Question

I need to do select * based on a list of input ids, what’s the best way of batch select? here’s what I have

StringBuilder inClause = new StringBuilder();
boolean firstValue = true;
for (int i=0; i < batchSize; i++) {
  inClause.append('?');
  if ( firstValue ) {
    firstValue = false;
  } else {
    inClause.append(',');
  }
}
PreparedStatement stmt = conn.prepareStatement(
    "select id, name from users where id in (" + inClause.toString() + ')');


for (int i=0; i < batchSize; i++) {
  stmt.setInt(i);  // or whatever values you are trying to query by
}

Answer 1

It looks pretty fine to me. Just spotted a logical bug there in this block of code,

boolean firstValue = true;
for (int i=0; i < batchSize; i++) {
  inClause.append('?');
  if ( firstValue ) {
    firstValue = false;
  } else {
    inClause.append(',');
  }
}

It will not append a , after the first element. And there would be a , after the last. So, you need not care about that , here. Just do it this way

for (int i=0; i < batchSize; i++) {
  inClause.append('?, ');
}

Then chop last two characters like this,

PreparedStatement stmt = conn.prepareStatement(
    "select id, name from users where id in (" + 
    inClause.substring(0, inClause.length()-2) + ')');