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Outer Joins

Whereas an INNER JOIN will find combinations of matching rows from joined tables, the OUTER JOIN also finds the instances where a row in one table has no match in another table. An OUTER JOIN find matches (just like an INNER JOIN), but also identifies mismatches. Two forms of outer joins are LEFT JOIN and RIGHT JOIN . These do not use the comma separator or the INNER JOIN keywords. They each answer the same kinds of questions, but differ slightly in their syntax. That is a LEFT JOIN can always be re-written into a equivalent RIGHT JOIN. (In the following sections, the terms left table and right table refer to the tables named first and second in the FROM clause, respectively.)  Left Outer Join A left join is a type of outer join, written using the LEFT JOIN keywords. A left join treats the left table (the first one named) as a reference table and produces output for each row selected from it, whether or not the row is matched by rows in the right table. Like a join writt