Fetch column from result set having expression based SELECT without alias name - sql

Question

This should be a Basic Question, but I was unable to answer when asked for. Sorry if it is too broad!
SELECT * FROM
(
SELECT MYGROUPKEY,MAX(MYCOLUMN)/MIN(MYCOLUMN) FROM
MYTABLE
GROUP BY MYGROUPKEY
) MYSUBQUERY;
This Query is Syntactically Valid. But how would someone fetch the second column of this resultset, if they have to attempt with some identifier? (The column was never provided with an alias). How can it be formatted in SQL*Plus. (COL <column_name> FORMAT A<num>)
Will that be first few characters of the text MAX(MYCOLUMN)/MIN(MYCOLUMN).
I understand, RDBMS concept doesnt like this. But still why would the Database execute this SQL then? It never name the resultset's columns unless the query carry it?

Answer 1

Assuming that the expression is less than 30 characters
SQL> column "MAX(MYCOLUMN)/MIN(MYCOLUMN)" format <<some format>>
should work. You can realistically look at the raw column header in SQL*Plus (before applying any formatting) to see what the assigned alias is. Depending on the actual expression, there can be various rules applied-- eliminating spaces, for example, and limiting the expression to 30 characters. It's generally easier to copy & paste from SQL*Plus and surround the assigned alias with double quotes.

Answer 2

There is an implicit alias which is the expression with any whitespace removed. But as the expression contains characters which aren't invalid for an identifier it has to be quoted, as "MAX(MYCOLUMN)/MIN(MYCOLUMN)". You can refer to that in an SQL*Plus column command too.
e.g.
SELECT MYGROUPKEY, "MAX(MYCOLUMN)/MIN(MYCOLUMN)" FROM
(
SELECT MYGROUPKEY,MAX(MYCOLUMN) / MIN(MYCOLUMN)
FROM MYTABLE
GROUP BY MYGROUPKEY
) MYSUBQUERY;
SQL Fiddle.
There is a further wrinkle though. As identifieres can only be 30 characters, if the expression is longer than that the implicit alias is also too long to be referred to within the SQL itself. This would get an ORA-00972:
SELECT MYGROUPKEY, "MAX(MYCOLUMN)/MIN(MYCOLUMN)*(1000/1000)" FROM
(
SELECT MYGROUPKEY,MAX(MYCOLUMN) / MIN(MYCOLUMN) * (1000 / 1000)
FROM MYTABLE
GROUP BY MYGROUPKEY
) MYSUBQUERY;
It's better to add your own aliases, even for shorter expressions.
But you can sometimes still use the longer alias, for example through JDBC, where it appears in the metadata (as it does here as the column label). A section of a test that accesses the same data as the SQL Fiddle:
pStmt = (OraclePreparedStatement) conn.prepareStatement(
"SELECT MYGROUPKEY,MAX(MYCOLUMN) / MIN(MYCOLUMN) * (1000 / 1000) "
+ "FROM MYTABLE GROUP BY MYGROUPKEY");
rSet = (OracleResultSet) pStmt.executeQuery();
OracleResultSetMetaData rsmd = (OracleResultSetMetaData) rSet.getMetaData();
for( int i=1; i <= rsmd.getColumnCount(); i++ ) {
System.out.println( "Column label: " + rsmd.getColumnLabel(i));
System.out.println( "Column Type: " + rsmd.getColumnTypeName(i));
System.out.println();
}
while (rSet.next())
{
System.out.println("MYGROUPKEY: " + rSet.getInt("MYGROUPKEY"));
System.out.println("MAX(MYCOLUMN)/MIN(MYCOLUMN)*(1000/1000): "
+ rSet.getInt("MAX(MYCOLUMN)/MIN(MYCOLUMN)*(1000/1000)"));
}
Produces output:
Column label: MYGROUPKEY
Column Type: NUMBER
Column label: MAX(MYCOLUMN)/MIN(MYCOLUMN)*(1000/1000)
Column Type: NUMBER
MYGROUPKEY: 1
MAX(MYCOLUMN)/MIN(MYCOLUMN)*(1000/1000): 2
Note the the column label/name reported n the metadata is the same as shown in the Fiddle, the expression with whitespace removed; and that I can use that as the argument for getInt() even though it's longer than 30 characters.