Creating a left join condition using where clause in sql

Question

I want to create a condition in where clause, that can imitate left join. For example:

From Table1, Table2 Where table1.ID *= table2.ID

needs to be written as

From Table1, Table2
Where Table1.ID = Table2.ID AND (some condition to get the rows from table1 where match does not exist)

I want to do this, because the existing query builder function calls like dozen functions to create the final query, and same function is called from 100+ pages. So, updating FROM clause to use Left Join will break the whole flow, and cause a whole lot of headache. I would rather just update the WHERE clause itself.

Answer 1

In general, a where clause cannot increase the number of rows in the result set. So, you cannot formally reproduce a left join in the where clause.

That said, it is possible that this would do what you want:

From Table1, Table2
Where (Table1.ID = Table2.ID or
       not exists (select 1 from table2 t2 where t2.id = table1.id) 
      )

This doesn't quite formally replicate the left join . For instance, if Table2 is empty, then the from clause returns no rows at all, so nothing is returned. However, it might work for your case.

Answer 2

CREATE TABLE Table1
    (`Name` varchar(5), `Age` int)
;

INSERT INTO Table1
    (`Name`, `Age`)
VALUES
    ('PVJ', 10),
    ('EPF', 12),
    ('CISCO', 13)
;

CREATE TABLE Table2
    (`Name` varchar(3), `Age` int)
;

INSERT INTO Table2
    (`Name`, `Age`)
VALUES
    ('PVJ', 10),
    ('EPF', 12),
    ('DVF', 13)

select Table1.Name,(select Table2.Age from Table2 where Table1.Name=Table2.Name)  Age from table1

http://sqlfiddle.com/#!2/7dbd4/17

Answer 3

From my current understanding of relational-algebra, this is not possible. The left outer join only joins where a counterpart int the right table exists.

You could use two Queries, one being the join, and one being a select with "where not_exists(select 1 from table2 where table2.id=table1.id)" You then union those two sets, this will result in your outer join.