PL/SQL can you INSERT INTO (SELECT GROUP BY)? - sql

Question

I am tasked with creating an Oracle PL/SQL procedure "summarizer" that creates a summary table (mean, median etc. for each column) from a table.
The original table (mix of numeric and categorical columns):
column_1 | column_2| column_3| column_4
---------------------------------------
The desired output:
table_name | column_name | mean | tally
---------------------------------------
To do this, I am attempting to use a cursor that iterates over the USER_TAB_COLUMNS table (contains table_name, column_name) to INSERT the aggregations for each column_name INTO the SUMMARY_TABLE.
Here is my progress so far:
CREATE OR REPLACE PROCEDURE summarizer
IS
CURSOR column_cursor IS
SELECT TABLE_NAME, COLUMN_NAME, DATA_TYPE
FROM USER_TAB_COLUMNS
WHERE TABLE_NAME = 'BANK';
BEGIN
EXECUTE IMMEDIATE 'DROP TABLE SUMMARY_TABLE';
EXECUTE IMMEDIATE 'CREATE TABLE SUMMARY_TABLE (
table_name VARCHAR(20),
column_name VARCHAR(20),
data_type VARCHAR(20),
mean NUMBER,
tally NUMBER,
PRIMARY KEY (table_name, column_name))';
FOR colname IN column_cursor LOOP
INSERT INTO SUMMARY_TABLE
SELECT
colname.table_name AS table_name,
colname.column_name AS column_name,
colname.data_type AS data_type,
mean(colname.column_name) AS mean,
count(colname.column_name) AS tally
FROM BANK
GROUP BY table_name, column_name, data_type;
END LOOP;
COMMIT;
END summarizer;
This produces the error:
Error report -
20/12 PL/SQL: SQL Statement ignored
28/51 PL/SQL: ORA-00904: "DATA_TYPE": invalid identifier
I am unable to locate the cause of this error. I believe it could be something to do with a GROUP BY in the INSERT INTO statement. Can anyone help?

Answer 1

You need dynamic SQL to use variables in a query string.
Consider:
FOR colname IN column_cursor LOOP
EXECUTE IMMEDIATE
'INSERT INTO SUMMARY_TABLE
SELECT
''' || colname.table_name || ''' AS table_name,
''' || colname.column_name || ''' AS column_name,
''' || colname.data_type || ''' AS data_type,
avg(' || colname.column_name || ') AS mean,
count(' || colname.column_name || ') AS tally
FROM BANK';
END LOOP;
Side notes:
the constant values (table name, column name and datatype) need to be enclosed in quotes
there is no mean() aggregate function in Oracle (nor in most other RDBMS): you can use avg() instead
you don't actually need a group by clause in your insert ... select ... query, since the non-aggregated columns are constants
Demo on DB Fiddle:
-- create the table and insert a few records
create table bank(id int, val int);
begin
insert into bank values(1, 1);
insert into bank values(2, 1);
insert into bank values(3, 1);
end;
/
-- create the summary table (it must exist since the procedure drops it)
create table summary_table(id int);
-- create the procedure
CREATE OR REPLACE PROCEDURE summarizer
IS
CURSOR column_cursor IS
SELECT TABLE_NAME, COLUMN_NAME, DATA_TYPE
FROM USER_TAB_COLUMNS
WHERE TABLE_NAME = 'BANK';
BEGIN
EXECUTE IMMEDIATE 'DROP TABLE SUMMARY_TABLE';
EXECUTE IMMEDIATE 'CREATE TABLE SUMMARY_TABLE (
table_name VARCHAR(20),
column_name VARCHAR(20),
data_type VARCHAR(20),
mean NUMBER,
tally NUMBER,
PRIMARY KEY (table_name, column_name))';
FOR colname IN column_cursor LOOP
EXECUTE IMMEDIATE
'INSERT INTO SUMMARY_TABLE
SELECT
''' || colname.table_name || ''' AS table_name,
''' || colname.column_name || ''' AS column_name,
''' || colname.data_type || ''' AS data_type,
avg(' || colname.column_name || ') AS mean,
count(' || colname.column_name || ') AS tally
FROM BANK';
END LOOP;
COMMIT;
END summarizer;
/
-- run the procedure
begin
summarizer;
end;
/
-- check the results
select * from summary_table
TABLE_NAME | COLUMN_NAME | DATA_TYPE | MEAN | TALLY
:--------- | :---------- | :-------- | ---: | ----:
BANK | ID | NUMBER | 2 | 3
BANK | VAL | NUMBER | 1 | 3