How to create table as select from multiple tables? - mysql

Question

How do I go about creating a table from multiple tables having variable columns (union cannot be used)?
mysql> select * from a;
+------+------+
| id | name |
+------+------+
| 1 | Ruby |
+------+------+
mysql> select * from b;
+------+------+
| pid | name |
+------+------+
| 1 | Java |
| 1 | C++ |
+------+------+
What I want...
+------+------+------+------+
| id | name | pid | bname|
+------+------+------+------+
| 1 | Ruby | | |
| | | 1 | C++ |
| | | 1 | Java |
+------+------+------+------+
What I have tried.
mysql> create table t as select * from a; # worked fine
mysql> alter table t as select pid, name as bname from b; #didn't work

Answer 1

You can do this in this rather arcane way:
select a.*, b.*
from a left join
b
on 1 = 0
union all
select a.*, b.*
from b left join
a
on 1 = 0;
This is one way, in MySQL of implementing:
select a.*, b.*
from a full outer join
b
on 1 = 0;
But, it requires a union. You could do it in two steps if you want to avoid the union:
create table ab as
select a.*, b.*
from a left join
b
on 1 = 0;
insert into ab
select a.*, b.*
from b left join
a
on 1 = 0;

Answer 2

There's no problem to use union, you just need to select nulls as the columns that aren't in the table:
CREATE TABLE t AS
SELECT id, name, null AS pid, null AS bname
FROM a
UNION ALL
SELECT null, null, pid, bname
FROM b

Answer 3

Create the new table, containing all four columns. Then
INSERT INTO t (id, name) SELECT * FROM a;
INSERT INTO t (pid, bname) SELECT * FROM b;
The first line will put everything from a in there, and leave the other two fields as NULL; the second will do the corresponding thing from b. You could do the whole thing in one CREATE/UNION statement, but if this is a one-off operation, it's probably easier to understand what you're doing if you do it in stages like this.

Answer 4

You can try this this as an example.
Create a new table (name, date of birth) by joining two tables (student id, name) and (student id, date of birth).
Create first table:
create table stu1(student_id char(5) primary key,name varchar(45))ENGINE=innodb;
Create second table:
create table stu2(student_id char(5) primary key,dob date)ENGINE=innodb;
Create third table (by joining two tables):
create table stu3 AS(select stu1.name,stu2.dob from stu1,stu2
where stu1.student_id=stu2.student_id);