor to next row of query results Ends query processing; closes the cursor
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Basic column-value retrieval getInt() getShort() getLong() getFloat() getDouble() getString() getBoolean() getDate() getTime() getTimestamp() getByte() getBytes() getObject() Large object retrieval getAsciiStream() Gets input stream object for processing a character large object (CLOB)column Retrieves integer value from specified column Retrieves short integer value from specified column Retrieves long integer value from specified column Retrieves floating point numeric value from specified column Retrieves double-precision floating point value from specified column Retrieves character string value from specified column Retrieves true/false value from specified column Retrieves date value from specified column Retrieves time value from specified column Retrieves timestamp value from specified column Retrieves byte value from specified column Retrieves fixed-length or variable-length BINARY data from specified column Retrieves any type of data from specified column
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GetBinaryStream() Gets input stream object for processing a binary large object (BLOB) column Other functions getMetaData() getWarnings() Table 19-17. Returns a ResultSetMetaData object with meta data for query Retrieves SQL warnings associated with the ResultSet
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JDBC ResultSet Object Methods
19:
SQL APIs
Here is a very simple Java program excerpt that shows how the objects and methods you have seen so far combine to perform simple query processing. It retrieves and prints out the office number, city, and region for each office in the OFFICES table:
// The connection object, strings and variables Connection dbconn; // the database connection Int num; // returned office number String city; // returned city String reg; // returned region String str1 = SELECT OFFICE, CITY, REGION FROM OFFICES ;
<code in here creates the connection>
// Create a statement object for executing the query Statement stmt = dbconn.createStatement(); // Carry out query method returns a ResultSet object ResultSet answer = stmt.executeQuery(str1); // Loop through ResultSet a row at a time while (answer.next()) { // Retrieve each column of data num = answer.getInt( OFFICE ); city = answer.getString( CITY ); reg = answer.getString(3); // Print the row of results System.out.println(city + } // Explicitly close the cursor and connection answer.close(); dbconn.close();
+ num +
+ reg);
The methods used are straightforward and parallel the query-processing steps already seen for embedded SQL and C/C++ APIs. The ResultSet object maintains a cursor to note its current position within the query results. Its next method advances the cursor, row by row, through them. There is an explicit JDBC get method call to retrieve each column of data for each row. Java s strong typing and memory-protection schemes make this approach a requirement, but it carries significantly higher overhead than the C/C++ approach of binding program variables and having the database API automatically populate those variables when a next row is fetched. Finally, the close method call ends query processing.
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The example also shows the two alternative methods for specifying which column s value should be retrieved by each get method call. You can specify the name of the column to be retrieved (used for the OFFICE and CITY columns), or its ordinal position within the columns of results (used for the REGION column). JDBC delivers this capability by overloading each of the get methods one version takes a string (column name) argument; the other takes an integer (column number) argument.
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