View error in PostgreSQL

Question

I have a large query in a PostgreSQL database. The Query is something like this:

SELECT * FROM table1, table2, ... WHERE table1.id = table2.id...


        

Answer 1

That happens because a view would have two id named columns, one from table1 and one from table2, because of the select *.

You need to specify which id you want in the view.

SELECT table1.id, column2, column3, ... FROM table1, table2 
WHERE table1.id = table2.id

The query works because it can have equally named columns...

postgres=# select 1 as a, 2 as a;
 a | a
---+---
 1 | 2
(1 row)

postgres=# create view foobar as select 1 as a, 2 as a;
ERROR:  column "a" duplicated
postgres=# create view foobar as select 1 as a, 2 as b;
CREATE VIEW

Answer 2

If only join columns are duplicated (i.e. have the same names), then you can get away with changing:

select *
from a, b
where a.id = b.id

to:

select *
from a join b using (id)

Answer 3

If you got here because you are trying to use a function like to_date and getting the "defined more than once" error, note that you need to use a column alias for functions, e.g.:

to_date(o.publication_date, 'DD/MM/YYYY') AS publication_date

Answer 4

No built-in way in the language to solve it (and frankly, * is a bad practice in general because it can cause latent defects to arise as the table schemas change - you can do table1.*, table2.acolumn, tabl2.bcolumn if you want all of one table and selectively from another), but if PostgreSQL supports INFORMATION_SCHEMA, you can do something like:

DECLARE @sql AS varchar

SELECT @sql = COALESCE(@sql + ', ', '') 
    + '[' + TABLE_NAME + '].[' + COLUMN_NAME + ']'
    + CHAR(13) + CHAR(10)
FROM INFORMATION_SCHEMA.COLUMNS
WHERE TABLE_NAME IN ('table1', 'table2')
ORDER BY TABLE_NAME, ORDINAL_POSITION

PRINT @sql

And paste the results in to save a lot of typing. You will need to manually alias the columns which have the same name, of course. You can also code-gen unique names if you like (but I don't):

SELECT @sql = COALESCE(@sql + ', ', '') 
    + '[' + TABLE_NAME + '].[' + COLUMN_NAME + '] '
    + 'AS [' + TABLE_NAME + '_' + COLUMN_NAME + ']'
    + CHAR(13) + CHAR(10)
FROM INFORMATION_SCHEMA.COLUMNS
WHERE TABLE_NAME IN ('table1', 'table2')
ORDER BY TABLE_NAME, ORDINAL_POSITION