Databases: Queries with Java Dr Andy Evans

JDBC SQL Three methods: Statements: Standard, simple, SQL. PreparedStatements: Compiled SQL statements that are altered to new data through input parameters. Useful, for example, in looped structures. CallableStatements: for SQL procedures stored in the database. Produce ResultSet objects.

Example: Select Statement st = con.createStatement(); ResultSet rs = st.executeQuery ("SELECT a,b,c FROM Table1"); Three different execution methods: executeQuery : simple SQL queries. executeUpdate : anything that changes or creates a Table, e.g. UPDATE. Returns number of rows effected. execute : Complex, multi-return queries. st.close(); polite!

Example: Creating tables String createTable = "CREATE TABLE Results (" + "Address varchar(255)," + "Burglaries int" + ")"; Statement st = null; try { st = conn.createStatement(); st.execute (createTable); } catch (SQLException ex) { ex.printStackTrace(); }

Auto-commit You can run multiple queries on the same statement. By default Connection objects are set to auto-commit – all changes are solidified after single statements are run. conn.setAutoCommit(booleanAutoCommit); If set to false (off) the changes will only be solidified when commit called: conn.commit(); Until then you can rollback all changes since the last commit, by calling: conn.rollback(); Also options to setup and rollback to savepoints.

Escape characters Remember that some characters, like “ _ ”, are wildcards. If we want to use these literally, they need a backslash infront of them “ \_ ”. However, backslash is a String escape character in Java, so you can define what is a special escape character to ignore in statement execution: stmt.executeQuery( "SELECT name FROM Table1 WHERE Id LIKE '\_%' {escape '\'}"); In some cases the escape occurs in a table name, in which case treat literally by enclosing in [] e.g. [Sheet1$]

ResultSets Links to a cursor on the database and converts data to Java types. ResultSet rs = st.executeQuery ("SELECT a, b, c FROM Table1"); while (rs.next()) { int i = rs.getInt("a"); or String s = rs.getString("b"); or Object o = rs.getObject("c"); } Use object if unsure, and cast. A new SQL query will close the current ResultsSet. Can also use column index number (starting 1).

ResultSets Standard results sets only let you read from beginning to end a line at a time. If you want to write to the data (without using SQL UPDATE) or go back and forwards, you need a scrollable statement. Statement st = conn.createStatement( ResultSet.TYPE_SCROLL_SENSITIVE, ResultSet.CONCUR_UPDATABLE, ResultSet.HOLD_CURSORS_OVER_COMMIT); Only need to worry about this if you’re not auto- committing.

Scrollable ResultSet rs.beforeFirst(); rs.afterLast(); rs.next() rs.previous() Looping: while (!rs.isAfterLast()) { Finding number of rows: rs.last(); int numberOfRows = rs.getRow();

Scrollable ResultSet st.setFetchSize(25); Fetch 25 rows at a time. rs.absolute(2); Move to row 2 rs.updateInt(3, 10); Update col 3 of current row. rs.updateInt("Name",10); Update by col name. also updateObject, updateString, updateFloat and others. rs.updateRow(); Must call this after updating data in all the columns you want to change for that row. You also need to call rs.close() to commit all changes.

Inserting a row rs.moveToInsertRow(); rs.updateString(1, "Bob"); rs.updateObject(2, someObject); rs.insertRow(); rs.first(); (In general the cursor points at the row used prior to insert.) st.commit(); If needed. rs.close();

Database metadata Especially useful in debugging is getting metadata about the database: DatabaseMetaData md = conn.getMetaData(); ResultSet rs = md.getTables( null, null, "%", null); while (rs.next()) { System.out.println(rs.getString(3)); }

Further info Online guide: art/GettingStartedTOC.fm.html