How to assign a values to a column made in a select query

Question

In a select query I can make-up columns but how can I assign values for them?

Example

select a.col1, a.col2, 'column3'
from A a
  union
select b.col2, b.col3, b.col3 as `column3`
from B b

I want to assign a default value of -1 to the column3 column I made in the first query. Also, I want the title of the column to still be column3 . is this possible?

Answer 1

Try this

select a.col1, a.col2, -1 as column3
from A a
  union
select b.col2, b.col3, b.col3
from B b

Or this if b.col3 is varchar

select a.col1, a.col2, '-1' column3
from A a
  union
select b.col2, b.col3, b.col3
from B b

If A and B table has the same values for the tree columns the database will do an DISTINCT to avoid that, if you want, use UNION ALL

Answer 2

To create a dummy column and assgin values to it we can use

select a.col1, a.col2, '-1' as col3 from A a

Answer 3

You are most of the way there:

select a.col1, a.col2, -1 as 'column3'
from A a
  union
select b.col2, b.col3, b.col3
from B b