Cannot use a Like query in a JDBC prepared statement?

Question

OK, first the query code and query:

ps = conn.prepareStatement("select instance_id, ? from eam_measurement where resource_id in (select RESOURCE_ID from eam_res_grp_res_map where resource_group_id = ?) and DSN like '?' order by 2");
ps.setString(1,"SUBSTR(DSN,27,16)");
ps.setInt(2,defaultWasGroup);
ps.setString(3,"%Module=jvmRuntimeModule:freeMemory%");
rs = ps.executeQuery();
while (rs.next()) { bla blah blah blah ...

Returns an empty resultSet.

Through basic debugging I have found its the 3rd bind that is the problem i.e. DSN like '?'

I have tried all kinds of variations, the most sensible of which seemed to be using:

DSN like concat('%',?,'%')

bit that doesn' work as I am missing the ' ' either side of the concatenated string so I try

DSN like ' concat('%',Module=P_STAG_JDBC01:poolSize,'%') ' order by 2

but I just can't seem to find a way to get them in that works.

What am I missing? :) Thanks!

Answer 1

Omit the ' around the ?. Without the ', ? is a placeholder for a parameter. With it, it's an SQL string (i.e. the same as "?" in Java).

Then you must concatenate the string on the Java side; you can't pass SQL functions as parameters to queries; only basic values (like string, integer, etc) because the JDBC driver will convert the parameter to the SQL type the database expects and it cannot execute SQL functions in this step.

Answer 2

Hi SeerUK,

There are two problems with your statement. You have to understand how bind variables work. The query is not processed by substituing the characters ? with your parameters. Instead, the statement is compiled with placeholders and then, during execution, the actual values of the parameters are given to the DB.

In other words, you parse the following query:

SELECT instance_id, :p1
  FROM eam_measurement
 WHERE resource_id IN (SELECT RESOURCE_ID 
                         FROM eam_res_grp_res_map 
                        WHERE resource_group_id = :p2)
   AND DSN LIKE '?'
 ORDER BY 2

I'm pretty sure the last parameter will be ignored because it is in a delimited character string. Even if it is not ignored, it does not make sense to have ' characters around because Oracle won't bind a parameter in a string (I'm surprised it hasn't raised any error, do you catch exceptions ?).

Now if you replace your DNS LIKE '?' with DSN LIKE ? and bind "%Module=jvmRuntimeModule:freeMemory%" this will make sense and should return the correct rows.

You still have the problem with your first parameter, it won't do what you expect, i-e the query that will be executed will be equivalent to the following query:

SELECT instance_id, 'SUBSTR(DSN,27,16)'
  FROM ...

which is not at all the same as

SELECT instance_id, SUBSTR(DSN,27,16)
  FROM ...

I would suggest parsing (=prepareStatement) the following query if you expect the SUBSTR to be dynamic:

SELECT instance_id, SUBSTR(DSN,?,?)
  FROM eam_measurement
 WHERE resource_id IN (SELECT RESOURCE_ID 
                         FROM eam_res_grp_res_map 
                        WHERE resource_group_id = ?)
   AND DSN LIKE ?
 ORDER BY 2

Answer 3

First, the PreparedStatement placeholders (those ? things) are for column values only, not for table names, column names, SQL functions/clauses, etcetera. Better use String#format() instead. Second, you should not quote the placeholders like '?', it would only malform the final query. The PreparedStatement setters already do the quoting (and escaping) job for you.

Here's the fixed SQL:

private static final String SQL = "select instance_id, %s from eam_measurement"
    + " where resource_id in (select RESOURCE_ID from eam_res_grp_res_map where"
    + " resource_group_id = ?) and DSN like ? order by 2");

Here is how to use it:

String sql = String.format(SQL, "SUBSTR(DSN,27,16)"); // This replaces the %s.
preparedStatement = connection.prepareStatement(sql);
preparedStatement.setInt(1, defaultWasGroup);
preparedStatement.setString(2, "%Module=jvmRuntimeModule:freeMemory%");

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See also:

• Sun JDBC tutorial: Using Prepared Statements
• Format string syntax