Let the result set metadata help you. It provides important information about the structure and content of the results.
Metadata information is valuable for formatting query results,
because it tells you several important things about the columns (such
as the names and display widths), even if you don’t know what the
query was. For example, you can write a general-purpose function that
displays a result set in tabular format with no knowledge about what
the query might have been. The following Java code shows one way to do
this. It takes a result set object and uses it to get the metadata for
the result. Then it uses both objects in tandem to retrieve and format
the values in the result. The output is similar to that produced by
mysql: a row of column headers
followed by the rows of the result, with columns nicely boxed and
lined up vertically. Here’s a sample of what the function displays,
given the result set generated by the query
SELECT
id,
name,
birth
FROM
profile
:
+----------+--------------------+----------+ |id |name |birth | +----------+--------------------+----------+ |1 |Fred |1970-04-13| |2 |Mort |1969-09-30| |3 |Brit |1957-12-01| |4 |Carl |1973-11-02| |5 |Sean |1963-07-04| |6 |Alan |1965-02-14| |7 |Mara |1968-09-17| |8 |Shepard |1975-09-02| |9 |Dick |1952-08-20| |10 |Tony |1960-05-01| |11 |Juan |NULL | +----------+--------------------+----------+ Number of rows selected: 11
The primary problem an application like this must solve is to
determine the proper display width of each column. The
getColumnDisplaySize()
method returns
the column width, but we actually need to take into consideration
other pieces of information:
The length of the column name has to be considered (it might be longer than the column width).
We’ll print the word “NULL” for NULL
values, so if the column can
contain NULL
values, the
display width must be at least four.
The following Java function, displayResultSet()
, formats a result set, taking the preceding factors into
account. It also counts rows as it fetches them to determine the row
count, because JDBC doesn’t make that value available directly from
the metadata.
public static void displayResultSet (ResultSet rs) throws SQLException { ResultSetMetaData md = rs.getMetaData (); int ncols = md.getColumnCount (); int nrows = 0; int[] width = new int[ncols + 1]; // array to store column widths StringBuffer b = new StringBuffer (); // buffer to hold bar line // calculate column widths for (int i = 1; i <= ncols; i++) { // some drivers return -1 for getColumnDisplaySize(); // if so, we'll override that with the column name length width[i] = md.getColumnDisplaySize (i); if (width[i] < md.getColumnName (i).length ()) width[i] = md.getColumnName (i).length (); // isNullable() returns 1/0, not true/false if (width[i] < 4 && md.isNullable (i) != 0) width[i] = 4; } // construct +---+---... line b.append ("+"); for (int i = 1; i <= ncols; i++) { for (int j = 0; j < width[i]; j++) b.append ("-"); b.append ("+"); } // print bar line, column headers, bar line System.out.println (b.toString ()); System.out.print ("|"); for (int i = 1; i <= ncols; i++) { System.out.print (md.getColumnName (i)); for (int j = md.getColumnName (i).length (); j < width[i]; j++) System.out.print (" "); System.out.print ("|"); } System.out.println (); System.out.println (b.toString ()); // print contents of result set while (rs.next ()) { ++nrows; System.out.print ("|"); for (int i = 1; i <= ncols; i++) { String s = rs.getString (i); if (rs.wasNull ()) s = "NULL"; System.out.print (s); for (int j = s.length (); j < width[i]; j++) System.out.print (" "); System.out.print ("|"); } System.out.println (); } // print bar line, and row count System.out.println (b.toString ()); System.out.println ("Number of rows selected: " + nrows); }
If you want to be more elaborate, you can also test whether a
column contains numeric values, and format it as right-justified if
so. In Perl DBI scripts, this is easy to check, because you can access
the mysql_is_num
metadata
attribute. For other APIs, it is not so easy unless there is some
equivalent “column is numeric” metadata value available.
If not, you must look at the data-type indicator to see whether it’s
one of the several possible numeric types.
Another shortcoming of the displayResultSet()
function is that it
prints columns using the width of the column as specified in the table
definition, not the maximum width of the values actually present in
the result set. The latter value is often smaller. You can see this in
the sample output that precedes the listing for displayResultSet()
. The id
and name
columns are 10 and 20 characters wide,
even though the widest values are only two and seven characters long,
respectively. In Perl, Ruby, PHP, and DB-API, you can get the maximum
width of the values present in the result set. To determine these
widths in JDBC, you must iterate through the result set and check the
column value lengths yourself. This requires a JDBC 2.0 driver that
provides scrollable result sets. If you have such a driver (MySQL
Connector/J is one), the column-width calculation code in the displayResultSet()
function can be
modified as follows:
// calculate column widths for (int i = 1; i <= ncols; i++) { width[i] = md.getColumnName (i).length (); // isNullable() returns 1/0, not true/false if (width[i] < 4 && md.isNullable (i) != 0) width[i] = 4; } // scroll through result set and adjust display widths as necessary while (rs.next ()) { for (int i = 1; i <= ncols; i++) { byte[] bytes = rs.getBytes (i); if (!rs.wasNull ()) { int len = bytes.length; if (width[i] < len) width[i] = len; } } } rs.beforeFirst (); // rewind result set before displaying it
With that change, the result is a more compact query result display:
+--+-------+----------+ |id|name |birth | +--+-------+----------+ |1 |Fred |1970-04-13| |2 |Mort |1969-09-30| |3 |Brit |1957-12-01| |4 |Carl |1973-11-02| |5 |Sean |1963-07-04| |6 |Alan |1965-02-14| |7 |Mara |1968-09-17| |8 |Shepard|1975-09-02| |9 |Dick |1952-08-20| |10|Tony |1960-05-01| |11|Juan |NULL | +--+-------+----------+ Number of rows selected: 11
3.20.224.107