Retrieving Data withSQL Queries - Java Database Programming Bible - page 236

Correlated queries are executed repeatedly (once for each row of the table identified in the outer-level
query), so they can be extremely inefficient. It is frequently worthwhile to rewrite correlated queries as
joins wherever possible, though in some cases the SQL engine may be able to optimize the correlated
subquery.

The next section explains how the SQL queries discussed in this chapter can be used in a JDBC
application.

JDBC ResultSets

The JDBC ResultSet holds the data, arranged in rows and columns, returned by a query. A
ResultSet maintains a cursor that points to the current row of data. The cursor moves down one row
each time the next() method is called. You access the data by sequencing through the rows and
requesting data from the columns using getter methods, either by column name or by column
number. In general, using the column number will be more efficient than using the column name .

Caution

Columns are numbered from 1, not from 0.

The JDBC ResultSet provides getter methods that convert column data from SQL data types to the
specified Java types. Each getter method comes in these two flavors:

getXXx(String columnName)

getXXX(int columnNumber)

For clarity only one variant is shown in the getter method summary in Table 7-6 .
Table 7-6: ResultSet getter Methods

Data Type Method

getBigDecimal(String columnName, int scale)

BigDecimal

boolean getBoolean(String columnName)

byte getByte(String columnName)

getBytes(String columnName)

byte[]

getDouble(String columnName)

double

getFloat(String columnName)

float

int getInt(String columnName)

java.io.InputStream getAsciiStream(String columnName)

getUnicodeStream(String columnName)

java.io.InputStream

getBinaryStream(String columnName)

java.io.InputStream

getDate(String columnName)

java.sql.Date

java.sql.Time getTime(String columnName)