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Dynamic SQL Query in Java

I have a function
public void executeMyQuery(Connection con) {
PreparedStatement ps = con.prepareStatement("SELECT * FROM STUDENT WHERE ID = ?");
ps.setInt(1, 7);
ps.executeQuery();
}
if i will run this it will work fine. But I want to do like this.
if I will setInt it should include WHERE clause. (returns matched
row)
if I don't setInt it should exclude WHERE clause. (returns whole table)
Or is there is any way to dynamically remove or modify the string after WHERE Clause.

Disadvantages of using string based SQL for dynamic SQL
Other answers have shown how to achieve dynamic SQL using string based JDBC usage. There are many disadvantages to building SQL strings dynamically using string concatenation, including:
High risk of SQL injection if you accidentally concatenate user input to your SQL queries
Difficult to avoid syntax errors in non-trivial cases, when dynamic SQL gets more complex
Also, when you're using plain JDBC (which only supports indexed bind variables), rather than some utility such as Spring JdbcTemplate, MyBatis, jOOQ, etc. you will have to manually match ? placeholders with their corresponding indexes, which is another subtle source of errors.
Using a query builder
At some point, when you implement dynamic SQL queries more often, query builders will definitely help. The most popular ones are:
jOOQ (for dynamic SQL querying)
JPA Criteria Query (for dynamic JPAL querying)
There are many other options that are more or less well maintained. For very trivial cases (like the one in your question), you can also build your own simple predicate builder.
Disclaimer: I work for the company behind jOOQ.

You have to build your query dynamically, at the beginning of the method check whether id is null or equal 0. To make it easier you can use trick in where clause with 1=1 so where clause can be included all the time in the query.
public void executeMyQuery( Connection con, Integer id) {
String query = "SELECT *FROM STUDENT WHERE 1=1";
if(id != null){
query += "AND ID = ?";
}
PreparedStatement ps = con.prepareStatement(query);
if(id != null){
ps.setInt(1, id);
}
ps.executeQuery();}

ifNull/Coalesce work nicely for this, if you pass a null, it will select where the field equals itself.
SELECT *
FROM STUDENT
WHERE 1 = 1
and ID = ifNull(:ID, ID)
I'd also suggest something other than using ? for your variables, fine when you a couple but as you get a ton of them, difficult to keep track or modify. I've found https://github.com/dejlek/jlib/blob/master/src/main/java/com/areen/jlib/sql/NamedParameterStatement.java pretty easy, made a few modifications to fit my particular needs but SQL is much easier to read and doing substitutions in intellij db console are much easier when developing the SQL.

You can have two PreparedStatements defined in your program - one without the WHERE ID = ? clause, and another one with it.
Moreover, you are supposed to keep your PreparedStatements and re-use, so you better store them as a field, etc.
And then, when needing to get the data - call either the first prepared statement, or the second one.

Michael Dz is close to the solution in his answer, but there is a problem in the code : he calls setInt on a non existing preparedStatement.
Try something like this :
public void executeMyQuery( Connection con, int Id) {
StringBuffer sql = new StringBuffer();
sql.append("Select * FROM STUDENT");
if(Id > -1) {
sql.append(" Where ID = ?");
}
preparedStatement ps = con.prepareStatement(sql.toString());
if(ID > -1) {
ps.setInt(1, Id);
}
ps.executeQuery(); // You might want to catch the result of the query as well
}
Hope this helps !

Appscan source edition - SQL Injection

I am using Appscan source edition for Java Secure Coding. It is reporting an SQL injection in my application. The issue is that we are generating the query dynamically in code so I cannot use a prepared statement. Instead I have to us e Esapi.encoder().encodeForSql(new OracleCodec(), query). AppScan does not consider this to mitigate the SQL injection issue.
final String s = "SELECT name FROM users WHERE id = " +
Esapi.encoder().encodeForSql(new OracleCodec(), userId);
statement = connection.prepareStatement(s);
This code additionally does not work for ESAPI.encoder()
How can I resolve this issue?

what you should do is
final String s = "SELECT name FROM users WHERE id = ?"
statement = connection.prepareStatement(s);
statement.setString(1, userId);

The documentation for encodeForSQL recommends a PreparedStatement which you can still use which dynamically generated queries:
Encode input for use in a SQL query, according to the selected codec
(appropriate codecs include the MySQLCodec and OracleCodec). This
method is not recommended. The use of the PreparedStatement interface
is the preferred approach. However, if for some reason this is
impossible, then this method is provided as a weaker alternative. The
best approach is to make sure any single-quotes are double-quoted.
Another possible approach is to use the {escape} syntax described in
the JDBC specification in section 1.5.6. However, this syntax does not
work with all drivers, and requires modification of all queries.
Let's check what the encoder is doing to see why your code have an injection vulnerability. The encoder calls encodeCharacter in the oracle codec which simply replaces single quotes with two single quotes:
public String encodeCharacter( char[] immune, Character c ) {
if ( c.charValue() == '\'' )
return "\'\'";
return ""+c;
}
This only makes sense if the value is inside single quotes, which string values would be. If id is actually an integer and you wanted to concatenate it with the query, then you would convert it to an int type first instead of using this encoder.

Does Hibernate's createCriteria() sanitize input?

Came across some code today that uses Hibernate to perform a query. The query uses a value submitted from a form. It made me curious as to whether or not this sort of code "sanitizes" its input.
public List<School> search(String query) {
Session session = this.getCurrentSession();
query = "%" + query + "%";
Criteria criteria = session.createCriteria(getPersistentClass());
criteria.createAlias("country", "a");
Criterion nameCriterion = Restrictions.ilike("name", query);
Criterion cityCriterion = Restrictions.ilike("city", query);
Criterion countryCriterion = Restrictions.ilike("a.name", query);
Criterion criterion = Restrictions.or(Restrictions.or(nameCriterion, cityCriterion), countryCriterion);
criteria.add(criterion);
return criteria.list();
}
Is this safe?

Hibernate Criteria Queries are quiet safe in terms of Sql Injection since they pass strings as parameter while performing any fetch. Even, Hql is quiet safe unless you build the query via string literal.
For more details, you should take a look at queries getting fired at the database level by switching on hibernate sql logging.

If you think to SQL injection attacks, then yes, Hibernate Criteria API is safe.
It will generate the underlying query by first compiling it from the specified query fields and only after apply the query parameters (It should use a classical PreparedStatement). This way the JDBC driver will know which part of the query are fields and which part are parameters. Then the driver will take care to sanitize the parameters.
Tough you should take care with the SQL restrictions applied on the Criteria, if you need to place parameters there. For example
String vulnerable = //parameter from user interface
criteria.add(
Restrictions.sqlRestriction("some sql like + vulnerable") //vulnerable
criteria.add(
Restrictions.sqlRestriction("some sql like ?",
vulnerable, Hibernate.STRING)) //safe
In this case the vulnerable parameter could "leak" in to the query fields part and be bypassed by JDBC driver checking as in a normal vulnerable SQL query.

Hibernate is useful to sanitizing inputs but sanitizing inputs is not considered the best practice for preventing SQL injection attacks. As your code develops over time, you will need to remember to change your Hibernate sanitation as your database and client-side application change; this leaves a lot of room for error and any one mistake can compromise your database.
To prevent SQL injection attacks, it is better to use prepared statements. In a prepared statement, your client-side application will make a non-SQL request and let your server generate your SQL statement.
For example, if a user wants all users in the city "Dallas" then your client-side application should make a request similar to username equals "Dallas" and then your server can generate:
SELECT * FROM users WHERE name='Dallas'

using java variable in sql statement

I'm trying to build a web page to better learn Java and SQL. My question is, is there a way in Java to make a generic SQL select statement? For example:
SELECT var1 FROM var2 WHERE var3=var4
or something of the sort.
My idea is to fill the vars with user selected items from the web page. I know this can be done in PHP using the Post method, but I'm not using PHP. Also, I've read about the Prepared Statement in Java, but seems only to work when the used after the comparison operator; ex:
SELECT * FROM table Where attr = ? &
Also, I do know i can do the hard coded version of "SELECT " + var1 + "FROM " + var2 + "WHERE attr = " + var3 + " " but that doesn't seem very generic and prone to a lot of errors.
Incase: I'm trying to build this test page using HTML & JSP.

What you are doing with the ? is parameterizing the query. The query can only be parameterized for values not names of tables or columns.
Every time you run a query. The database has to create a query plan. If you are running the same query again and again, you can reduce this overhead by creating a PreparedStatement.
The first execution of PreparedStatement will generate the query plan. The subsequent executions will reuse the same plan.
Same query here means, it is identical in all respects except values used in where clause, expressions etc.
If you change the Column or Table name or modify the structure of the query, then it is a different query and will require a different query plan. A PreparedStement is not useful in this case and you should stick to the hardcoded version you talked about. Because of this reason you will get an error if you try to parameterize Table or Column names in PreparedStement.
Having said that. It is not advisable to take such a generic approach for queries. If your queries are that simple, you can benefit from ORM tools. You would not have to maintain even a line of SQL. For complex queries you have an option of using ORM specific query language or JPQL or Native SQL. Look for JPA + Hibernate

Your specific usage is not permitted by JDBC. You need to hard code the table name when creating the prepared statement. If you really do want to do that I suggest you use String concatenation to create the SQL statements and then create a PreparedStatement with parameters to handle the where part. In case you are wondering why bother with PreparedStatements in the specific solution, it's to avoid SQL injection.

You can use PreparedStatement to achive your objective.
For example -
String query = "SELECT * FROM table Where attr = ?";
PreparedStatement pt = con.prepareStatement(query);
pt.setString(1, attribete);
pt.executeUpdate();

There is no such direct provision in any of SQL packaged classes or others to replace table, column names along with query parameter values, in a query string, using a single method.
You require to depend on both PreparedStatement and any of String methods replace(...) and replaceFirst(...) to achieve your requirement.
String sql = "Select $1, $2 from $3 where $4=? and $5=?";
sql = sql.replaceFirst( "$1", "col1_name" );
sql = sql.replaceFirst( "$2", "col2_name" );
sql = sql.replaceFirst( "$3", "table_name" );
sql = sql.replaceFirst( "$4", "col4_name" );
sql = sql.replaceFirst( "$5", "col5_name" );
// .. and so on
PreparedStatement pst = con.prepareStatement( sql );
// use relevant set methods to set the query parametrs.
pst.setXXX( 1, value_for_first_query_parameter ); // from a variable or literal
pst.setXXX( 2, value_for_second_query_parameter); // from a variable or literal
// ... and so on

If you are using JDBC, can try this
PreparedStatement statement = connection.prepareStatement("SELECT ? FROM ? WHERE ?=? ");
then
statement.setString(1, "column_name");
statement.setString(2, "table_name");
statement.setString(3, "column_name");
statement.setBigDecimal(4, 123);
If you are using other ORM like Hibernate or JPA, I believe there are also ways to do.

Error with simple Parameterized Query - Java/ SQL

Following on from one of my previous questions to do with method design I was advised to implemented my SQL queries as a parameterized query as opposed to a simple string.
I've never used parameterized queries before so I decided to start with something simple, take the following Select statement:
String select = "SELECT * FROM ? ";
PreparedStatement ps = connection.prepareStatement(select);
ps.setString(1, "person");
This gives me the following error: "[SQLITE_ERROR] SQL error or missing database (near "?": syntax error)"
I then tried a modified version which has additional criteria;
String select = "SELECT id FROM person WHERE name = ? ";
PreparedStatement ps = connection.prepareStatement(select);
ps.setString(1, "Yui");
This version works fine, in the my first example am I missing the point of parameterized queries or am I constructing them incorrectly?
Thanks!

Simply put, SQL binds can't bind tables, only where clause values. There are some under-the-hood technical reasons for this related to "compiling" prepared SQL statements. In general, parameterized queries was designed to make SQL more secure by preventing SQL injection and it had a side benefit of making queries more "modular" as well but not to the extent of being able to dynamically set a table name (since it's assumed you already know what the table is going to be).

If you want all rows from PERSON table, here is what you should do:
String select = "SELECT * FROM person";
PreparedStatement ps = connection.prepareStatement(select);
Variable binding does not dynamically bind table names as others mentioned above.
If you have the table name coming in to your method as a variable, you may construct the whole query as below:
String select = "SELECT * FROM " + varTableName;
PreparedStatement ps = connection.prepareStatement(select);
Parameterized queries are for querying field names - not the table name!

Prepared statements are still SQL and need to be constructed with the appropriate where clause; i.e. where x = y. One of their advantages is they are parsed by the RDMS when first seen, rather than every time they are sent, which speeds up subsequent executions of the same query with different bind values.