SQL，sql教程
Inner joins return rows only when there is at least one row from both tables that matches the join condition. Inner joins eliminate the rows that do not match with a row from the other table. Outer joins, however, return all rows from at least one of the tables or views mentioned in the FROM clause, as long as those rows meet any WHERE or HAVING search conditions. All rows are retrieved from the left table referenced with a left outer join, and all rows from the right table referenced in a right outer join. All rows from both tables are returned in a full outer join.SQL Server uses the following ISO keywords for outer joins specified in a FROM clause:
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LEFT OUTER JOIN or LEFT JOIN
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RIGHT OUTER JOIN or RIGHT JOIN
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FULL OUTER JOIN or FULL JOIN
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