如何在Java中串联SQL查询？

Question

I'm beginning to learn sql and java and I have a problem.

The logic for the code is:

The first part of the sql statement should be "a.stdn_code_ts" and as more elements (in this case student code) are included, I need to concatenate with and OR-Statement.

student codes could be a single value or a range, say for example: '567777' is valid as well as '567777-876677'.

If it is single value, just add "=" then the student code. In the example if the user entered '567777' then the query should be something like "a.stdnt_code_ts = '567777'"

If it is a range, add the first student code then "BETWEEN" second code. Ie: if the user entered '567777-876677', the query should be "a.stdnt_code_ts BETWEEN '567777' AND '876677'".

and as I mentioned above, if there are 2 or more student codes the query should be concatenated with an "OR a.stdnt_code_ts" then checks again if it is a single value or a range.

I already have this code and got stuck:

private void formatStudentCode(Connection connection) throws Exception {
    studentCode = "a.stdnt_code_ts ";

    for(int i=0; i < stdntCode.size(); i++) {
        if (stdntCode.get(i).indexOf("-")==-1) {
            studentCode += "= '" + stdntCode.get(i) + "'";
        }
        else {
            String [] range=stdntCode.get(i).split("-");
            studentCode += "BETWEEN '" + range[0] + "' AND '" + range[1] + 
            "'";
        }
    }
}

Answer 1

First, this code is incomplete, so I'll need to make some guesses. But let's try.

Let's fix the loop first:

String sql = "SELECT * FROM students";

List<String> terms = new ArrayList<>();
List<String> arguments = new ArrayList<>();
// Don't need index here, for each loop is better
for (String code : codes) {
    // No need for IndexOf
    if (code.contains("-")) {
        terms.add("stdnt_code_ts between ? and ?");
        String[] split = code.split("-");
        arguments.add(split[0]);
        arguments.add(split[1]);
    }
    else {
        // Don't concatenate SQL query parameters
        terms.add("stdnt_code_ts = ?");
        arguments.add(code);
    }
}

Now to put our OR :

if (terms.size() > 0) {
   sql += " WHERE " + Strings.join(terms, " OR ");
}

Now to add actual parameters for each question mark:

PreparedStatement preStmt = conn.prepareStatement(sql);
int count = 0;

for (String code : arguments) {
    preStmt.setString(++count, code);
}

And finally to execute the query:

ResultSet rs = preStmt.executeQuery();

Note that I'm not running this code, so I may miss a line or two, but that's the general idea of how it should be done correctly.