How to Modify the Size of a Datatype in a Target table A matching it with the source table B for a same Column Name - sql

Question

Table A(Target) Name Varchar2(40)
Table B(source) Name Varchar2(60)
I need a script where A and B table should be compared and if the Datatype size dosen't match, then the datatype size in the A table should be altered to that of Datatype size of Table B.
Result
Table A(Target) Name Varchar2(60)
Table B(source) Name Varchar2(60)

Answer 1

It is the ALTER TABLE you'd use.
Sample table:
SQL> create table table_a (name varchar2(40));
Table created.
SQL> insert into table_a values ('Littlefoot');
1 row created.
How to modify column's size:
SQL> alter table table_a modify name varchar2(60);
Table altered.
No problem with making it larger, but - you might have problems if you'd want to make it smaller and length of some data in that column is longer than desired target size. In this example, 'Littlefoot' has 10 letters so making the column smaller than that raises an error:
SQL> alter table table_a modify name varchar2(5);
alter table table_a modify name varchar2(5)
*
ERROR at line 1:
ORA-01441: cannot decrease column length because some value is too big
SQL>
If you want to write some code which will do that job for you, you'll have to query user_tab_columns and use dynamic SQL (i.e. execute immediate). For example:
SQL> create table table_a (id number, name varchar2(40));
Table created.
SQL> create table table_b (id number, name varchar2(60));
Table created.
SQL> begin
2 for cur_r in (select a.column_name, a.data_length a_len,
3 b.data_length b_len
4 from user_tab_columns a join user_tab_columns b on a.column_name = b.column_name
5 where a.table_name = 'TABLE_A'
6 and b.table_name = 'TABLE_B'
7 and a.data_type = 'VARCHAR2'
8 )
9 loop
10 if cur_r.a_len < cur_r.b_len then
11 execute immediate 'alter table table_a modify ' || cur_r.column_name || ' varchar2(' ||
12 cur_r.b_len ||')';
13 end if;
14 end loop;
15 end;
16 /
PL/SQL procedure successfully completed.
What's the outcome?
SQL> desc table_a;
Name Null? Type
----------------------------------------- -------- ----------------------------
ID NUMBER
NAME VARCHAR2(60) --> this
SQL> desc table_b;
Name Null? Type
----------------------------------------- -------- ----------------------------
ID NUMBER
NAME VARCHAR2(60) --> this
SQL>
Columns' sizes match.
Of course, that simple code might require additional settings (if you'd want to affect other datatypes), but that should give you initial idea.