the type of table join

[ b]join:[/b] return rows at least one match in tables
inner join is the most common join type used in applications ,and can be regarded
the default join type
outer join[u][/u]: outer join does not requires each record in the two records,outer join retains each record,
full join:return rows from the left join,even if there are no matches in right rows
left join: return rows from the right join, even if there are no matches in left rows
right join:return rows  from all tables