qr barcode generator vb.net JDBC Statement Object Methods in Software

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JDBC Statement Object Methods
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Note that the Connection and Statement methods called in this program excerpt can cause errors, and the excerpt does not show any error-handling code. If an error occurs, the JDBC driver will throw a SQLException exception. Normally, an excerpt like the previous one (or parts of it) will appear within a try/catch structure to handle the possible exception. For simplicity, the enclosing try/catch structure is suppressed in this and the next several examples. Error-handling techniques are described in the Error Handling in JDBC section later in this chapter.
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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 some of the methods provided by the ResultSet object. 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 the office number, city, and region for each office in the OFFICES table:
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// 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);
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Method Cursor motion next() close() getInt() getShort() getLong() getFloat() getDouble() getString() getBoolean() getDate() getTime() getTimestamp() getByte() getBytes() getObject() Large object retrieval getAsciiStream() GetBinaryStream() Other functions getMetaData() getWarnings()
TABLE 19-17
Description Moves cursor to next row of query results Ends query processing; closes the cursor 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 Gets input stream object for processing a character large object (CLOB) column Gets input stream object for processing a binary large object (BLOB) column Returns a ResultSetMetaData object with metadata for query Retrieves SQL warnings associated with the ResultSet
Basic column-value retrieval
JDBC ResultSet Object Methods
The methods used are straightforward and they 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 the next row is fetched. Finally, the close() method call ends query processing.
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