本文详细介绍了SQL中不同类型的连接方式，包括内连接、外连接、左连接和右连接等，并解释了每种连接如何工作及应用场景。对于理解数据库查询逻辑至关重要。
[[u]b]join:[/b] [/u] 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
[b]outer join[/b][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
inner join is the most common join type used in applications ,and can be regarded
the default join type
[b]outer join[/b][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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