SQL Use alias in Where statement

Question

I wounder how I could use an alias in a where statement.

Example :

SELECT SUBSTRING(Column1, 1, 4) + SUBSTRING(Column1, 4, 3)  AS Col1
FROM MyTable
W         


        

Answer 1

With PostgreSQL 9.3+ OR Oracle 12c, there is now lateral join that allows creating an alias.

Lateral joins are joints inside witch you can reference preceding tables.

SELECT col1, col2,col3
FROM MyTable m
JOIN LATERAL (
    SELECT SUBSTRING(m.Column1, 1, 4) + SUBSTRING(Column1, 4, 3)  AS Col1 
) x ON true
WHERE Col1 = 'MySearch'

With this syntax, you don't have to use '*' that can be non-performing or recopy all the columns.