Compare two tables and find the missing column using left join - select

Question

I wanted to compare the two tables employees and employees_a and find the missing columns in the table comployees_a.
select a.Column_name,
From User_tab_columns a
LEFT JOIN User_tab_columns b
ON upper(a.table_name) = upper(b.table_name)||'_A'
AND a.column_name = b.column_name
Where upper(a.Table_name) = 'EMPLOYEES'
AND upper(b.table_name) = 'EMPLOYEES_A'
AND b.column_name is NULL
;
But this doesnt seems to be working. No rows are returned.
My employees table has the below columns
emp_name
emp_id
base_location
department
current_location
salary
manager
employees_a table has below columns
emp_name
emp_id
base_location
department
current_location
I want to find the rest two columns and add them into employees_a table.
I have more than 50 tables like this to compare them and find the missing column and add those columns into their respective "_a" table.

Answer 1

Missing columns? Why not using the MINUS set operator, seems to be way simpler, e.g.
select column_name from user_tables where table_name = 'EMP_1'
minus
select column_name from user_tables where table_name = 'EMP_2'

Answer 2

Thirstly, check if user_tab_columns table contains columns of your tables (in my case user_tab_columns is empty and I have to use all_tab_columns):
select a.Column_name
From User_tab_columns a
Where upper(a.Table_name) = 'EMPLOYEES'
Secondly, remove line AND upper(b.table_name) = 'EMPLOYEES_A', because upper(b.table_name) is null in case a column is not found. You have b.table_name in JOIN part of the SELECT already.
select a.Column_name
From User_tab_columns a
LEFT JOIN User_tab_columns b
ON upper(a.table_name) = upper(b.table_name)||'_A'
AND a.column_name = b.column_name
Where upper(a.Table_name) = 'EMPLOYEES'
AND b.column_name is NULL

Answer 3

You do not need any joins and can use:
select 'ALTER TABLE EMPLOYEES_A ADD "'
|| Column_name || '" '
|| CASE MAX(data_type)
WHEN 'NUMBER'
THEN 'NUMBER(' || MAX(data_precision) || ',' || MAX(data_scale) || ')'
WHEN 'VARCHAR2'
THEN 'VARCHAR2(' || MAX(data_length) || ')'
END
AS sql
From User_tab_columns
Where Table_name IN ('EMPLOYEES', 'EMPLOYEES_A')
GROUP BY COLUMN_NAME
HAVING COUNT(CASE table_name WHEN 'EMPLOYEES' THEN 1 END) = 1
AND COUNT(CASE table_name WHEN 'EMPLOYEES_A' THEN 1 END) = 0;
Or, for multiple tables:
select 'ALTER TABLE ' || MAX(table_name) || '_A ADD "'
|| Column_name || '" '
|| CASE MAX(data_type)
WHEN 'NUMBER'
THEN 'NUMBER(' || MAX(data_precision) || ',' || MAX(data_scale) || ')'
WHEN 'VARCHAR2'
THEN 'VARCHAR2(' || MAX(data_length) || ')'
END
AS sql
From User_tab_columns
Where Table_name IN ('EMPLOYEES', 'EMPLOYEES_A', 'SOMETHING', 'SOMETHING_A')
GROUP BY
CASE
WHEN table_name LIKE '%_A'
THEN SUBSTR(table_name, 1, LENGTH(table_name) - 2)
ELSE table_name
END,
COLUMN_NAME
HAVING COUNT(CASE WHEN table_name NOT LIKE '%_A' THEN 1 END) = 1
AND COUNT(CASE WHEN table_name LIKE '%_A' THEN 1 END) = 0;
fiddle