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

use a view or a derived table.

Using a derived table, your example would look like:

select col1 
from 
(SELECT SUBSTRING(Column1, 1, 4) + SUBSTRING(Column1, 4, 3)  AS Col1
FROM MyTable) 
where col1='Mysearch'

Answer 2

Use a subquery:

SELECT * 
FROM 
  (SUBSTRING(Column1, 1, 4) + SUBSTRING(Column1, 4, 3) AS Col1 FROM MyTable)
WHERE Col1 = 'MySearch'

Answer 3

The answer is you can't - you can do this

SELECT 
    SUBSTRING(Column1, 1, 4) + SUBSTRING(Column1, 4, 3)  AS Col1
FROM 
    MyTable
WHERE 
    SUBSTRING(Column1, 1, 4) + SUBSTRING(Column1, 4, 3) = 'MySearch'

Answer 4

You can do this:

SELECT Col1
FROM ( SELECT SUBSTRING(Column1, 1, 4) + SUBSTRING(Column1, 4, 3) AS Col1 ) AS x
WHERE Col1 = 'MySearch'

Answer 5

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.

Answer 6

Not possible, but you can do the following:

SELECT SUBSTRING(Column1, 1, 4) + SUBSTRING(Column1, 4, 3)  AS Col1
FROM MyTable
WHERE SUBSTRING(Column1, 1, 4) + SUBSTRING(Column1, 4, 3) = 'MySearch'

No subqueries or hacks required