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-commitYou 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 charactersRemember 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:http://docs.oracle.com/javase/1.4.2/docs/guide/jdbc/getstart/GettingStartedTOC.fm.html