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the type of table join

 
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[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
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