Is it possible to get overview of a CTE? - sql

Question

In sql developer I can do
desc table
and get
describe table
Name Null Type
---------------------- ---- -------------
DATE_TIME DATE
KEY VARCHAR2(11)
Is something like this possible with a CTE? For example if there are 10 columns in table, I will get the description of all of them. But what if I want the description of only 2 columns.
with alias as (select col1, col2 from table) desc alias
This doesn't work.

Answer 1

You could apply a trick:
CREATE VIEW dummy AS
WITH ..
SELECT ..
And then:
DESC dummy

Answer 2

No, you can't do that.
A CTE is not stored anywhere in the system catalogs. It's essentially the same as a derived table:
with alias as (select col1, col2 from table)
select * from alias
is the same as
select *
from ( select col1, col2 from table) alias
And you can't describe that derived table either.
The only way you can do something remotely similar is to select from the CTE but with a where condition that selects not rows.
with alias as (select col1, col2 from table)
select * from alias
where 0 = 42;
Then you would at least see the column names and data types (the details of that depend on the SQL client you are using)

Answer 3

You can do this without creating a view, with something a bit more complicated:
create table yourTable(
DATE_TIME DATE,
KEY VARCHAR2(11)
);
You can use DBMS_SQL to describe the columns of a select query:
SQL> DECLARE
2 l_Cursor INTEGER DEFAULT dbms_sql.open_cursor;
3 l_Stmt VARCHAR2(4000);
4 l_colCnt NUMBER DEFAULT 0;
5 l_descTbl dbms_sql.desc_tab;
6 BEGIN
7 dbms_sql.parse(l_Cursor, 'SELECT key FROM yourTable', dbms_sql.native);
8 dbms_sql.describe_columns(l_Cursor, l_colCnt, l_descTbl);
9 --
10 FOR i IN 1..l_colCnt LOOP
11 dbms_output.put_line('Column '||l_descTbl(i).col_name || ' - type '||l_descTbl(i).col_type);
12 END LOOP;
13 --
14 dbms_sql.close_cursor(l_Cursor);
15 END;
16 /
Column KEY - type 1
PL/SQL procedure successfully completed.
The result must be interpreted by this table:
Datatype Number
VARCHAR2 1
NVARCHAR2 1
NUMBER 2
INTEGER 2
LONG 8
ROWID 11
DATE 12
RAW 23
LONG RAW 24
CHAR 96
NCHAR 96
MLSLABEL 106
So, it is saying that the column KEY has type 1, that is a VARCHAR2 .
The same way you can have more informations, about size for example.