Selecting from two tables with a same named column

Question

SELECT bar.*, baz.foo FROM bar LEFT JOIN baz if I have a column called foo in bar, what is the standard SQL behaviour? Will the result set contain only foo from baz or...?

Answer 1

It will contain column foo from both tables.

Since your question is tagged 'standards', I would suggest using an explicit alias for 2 columns with the same name in a result set.

Answer 2

It will contain two output columns called foo ; one with values from table bar and one values from table baz .

If you do a natural join, then the foo columns will be used in the join and only one value will appear in the output - but you did a left outer join, not a natural join.

You do need to specify the join condition with an ON clause or a USING clause (but not every DBMS supports USING).

Answer 3

It will contain 2 x foo columns. This isn't a problem for SQL, but it tends to break client code. Simply alias it

SELECT bar.*, baz.foo AS foo2...
SELECT bar.*, foo2 = baz.foo...

While we're on standards, SELECT * isn't good practice: use named columns. It's been discussed to death already in other questions...