Java从列表中提取值并将其作为列名称包含在SQL查询中

Question

I have a function hasColumnBeenIdentified() which takes an arraylist()(columnNameList) as one of the arguments to this method. The logic the method performs is that the method has to connect to the database and execute a Query. The Query that it executes is a select query in which the column names will be retrieved from the arraylist which is passed as an argument to this method. The Query will look like below

 String query = "Select columnNameList FROM "+ currTableName + " " + 
                "WHERE ROWNUM <= " + numberOfSamplesForInstaceMatching;

I have two issues here.

1. how to extract the values from the list(columnNameList) and use it in the query and execute the query. Ie the ColumnNameList contains the list of columns whose values needs to be retrieved from the db. I need to know how to insert the columnnames in the query.
2. After executing the above query, once we get the result set, I would like to know how to extract the column values from the resultset and store it in a hashmap.

Note: The method hasColumnBeenIdentified() will get invoked multiple times ie it will get invoked based on the number of tables in a particular schema. Hence I cannot customize the resultset nor the columnNamelist, since for each table, the number of columns will be vary.

Any help on this is much appreciated. Thanks in Advance

Answer 1

As per my Understand you need to coombine your column name list in query.For this you can iterate your list create comma separated string of your column name.

String columnStr = "";
for(String columnName : yourList<>)  
{  
   columnStr += columnName+",";  
}  
columnStr = columnStr.substring(0, columnStr.length()-1);  

Replace this string in your query.

String query = "Select "+columnStr+" FROM "+ currTableName + " " + 
            "WHERE ROWNUM <= " + numberOfSamplesForInstaceMatching;  

For getting column name back from ResultSet you need ResultSetMetaData like given below.

ResultSet rs = ...; //Your ResultSet
ResultSetMetaData rsm = rs.getMetaData();
String columnName = rsm.getColumnName("YourColumnIndex"); //Index start from 1  

For storing your result in HashMap you first need to decide howto store it into HashMap.

Edit :
You may use external library to join your list into String, one of them is
Apache's commons lang

Like this : String columnNames = StringUtils.join(YOUR_LIST,",");

Answer 2

You can create a String with the columnnames from the list and use this string in the query. Eg you can use following:

String columnsString  = org.apache.commons.lang.StringUtils.join(ColumnNameList, ",");
String query = "Select ColumnNameList FROM "+ currTableName + " " + 
            "WHERE columnName IN ( " + columnsString + ")";

Another possibility withou using a external library is to create the string in a for-loop:

StringBuilder builder = new StringBuilder();
builder.append(list.remove(0));

for( String s : ColumnNameList) {
    builder.append(", ");
    builder.append(s);
}